Topic outline

  • Unit 1: Chemical Bonding

    LEARNING OBJECTIVES                                                                               


    After reading this  unit, you will be able to:

    • explain the nature of ionic, covalent and metallic bonding.

    • state the typical physical properties of ionic compounds, and of covalent compounds.

    • explain the physical properties of metals in terms of their structure.

    KNOWLEDGE GAIN

    In 1985, a new all otrope of carbon Buck minsterfullerene was discovered. It has a cage-like ring structure which resembles a football. It is made of twenty hexagons and twelve pentagons.

    1.1 STABILITY OF ATOMS

    ACTIVITY 1.1: Showing Stability of Atoms

    • Take a glass full of water. Try adding water into it. Are you able to add?

    • Now take another glass of water but a quarter (one-fourth) filled.

    • Try adding water into it. Now, are you able to add or not?

    Perform the two activities in classroom and then discuss your answers among your classmates.
     

    In the above activity, you will observe that when the glass was already filled, there was no space to add more water into it. Thus, the water in the glass remained stable. A noble gas has a fully filled outermost shell just like the glass full of water. It has eight electrons in the outermost shells except helium (2 electrons).

    When atoms or the elements combine to form molecules, a force of attraction is developed between the atoms (or ions) which holds them together. The force which links the atoms (or ions) in a compound is called a chemical bond (or just “bond”). A bond is formed so that each atom acquires a stable electronic configuration similar to that of a noble gas.

    The atoms combine with one another to achieve the inert gas electron arrangement and become more stable. So, when atoms combine to form compounds, they do so in such a way that each atom gets 8 electrons in its outermost shell or 2 electrons in the outermost n shell.

    An atom can achieve the inert gas electron arrangement in three ways:

    By losing one or more electrons (to another atom). Atoms with 1, 2 or 3 electrons in the outermost shell lose electrons to achieve stability.

    By gaining one or more electrons (from another atom).  Atoms with five, six or seven electrons in the outermost shell gain three, two or one electron respectively to achieve stability.

    • By sharing one or more electrons (with another atom).  Atoms with four to seven electrons in outermost shell may achieve stability by sharing them with each other.

    EXERCISES 1.1

    1. What do you mean by a chemical bond?

    2. When a bond is formed, each atom acquires a stable configuration similar to _______ 

    3. Generally, metals lose electrons to achieve inert gas electron arrangement. (True or False)

    4. Which of the following is not a noble gas?  (a) Helium (b) Neon  (c) Hydrogen (d) Argon

    5. Among, phosphorus, sulphur, and calcium; which element achieves stability by losing electron.

    1.2 FORMATION OF IONS FROM ATOMS

    ACTIVITY 1.2: Illustrating Formation of Ion

    Divide the class into two groups. Half of the students hold positive plank cards and another half hold negative plank cards. Positive plank cards are protons and negative plank cards are electrons. Now perform the following and analyze:

    Students with 5 positive and 5 negative plank cards are grouped together. Their total charge being neutral in the group.

    • Now, one electron is removed from the group. 4 students are left holding negative plank cards.  Can you tell the net charge now of this group?

    • Add one electron to the neutral group.
     6 students are now holding negative plank cards. 

    Can you now tell what is the charge of this group?

    • Similarly, perform the above activity with 7 students and analyze the charge.

    An atom contains electrons, protons and neutrons.

    Protons carry positive charges, electrons carry negative charges and neutrons carry no charges. Every atom contains an equal number of “positively charged protons” and “negatively charged electrons”.

    Thus, an atom is electrically neutral.
                                       

    An ion is formed when an atom loses or gains one or more electrons. The atom may be of a metal or a non-metal.

    A metal readily loses its outermost electron or electrons to form a positive ion or cation. The number of positive charges carried on a cation is equal to the number of electron(s) lost by the metal atom. Examples are given in Table 1.3.

                                        

    Metal ions carry positive charges because the number of positively charged protons in the nucleus becomes greater than the number of negatively charged electrons surrounding it. For example, in a sodium atom there are 11 protons in the nucleus and 11 electrons surrounding it. Loss of one electron to form a sodium ion means that there are 11 protons but only 10 electrons. There is a net charge of 1+. This charge is written as a superscript at the right of the symbol of the element (Figure 1.1).

                                          Figure 1.1: Formation of a sodium ion.

    Hydrogen atoms can also lose an electron to form an ion with one positive charge. Some non-metals readily gain one or more electrons into their outermost shell to form a negative ion or anion. The number of negative charges an anion carries is equal to the number of electron(s) gained by the non-metal atom. Examples are given in Table 1.4.
         

    Non-metal ions carry negative charges because the number of negatively charged electrons surrounding the nucleus becomes greater than the number of positively charged protons in it. For example, in a chlorine atom there are 17 protons in the nucleus and 17 electrons surrounding it. Gain of one electron to form a chloride ion means that there are 18 electrons and only 17 protons. There is a net charge of –1. This charge is written as a superscript at the right of the symbol of the element (Figure 1.2).

                     

    Notice that when a non-metal forms an anion, the name changes slightly; chlorine forms a chloride ion, oxygen forms an oxide ion. Several common radicals exist as negative ions including nitrate (NO– 3), carbonate
     (CO2– 3 ) and phosphate (PO3 4 –).

    EXERCISE 1.2

     1. Define ion.

    2. Which of the following is an anion?  (a) Cl                            (b) Na

                                                                 (c) Mg2+                         (d) Al3+

    3. Why do metal ions carry positive charges?

    4. The number of negative charges an anion carries is equal to the number of electrons gained by the non-metal atom. (True or False)

    5. Give two examples of each:  (i) anion (ii) cation


    1.3 IONIC BONDING

    The compounds which are made up of ions are known as ionic compounds. In an ionic compound, the positively charged ions (cations) and negatively charged ions (anions) are held together by the strong electrostatic forces of attraction. The forces which hold the ions together in an ionic compound are known as ionic bonds or electrovalent bonds. Since an ionic bond consists of an equal number of positive ions and negative ions, the overall charge on an ionic compound is zero. For example, sodium chloride (NaCl) is an ionic compound which is made up of equal number of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl). Some of the common ionic compounds, their formulae and the ions present in them are given in Table 1.5.

     

     

    Ionic compounds are made up of a metal and a non-metal (except ammonium chloride which is an ionic compound made up of only non-metals). So, whenever a bond involves a metal and a non-metal, we call it ionic bond.

    EXERCISE 1.3

     1. Give two examples of ionic compounds. Write their chemical formulae.

    2. The overall charge on an ionic compound is zero. (True or False)

    3. Name the ions present in calcium nitrate.

    4. Ionic compounds are made up of a ______ and a ______ .

    5. Give an example of an ionic compound made up of only non-metals.

    1.4 FORMATION OF IONIC BOND

    An ionic bond changes the electronic configurations of the atoms. Metal atoms lose their outermost electron(s), forming cations. Non-metal atoms gain electron(s) to fill their outermost shell, forming anions. The electrostatic force of attraction between the oppositely charged ions holds the ions together. For example,

    (a) When a hot sodium atom is placed in chlorine gas, a reaction takes place

    Resulting in formation of sodium chloride

                                      

     (b) When a magnesium atom comes in contact with chlorine gas, it forms magnesium chloride.

     1. With the help of dot and cross, show the formation of CaCl2.

    1.5 PROPERTIES OF IONIC COMPOUNDS

    ACTIVITY 1.3: Illustrating Physical Properties of Ionic Compounds

    • Take a sample of sodium chloride or any other salt from the science laboratory.

    • What is the physical state of this salt?

    • Take a small amount of a sample on a metal spatula and heat directly on the flame as shown in figure (a).

                                                   

    (a) Testing melting point of sodium chloride

    • What did you observe? Did the sample impart any color to the flame? Does this compound melt

    • Try to dissolve the sample in water, petrol and kerosene. Is it soluble?

    • Make a circuit as shown in figure (b) and insert the electrodes into a solution of salt. What did you observe?

                                        

                   (b) Testing electrical conductivity of salt solution
                  • What is your inference about the nature of this compound?

    You may have observed the following general properties for ionic compounds:

    • Ionic compounds are usually crystalline solids.

    • Ionic compounds have high melting and high boiling points.


    The temperature at which a solid melts into liquids is called the melting point of the solid. The temperature does not change during melting.

    Boiling point is the temperature at which a liquid changes into a gas. The temperature of a liquid remains the same once boiling has started.

    • Ionic compounds are usually soluble in water but insoluble in organic solvents like petrol and kerosene.

    • Ionic compounds conduct electricity when dissolved in water or when melted. When we dissolve the ionic solid in water or melt it, the crystal structure is broken down to form ions. These ions help in conducting electricity.


    EXERCISE 1.5


     1. Why do ionic compounds conduct electricity when dissolved in water?

    2. Ionic compounds are insoluble in

                        (a) kerosene                     (b) petrol

                        (c) both (a) and (b)            (d) neither (a) nor (b)

    3. Ionic compounds have low melting and boiling points. (True or False)

    4. Ionic compounds are usually ______ solids

    1.6 COVALENT BONDING

    The chemical bond formed by sharing of electrons between two atoms is known as a covalent bond. The compounds containing covalent bonds are known as covalent compounds. A covalent bond is formed when both the reacting atoms need electrons to achieve the inert gas electron arrangement. Now, the non-metals have usually 5, 6 or 7 electrons in the outermost shells of their atoms. So, all the non-metal atoms need electrons to achieve the inert gas structure. They get these electrons by mutual sharing. Thus, whenever a non-metal combines with another non-metal, covalent bond is formed.

           

            

          

    1.7 FORMATION OF COVALENT BOND


    Covalent bonding between atoms of different elements. 

    (i) Carbon atom shares four electrons to form methane.

                            

     (ii) As in water molecule, 2 hydrogen atoms share electrons with oxygen atom.

                                  

                 

    1.8 PROPERTIES OF COVALENT COMPOUNDS


    ACTIVITY 1.4: Illustrating Physical Properties of Covalent Compounds

    Let us test some covalent compounds in different ways:

    • Take sample of cooking oil. Try to dissolve it in water and ethanol. Does it dissolve?

    • Have you ever observed a burning candle wax? If not, take a candle wax and observe it burning. How much time does it take to melt down?

    • Take a pan and add water to it. Let it boil. Do you know the boiling point of water?

    • Now add two electrodes to the water pan making a circuit. What did you observe? What would have happened if you would have added NaCl salt in the pan?

    • What can you now say about these covalent compounds?
    You have observed the following properties of covalent compounds:

    • Covalent compounds are usually liquids, gases or solids. For example, alcohol, benzene, water and cooking oil are liquids. Methane, ethane and chlorine are gases. Glucose, urea, and wax are solid covalent compounds.

    • Covalent compounds have usually low melting points and  low boiling points.

    • Covalent compounds are usually insoluble in water, but they are soluble in organic solvents. Some of the covalent compounds like glucose, sugar and urea are soluble in water.

    • Covalent compounds do not conduct electricity because they do not contain ions.

    ACTIVITY 1.5: Detecting an Ionic Bond or Covalent Bond

    • Take the sample such as common salt (NaCl) provided.

    • Try to dissolve it in water.

    • If it dissolves, chances are it is likely to be an ionic compound. But, you already know some covalent compounds like glucose, urea and sugar are soluble in water.

    • Now, perform electrical conductivity test.

    • If the NaCl sample dissolves in water, arrange a circuit with two electrodes and a bulb.

    • Figure  out whether the bulb glows or not. According to your observation conclude the bond present in the sample.

    • Make a report on the properties of ionic and covalent compounds.

                                            

      

      

    ACTIVITY 1.6: Identifying Ionic and Covalent Compounds

     Choose the ionic as well as covalent compounds from the bubbles and make a table in your exercise notebook.

                                       

    EXERCISE 1.8


     1. Some covalent compounds are solid. (True or False)

    2. Most covalent compounds are ______ in water but ______ in organic solvents.

    3. Name two covalent compounds which are soluble in water.

    4. Why most covalent compounds do not conduct electricity?

    5. Melting and boiling points of covalent compounds are

    (a) high  (b) low  (c) between 500°C and 1000°C  (d) cannot be determined.


    1.9 GIANT COVALENT STRUCTURES

    Diamond, graphite and silicon dioxide have giant covalent structures.


    1.9.1 Diamond and its Properties

    Diamond is a colorless transparent substance having extraordinary brilliance. Diamond is quite heavy. Diamond is extremely hard. It is the hardest natural substance known. Diamond does not conduct electricity. Diamond burns on strong heating to form carbon dioxide. It has a very high melting point. If we burn diamond in oxygen, then only carbon dioxide gas is formed and nothing is left behind. This shows that diamond is made up of carbon only. Since diamond is made up of carbon atoms only, its symbol is taken to be C.

                                     

    1.9.2 Graphite and its Properties


    Graphite is a greyish-black opaque substance. Graphite is lighter than diamond. Graphite is soft and slippery to touch. Graphite conducts electricity. Graphite burns on strong heating to form carbon dioxide. Like diamond, graphite also has very high melting point. If we burn graphite in oxygen, then only carbon dioxide gas is formed and nothing is left behind. This shows that graphite is made up of carbon only. Since graphite is made up of carbon atoms only, its symbol is taken to be C.

                                  

    1.9.3 Silicon Dioxide and its Properties

    Silicon dioxide (also known as Silica) has a giant covalent structure. Each silicon atom is covalently bonded to four oxygen atoms. Each oxygen atom is covalently bonded to two silicon atoms. This means that, overall, the ratio is two oxygen atoms to each silicon atom, giving the formula SiO2. Silicon dioxide is very hard. It has a very high melting point (1,610°C) and boiling point (2,230°C). It  is insoluble in water, and does not conduct electricity.

    These properties result from the very strong covalent bonds that hold the silicon and oxygen atoms in the giant covalent structure. Silicon dioxide is found as quartz in granite, and is the major compound in sandstone. The sand on a beach is made mostly of silicon dioxide.

                                                       

    1.9.4  Uses of Diamond, Graphite and Silicon Dioxide

    Uses of Diamond

    • Since diamond is extremely hard, it is used for cutting and grinding other hard materials. It is also used for drilling holes in the earth’s rocky layers. Diamond ‘dies’ are used for drawing thin wires like the tungsten filament of an electric bulb. 

    • Diamonds are used for making jewelry. The use of diamonds in making jewelry is because of their extraordinary brilliance. Diamond is also used in the tip of glass cutter. A sharp diamond-edged knife called keratome is used by eye surgeons to remove cataract from the eyes.

                      

    Uses of Graphite

    Due to its softness, powdered graphite is used as a lubricant for fast moving parts of machinery. Graphite can be used as a dry lubricant in the form of graphite powder or mixed with petroleum jelly to form graphite grease. Graphite powder can also be mixed with lubricant oils.
    Anode (Zinc Inner Case)
    Cathode (Graphite Rod)
    Paste of MnO ,
    NH Cl, and Carbon 2 4

                                                 

                                           

                                     Figure 1.7: Some of the uses of graphite.

    • Graphite is a good conductor of electricity due to which graphite is used for making carbon electrodes or graphite electrodes in dry cells and electric arcs. The black colored ‘anode’ of a dry cell is made of graphite. The carbon brushes of electric motors are also made of graphite.

    • Graphite is used for making the cores of our pencils called ‘pencil leads’ and black points. Graphite is black in color and quite soft. So, it marks black lines on paper. Due to this property, graphite is used for making pencil leads. For making pencil leads, graphite is usually mixed with clay.

    Uses of Silicon Dioxide

    • An estimated 95% of silicon dioxide produced is consumed in the construction industry,

       e.g. for the production of Portland cement

    • Silica is used primarily in the production of glass for windows, drinking glasses, beverage bottles, and many other uses.

      • The majority of optical fibers for telecommunication are also made from silica.

                                               

                                           Figure 1.8: Some of the uses of silicon dioxide

    EXERCISE 1.9

    1. Diamond and Graphite are two common all tropes of Carbon. (True or False) 

    2. Which of the following is correct? 

    (a) Diamond is the hardest substance known. 

    (b) Graphite has very low melting point. 

    (c) Graphite does not conduct electricity. 

    (d) Diamond burns on strong heating to form helium gas.

    3. Why are diamonds used for making jewelry?

    4. Graphite is used for making 

    (a) pencil lead

    (b) electrodes 

    (c) both (a) and (b)

    (d) none of these.

    5. Diamond and Graphite have very ______ melting point.

    1.10 METALLIC BONDING

    The force which binds various metal atoms together is called metallic bond. The metallic bond is neither a covalent bond nor an ionic bond because these bonds are not able to explain properties of metals.

    For example, metals are very good conductors of electricity but in solid state. Both ionic and covalent compounds cannot do so with the exception of graphite.


    1.11 FORMATION OF METALLIC BOND

    Loreutz proposed the theory of electron gas model or electron sea model for metallic bonding.

    In this model, the metal is pictured as an array of metal cations in a “sea” of electrons. The atoms in a metallic solid contribute their valence electrons to form a “sea” of electrons that surrounds metallic cations. Delocalized electrons are not held by any specific atom and can move easily throughout the solid. A metallic bond is the attraction between these electrons and the metallic cation.

                                         

    EXERCISE 1.10

    1. Name the scientist who proposed the theory of electron sea model.

    2. Metallic bond is neither a covalent bond nor an electrovalent bond. (True or False)

    3. The force which binds various metal atoms together is called ______ .

    4. Make a 3D structure of electron sea model.

    5. Write a short note on formation of metallic bonding.

    1.12 PROPERTIES OF METALLIC BOND


    ACTIVITY 1.7: Illustrating the Properties of Metals

    • Take samples of iron, copper, aluminum, sodium, carbon and iodine. Note the appearance of each sample.

    • Clean the surface of each sample by rubbing them with sand paper and note their appearance again.

    • Try to cut these elements with a sharp knife and note your observations.

    • Hold a piece of sodium with a pair of tongs.  Caution: Always handle sodium with care. Dry it by pressing between the folds of a filter paper.

    • Put it on a watch-glass and try to cut it with a knife.

    • What do you observe?

    • Place any one element on a block of iron and strike it four or five times with a hammer. What do you observe?

    • Repeat above steps with other elements. 

    • Record the change in the shape of these elements.

    • Which of the above elements are available in the form of wires?


    1.12.1 Properties of Metals


    ACTIVITY 1.8: Illustrating Conductivity of Heat and Electricity of Metals

    • Take an aluminum or copper wire. Clamp this wire on a stand, as shown in Figure (a).

    • Fix a pin to the free end of the wire using wax.

    • Heat the wire with a spirit lamp, candle or a burner near the place where it is clamped. 

    • What do you observe after some time?

    • Repeat the same with carbon or Sulphur.

    • Note your observations.  Does the element melt?

                                           

    • Consider elements aluminum, copper, Sulphur and carbon.

    • Set up an electric circuit as shown in Figure (b).

                                           


    • Place the element to be tested in the circuit between terminals A and B as shown. Does the bulb glow? What does this indicate?

    • Compile your observations regarding properties of elements in your exercise book.

     • Metals are good conductors of heat and electricity. This means that metals allow heat and electricity to pass through them easily. Silver metal is the best conductor of heat. Copper metal is a better conductor of heat than aluminum metal.

    • Metals are lustrous (or shiny). This means that metals have a shiny appearance.
    Chemical Bonding 17
     • Metals are usually strong. For example, iron metal (in the form of steel) is very strong  when freshly cut and is used in the construction of bridges, buildings and vehicles. Some metals are not strong. For example, sodium and potassium.

    • Metals are ductile. This means that metals can be drawn (or stretched) into thin wires.

    • Gold and silver are among the best ductile metals.

    • Metals are malleable. This means that metals can be hammered into thin sheets.
    The

    The cooking utensils are made of metals because metals are good conductors of heat.

    EXERCISE 1.11

    1. Name the metal which is the best conductor of electricity.

    2. Aluminum is a better conductor of heat than copper.  (True or False)

    3. Metals are _______. This means that they can be hammered into thin sheets.

    4. Why are cooking utensils made of metals?

    5. Which of the following statement(s) is/are correct for metals? 

    (a) Metals such as sodium and potassium are not strong. 

    (b) Iron is used in the construction of buildings. 

    (c) Gold and Silver are among the best  ductile metals. 

    (d) All of these.


    1.13 SUMMARY

    • An atom achieves a stable electronic configuration by losing, gaining or sharing electrons.

    ♦ Metal atoms with one, two or three electrons in the outermost shell lose  electron(s) to form positively charged ions (cations).

    ♦ Non-metal atoms with five, six or seven electrons in the outermost shell gain three, two and one electron(s) to form negatively charged ions (anions).

    ♦ Non-metal atoms with four to seven outermost electrons may gain electrons by sharing them with each other.

    • A chemical bond is a force that holds ions, molecules or atoms together. A bond is formed when each atom acquires a stable electronic configuration like noble gas.

    • The  electrostatic binding force is called an ionic bond or electrovalent bond.

    • Ionic compounds are formed by attraction of positive and negative ions. These compounds are crystalline solid. They conduct electricity. Ionic compounds have high melting and boiling points. 

    • A covalent bond forms between two or more atoms of non-metals that are unable to form ions.
     • Covalent compound is formed when atoms achieve a stable electronic configuration by sharing of electrons. Covalent compounds are solids, liquid or gases. Covalent compounds have low melting and boiling points.

    • The two forms of carbon that join covalently to form giant structure are diamond and graphite.

    • The force which binds various metal atoms together is called metallic bond.

    • Metals are generally hard, lustrous, strong, malleable and ductile. They conduct heat and electricity in both molten and solid state.

    1.14 GLOSSARY

    • Anion: a negatively charged ion.

    • Cation: a positively charged ion.

    • Crystal: a solid where the atoms form a periodic arrangement.

    • Diamond: one of the known all otropes of carbon.

    • Ductile: able to be drawn out into a thin wire.

    • Electronic configuration: the distribution of electrons of an atom.

    • Graphite: a grey crystalline allotropic form of carbon which occurs as a mineral in some rocks.

    • Malleable: able to be hammered or pressed into shape without breaking or cracking.

    • Noble gas: the gaseous elements helium, neon, argon, krypton, xenon, and radon.

    1.15 UNIT ASSESSMENT


    I. Multiple Choice Questions

    1. The number of electrons gained by non-metals to achieve noble gas electronic configuration is

    (a) one (b) two (c) three (d) all of these

    2. The electronic configuration of sodium ion is

    (a) 2,8,1 (b) 2,8,8 (c) 2,8 (d) 2,8,2

    3. The electronic configuration of chloride ion is

    (a) 2,8 (b) 2,8,8 (c) 2,8,7 (d) 2,8,3 

    4. Choose the ionic compound.

    (a) Calcium chloride   (b) Copper sulphate (c) Sodium hydroxide   (d) All of these

    5. Most ionic compounds are soluble in

    (a) water (b) petrol (c) kerosene (d) all of these

     6. Which of these is not a covalent compound?

    (a) Carbon dioxide (b) Methane (c) Ammonia (d) None of these

    7. Choose the correct statement.

    (a) Covalent compounds have low melting points (b) Ionic compounds have high melting points (c) Urea and glucose are solid covalent compounds (d) All of these

    8. Graphite is used for making ____________.

    (a) lubricant oils (b) pencil leads (c) both (a) and (b) (d) jewelry

    9. If we burn diamond, the product formed is ____________.

    (a) carbon dioxide   (b) hydrogen gas (c) hydrogen chloride gas (d) oxygen gas

    10. The force which binds various metal atoms together is called ____________.

    (a) metallic bond (b) covalent bond (c) ionic bond (d) none of these


    II. Open Ended Questions

     1. How can an atom achieve stability?

    2. Distinguish between covalent and ionic bond.

    3. Compare between the properties of ionic and covalent compounds.

    4. Explain the formation of sodium ion.

    5. Give five examples of each

           (a) Ionic compounds            (b) Covalent compounds

    6. Compare the conductivity of distilled water with sodium chloride solution.

    7. Write two uses of diamond.

    8. Draw the structure of graphite.

    9. Illustrate the physical properties of metals

    . III. Practical-based Questions

    1. Look at the figures and choose the correct statement.

     (a) Figure A is an example of  ionic compound

    (b) Figure B is not an example of covalent compound

    (c) Both Figure A and Figure B are covalent compounds

    (d) None of these

    2. The following figure illustrates the electronic configuration of

     (a) Lithium                 (b) Sodium                      (c) Chlorine                            (d) Helium

    3. shows the structure of ...........................

                                                   
     (a) the hardest substance known

    (b) an allotrope of carbon

    (c)  both (a) and (b)  

    (d) none of these

    4. Which of the following materials makes the circuit complete when inserted in between the crocodile clips?

                             

                              (a) Aluminum foil                                (b) Copper wire

                              (c) Both (a) and (b)                              (d) Sulphur

     5. In the given figure, arrow shows the

                                               

     (a) carbon rod                (b) iron rod               (c) brass rod                         (d) copper rod

    6. Which of the following depicts the molecule of water?

                        

        


     
                                
     

  • Unit 2: Trends in Properties of Elements in the Periodic Table

    LEARNING OBJECTIVES

    After reading this  unit, you will be able to: • describe trends in reactive elements with acids, water, and halogens. • explain the trends in the physical properties across a period and down a group.

     KNOWLEDGE GAIN

    Jons Jakob Berzelius was a Swedish chemist and one of the founders of modern chemistry. He proposed the first letter (or first letter and another letter) of the name of an element as its symbol.

    2.1 CLASSIFICATION OF ELEMENTS


    ACTIVITY 2.1: Distinguishing Metallic and Non-metallic Objects


    Collect five objects made of metals. Also collect five objects made of non-metals. Compare the physical properties of metallic objects and non-metallic objects.

    Observe Figure 2.1 of the periodic table. There are 118 chemical elements known at present. These elements are classified into metals, metalloids and non-metals. The metals appear at the left-hand side and middle part of the periodic table. The non-metals appear at the right-hand side of the periodic table (Figure 2.1). Metalloids lie in between metals and non-metals.


    ACTIVITY 2.2: Categorizing Elements into Metals, Non-metals and Metalloids


    In groups, classify all elements of periodic table into metals, metalloids and non-metals.

    In the periodic table, there is a regular variation in the properties of elements in groups and periods.


    2.1.1 Variation in Metallic and Non-metallic Character Across a Period

    Observe Figure 2.2 (a).

    In the third period of the periodic table shown in Figure 2.2(a), sodium, magnesium and aluminum are metals. The properties of silicon are in-between those of a metal and a non-metal; therefore, silicon is a metalloid. The next elements – phosphorus, Sulphur and chlorine are non-metals. The metallic character decreases from sodium to magnesium to aluminum; silicon is a metalloid; and the non-metallic character increases from phosphorus to Sulphur to chlorine. Thus, in the third period of the periodic table, sodium is the most metallic element whereas chlorine is the most non-metallic element. In general, we can say that the greatest metallic character is found in the elements on the extreme left side of a period and the greatest non-metallic character is found in the elements on the right side of a period.

    On moving from left to right in a period, the metallic character of elements decreases. The non-metallic character increases on moving from left to right in a period. On the left side in a period, we have metals and on the right side we have non-metals. Some elements in-between the metals and nonmetals are known as metalloids.


    EXERCISE 2.1

    1. On moving from left to right in a period, the metallic character of elements ____ . 

    2. How many metals are there in third period of periodic table?

    3. The non-metallic character increases on moving from left to right in a period.  (True or False)

    4. Choose the symbol of metalloid from the following. 

    (a) Na                             (b) Si                          (c) S                    (d) Ne

    Observe Figure 2.2(b). On going down in a group of the periodic table, the metallic character of elements increases. For example, when we move down in group 1 of the periodic table, the metallic character increases from lithium to francium.

                                            

    Thus, in group 1 of alkali metals, lithium is the least metallic element whereas francium is the most metallic element. It is obvious that the greatest metallic character is found in the elements in the lowest part of a group.

    We can also say that on going down in a group of the periodic table, the non-metallic character of elements decreases. Observe Figure 2.2(c). When we go down in group 17 of the halogen elements, the non-metallic character decreases from fluorine to iodine.  Thus, out of fluorine, chlorine, bromine and iodine, fluorine has the most non-metallic character whereas iodine has the least non-metallic character.

                                              

    EXERCISE 2.2


     1. On moving down in a group of the periodic table, the non-metallic character of elements ______ .

     2. In group 17, Iodine has the least nonmetallic character. (True or False)

    2.1.3 Metals

    The majority of known elements (about 80%) are metals. All the metals are solid, except mercury which is a liquid metal at room temperature and pressure. Metals are elements which conduct electricity and heat. Metals are also shiny, hard and produce ringing sound when struck. Metals are widely used in our daily life for a large number of  purposes. The common metals we use are iron, copper, aluminum, tin, zinc, gold, etc. (Figure 2.3). The electric fan, machines, bicycle, cars, aero plane, cooking utensils are all made of metals or mixture of metals.

                           


    During a chemical reaction, metals can form positive ions by losing electrons. Metals are the elements which form positive ions by losing electrons.

    For example, sodium (Na) is a metal which forms positively charged sodium ion (Na+) by losing one electron.

    Metals are very important for the national economy of every country. The economy and prosperity of a country are dependent on the natural resources produced by the country. The main metal deposits in our country are tin, coltan and tungsten.

    The major metals in the earth’s crust are aluminium, iron, calcium, sodium, potassium and magnesium. Aluminium is the most abundant metal in the earth’s crust.

    EXERCISE 2.3


     1. Name the liquid metal.

    2. A majority of known ______ (about 80%) are metals.

    3. Metals are very important for the economic growth of every country.  (True or False)

    4. Which is the most abundant metal in the earth’s crust?  (a) Aluminium (b) Iron  (c) Sodium (d) Calcium

    5. The main metal deposits in our country are ______ and _____.

    2.1.4 Non-metals

    There are only 22 non-metals. Out of these, 10 non-metals are solid, 1 non-metal is liquid and the remaining 11 are gases. Thus, all nonmetals are solids and gases, except bromine which is a liquid non-metal.

    Non-metals are elements which do not conduct heat and electricity. The only exception is graphite. Non-metals are brittle, and have dull appearance. They are soft, not strong and not shiny.

              Carbon

                               

    During a chemical reaction, non-metals form negative ions by gaining electrons. So we define non-metals as elements which form negative ions by accepting electrons. For example, Chlorine (Cl) is a non-metal which forms negatively charged chloride ion (Cl–) by gaining one electron.

    Non-metals are small in number as compared to metals but they play an important role in our everyday life. In fact, life would not have been possible without the presence of nonmetals on earth. For example, oxygen (nonmetal) is essential for breathing to maintain life. Another non-metal carbon is one of the most important elements for existence of life on earth. This is because carbon compounds like proteins, carbohydrates, vitamins and fats are essential for growth and development of living organism. Non-metals are also used to make vegetable oil, acids, fertilisers, and fungicides.

    The major non-metals in the earth’s crust are oxygen, silicon, phosphorus and sulphur. Oxygen is the most abundant non-metal in the earth’s crust. Non-metals are the major constituents of air, earth’s crust and oceans. For example, oxygen and nitrogen are the major constituents of air. Oxygen and silicon are the major constituents of earth’s crust. Oxygen and hydrogen are the major constituents of oceans.

    EXERCISE 2.4


     1. Name the only non-metal which is liquid.

    2. How many non-metals are found in gaseous state?

    3. The only non-metal which conducts electricity is graphite. (True or False)

    4. Which is the most abundant non-metal in the earth’s crust?

      (a) Carbon                (b) Oxygen                         (c) Sulphur                             (d) Nitrogen

    5. Non-metals are used to make ______ and ______ .

    2.1.5 Metalloids

    Metalloids are the elements found along the stair-step line that distinguishes metals from non-metals. Metalloids have properties of both metals and non-metals. Some of the metalloids, such as silicon and germanium, are semiconductors. This means that they can carry an electrical charge under special conditions. This property makes metalloids useful in computers and calculators. The metalloids (Figure 2.4) are:

    • Boron                                                                       • Silicon 

    • Germanium                                                               • Arsenic

    • Antimony                                                                   • Tellurium

     • Polonium

    Metalloids tend to be economically important because of their unique conductivity properties (they only partially conduct electricity), which makes them valuable in the semiconductor and computer chip industry.

                  

             EXERCISE 2.5

     1. Name two metalloids.

    2. Metalloids have properties of both metals and non-metals. (True or False)

    3. Silicon is used in making ______.

    4. Which of the following is/are metalloid(s)?

    (a) Boron (b) Germanium  (c) Antimony (d) All of these

    EXERCISE 2.5


     1. Name two metalloids.

    2. Metalloids have properties of both metals and non-metals. (True or False)

    3. Silicon is used in making ______.

    4. Which of the following is/are metalloid(s)? 

    (a) Boron (b) Germanium  (c) Antimony (d) All of these

    2.2 PHYSICAL PROPERTIES OF METALS

    ACTIVITY 2.3: Illustrating Physical Properties of Metals

     • Collect some metallic objects.

    • Note physical properties of collected objects (Get more information from internet, if available).

    • Make a report on the findings.

    All metals, except mercury, are solid at room temperature. For example, gold, silver, iron, copper, aluminium, etc. Metals are usually grey or silvery in color. The only exceptions are copper and gold.

    Copper is reddish-brown in color whereas gold is yellow.

    The other important physical properties of metals are:

    1. Metals are lustrous: Metals are lustrous, that is, they have a shining surface. The shining surface of metals makes them useful in making jewelry and decorative items. For example, gold and silver are used for making jewelry.

                                          

    A metal has a shining surface only when it is freshly cut. On exposure to air, metals lose their brightness due to the formation of oxide and carbonate on their surface. This is known as corrosion. If we rub the corroded metal (dull surface of metal) with sand paper, the outer corroded layer is removed and the metal object becomes shiny once again.

    ACTIVITY 2.4: Illustrating the Appearance of Metals


    Take samples of iron, copper, aluminium and magnesium. Note the appearance of each sample.

    Clean the surface of each sample by rubbing them with sand paper and note their appearance again.

    Deduction

    Metals, in their pure state, have a shining surface. This property is called metallic lustre.
     

     2. Metals are hard and strong: Most of the metals are hard but all metals are not equally hard. The hardness of metals varies from metal to metal. Metals such as iron, aluminium, and copper are very hard. They cannot be cut with a knife. Sodium and potassium are soft metals. These metals can be cut easily with a knife. 

    Generally metals are strong. But some metals like sodium and potassium are soft. Metals can hold large weight without breaking. For example, iron in the form of steel is very strong. Due to this, iron metal is used in machines, chains and vehicles.

    ACTIVITY 2.5: Illustrating the Variation of Hardness in Metals


     •
    Take small pieces of iron, copper, aluminium, and magnesium. Try to cut these metals with a sharp knife and note your observations.

    • Hold a piece of sodium metal with a pair of tongs.

    Caution: Always handle sodium metal with care.

                   Dry it by pressing between the folds of a filter paper.

                   Put it on a watch glass and try to cut it with a knife.

                   What do you observe?


    Explanation

    You will find that metals are generally hard. The hardness varies from metal to metal.

    3. Metals are malleable:Metals can be beaten (hammered) into very thin sheets without breaking. This property of metals is called malleability. Gold and silver are the most malleable metals. These can be hammered into very thin sheets called foils. Aluminium foils are used for packing medicines and cigarettes.

     ACTIVITY 2.6: Illustrating the Malleability of Metals


     • Take pieces of iron, zinc, lead and copper.

    • Place any one metal on a block of iron and strike it four or five times with a hammer. What do you observe?

    • Repeat with other metals.

    • Record the change in the shape of these metals.


    Conclusion

    You will find that some metals can be hammered into thin sheets. This property is called malleability.

     4. Metals are ductile: Metals are ductile.

    They can be drawn into thin wires. All the metals are not equally ductile. Gold and silver are the best ductile metals. Just one gram of gold can be stretched into a wire of about 2 km  length. Copper and aluminium are also very ductile. Their wires are used in electrical wiring.

                                               

     5. Metals are good conductors of heat: Metals allow heat to pass through them easily. The conduction of heat is also known as thermal conductivity.
     

    ACTIVITY 2.7: Illustrating Thermal Conductivity in Metals


    Caution: Be careful while heating the objects.

    • Take a steel spoon, a brass key, aluminium or copper wire (10 cm), and iron rod.

    • Light the burner.

    • Hold one end of iron rod in your hand.

    • Keep the other end of iron rod to the flame of burner for 3 to 4 minutes as shown in figure.

                                                

     What do you feel?

    • Repeat the activity with other metallic objects.

    • State your observation in each case. Does the metal wire melt?

    Deduction

    The activity tells that metals are good conductors of heat and have high melting points. The best conductors of heat are silver and copper. Lead and mercury are comparatively poor conductors of heat.

     6. Metals are good conductors of electricity: Metals allow electricity to pass through them.

    Silver metal is the best conductor of electricity, copper metal is the next best conductor of electricity followed by gold, aluminium and tungsten.

    Metals are good conductors of electricity because they contain free electrons. These free electrons can move easily through the metal and conduct electric current. Thus, electrical conductivity is another characteristic property of metals. From the above discussion we conclude that metals are good conductors of heat and electricity.

    Metals are good conductors of electricity because they contain free electrons. These free electrons can move easily through the metal and conduct electric current. Thus, electrical conductivity is another characteristic property of metals. From the above discussion we conclude that metals are good conductors of heat and electricity.

    The electric wires that carry current in our homes have a covering of plastic such as poly vinyl chloride (PVC). Polyvinyl chloride is an insulator. It does not allow electric current to pass through it. The electric wires have a covering of an insulating material (like PVC) around them so that even if we happen to touch them, the current will not pass through our body and hence we will not get an electric shock.

    ACTIVITY 2.8: Showing that a Metal Conducts Electricity

    We take a dry cell, a torch bulb fitted in a holder and some connecting wires (copper wires) with crocodile clips, and connect them [as shown in Figure a] to make an electric circuit. There is a gap between the ends of the crocodile clips A and B so no current flows in the incomplete circuit shown in Figure (a) and hence the bulb does not light up. Let us now insert  (i) a piece of aluminium foil between the ends of crocodile clips A and B as shown in Figure (b) (ii) an iron rod between the ends of crocodile clips A and B as shown in Figure (b).

                 

    Deduction

    In both cases, we see that the bulb lights up at once. This means that both aluminium foil and iron rod allow electric current to pass through them. In other words, aluminium metal is a good conductor of electricity. Note that the connecting wires used in this experiment are made of copper metal. Since these copper connecting wires allow electric current to pass through them, copper metal is also a good conductor of electricity.

     7. Metals have high melting points: Metals melt and turn into liquid at very high temperatures. However, there are some exceptions. Sodium and potassium have low melting points. Melting points of some metals are given in Table 2.1.
     

     8. Metals have high densities: The density of a substance is defined as mass of the substance per unit volume. Metals have high densities. Thus, metals are heavy substances. However, aluminium, sodium and potassium have low densities. Densities of some metals are given in Table 2.1.

                               DO YOU KNOW?

    The melting point of gallium and caesium metals are so low that they start melting at temperatures  greater than 27.76°C.

          

    9. Metals are sonorous: Metals are sonorous means that they are capable of producing a ringing sound. The property of metals of being sonorous is called sonority. It is due to the property of sonority, metals are used for making bells and wires of violin.

                                  

    EXERCISE 2.6
     1. Which metal is yellow in color?

    2. Write two uses of metals.

    3. Define malleability.

    4. Why metals are good conductors of electricity?

    5. Metals have high

      (a) melting points    (b) densities      (c) both (a) and (b)             (d) None of these.

    2.3 PHYSICAL PROPERTIES OF NON-METALS

    ACTIVITY 2.9: Showing that a Non-metal does not Conduct Electricity

    We take a dry cell, a torch bulb fitted in a holder and some connecting wires (copper wires) with crocodile clips, and connect them as shown in Figure (a) to make an electric circuit. There is a gap between the ends of crocodile clips A and B so no current flows in the open circuit shown in figure (a).

    Let us now insert

    (i)  a piece of sulphur (which is a non-metal) between the crocodile clips A and B as shown in figure (b)

    (ii) a carbon rod between the crocodile clips A and B as shown in figure (b).

              
             

    Explanation

    In both cases, we see that the bulbs do not light up at all. This means that both sulphur and carbon do not allow electric current to pass through them and no current flows in the circuit. This activity shows that non-metals do not conduct electricity.

    Non-metals exist in all three physical states: solid, liquid and gaseous. For example, carbon, sulphur and phosphorus are solid. Bromine is a liquid. Oxygen, hydrogen and nitrogen are gases. Non-metals have many different colors.

    For example, phosphorus is red, black and white. Sulphur is yellow, chlorine is green.

    Some non-metals such as oxygen and hydrogen are colorless.

    The other important physical properties of non-metals are:

    1. Non-metals are not lustrous: Nonmetals do not have a shining surface. The only non-metals having a shining surface is iodine.

    2. Non-metals are neither hard nor strong: Most of the solid non-metals are soft, they can be broken easily. For example, sulphur and phosphorus. Only one non-metal carbon in the form of diamond is very hard. Diamond is the hardest substance known on earth.

    3. Non-metals are neither malleable nor ductile: Non-metals are brittle which means that they break into pieces when hammered or stretched. 

    Therefore, non-metals cannot be hammered with a hammer to form thin sheets. They cannot be stretched to form wires. 

    The property of breaking easily is called brittleness. Brittleness is the characteristic property of non-metals. Note: Brittleness is not applicable to liquid and gaseous non-metals.

    4. Non-metals do not conduct heat and electricity: Non-metals do not conduct heat and electricity because they have no free electrons which are necessary to conduct heat and electricity. However, there is one exception. Carbon in the form of graphite is a good conductor of electricity. Therefore, graphite is used for making electrodes.

    ACTIVITY 2.10: Illustrating Thermal Conductivity in Nonmetals

    Repeat Activity 2.7 with carbon rod (taken out from used cell) and a lump of sulphur.

    Activity 2.9 shows that non-metals do not conduct electricity. 

    In Activity 2.10, you will observe that both carbon and sulphur are poor conductors of heat

     5. Non-metals have low melting and boiling points: Non-metals have comparatively low melting and boiling points. Only one non-metal diamond (allotropic form of carbon) has high melting point. The melting point of diamond is 3500°C.

    6. Non-metals have low density: The density of non-metals is low, that is, they are light substances.  Densities of some non-metals are given in Table 2.2.

                                     

     7. Non-metals are non-sonorous: Nonmetals do not produce ringing sound when hit with an object.

    EXERCISE 2.7


     1. Non-metals exist in all three physical states. (True or False)

    2. ______ and ______ are colorless nonmetals.

    3. Which non-metal is used for making electrodes?

    4. Why do non-metals not conduct heat and electricity?

    5. Non-metals have low  (a) densities  (b) melting points  (c) boiling points  (d) All of these.

    2.4 TRENDS IN REACTIVITY FOR METALS AND NON-METALS

    2.4.1 Reactivity of Metals

    The chemical reactivity of metals increases on going down in a group of the periodic table. For example, in group 1 of alkali metals, the chemical reactivity increases from lithium to francium (radioactive).

                                         

    Thus, as we go down in a group of metals, the tendency of their atoms to lose electrons increases, and hence their chemical reactivity also increases.

    On moving from left to right in a period, the chemical reactivity of elements first decreases and then increases.

                                              

    In the third period of elements shown above, sodium is a very reactive element, magnesium is less reactive, whereas aluminium is still less reactive. Silicon is the chemically least reactive element in the third period. Now, phosphorus is quite reactive, sulphur is still more reactive, whereas chlorine is very reactive. From this discussion we conclude that in the third period of the periodic table, chemical reactivity first decreases from sodium to silicon and then increases from phosphorus to chlorine.

                                             

    2.4.2 Reactivity of Non-metals

    The chemical reactivity of non-metals decreases on going down in a group of the periodic table. For example, in group 17 of halogen elements (which are non-metals), the chemical reactivity decreases from fluorine to iodine.

    Thus, as we go down in a group of non-metals, the tendency of their atoms to gain electrons decreases, due to which their reactivity also decreases.

    EXERCISE 2.8 1.

    1.Explain, why chemical reactivity of metals increases on going down in a group of periodic table?

    2. Which of the following element is least reactive? 

    (a) Sodium (b) Silicon (c) Sulphur (d) Chlorine

    3. The chemical reactivity of non-metals decreases on going down in a group of periodic table. Why?

    4. In the third period of elements shown below Na Mg Al Si P S Cl  Sodium reacts vigorously but sulphur reacts less vigorously with oxygen.  

                                                                                     (True or False)

    2.5 CHEMICAL PROPERTIES OF METALS


    2.5.1 Reaction of Metals with Water


    ACTIVITY 2.11: Illustrating Reaction of Metals with Water
    Caution:
    Do not touch sodium and potassium with bare hands. They cause severe burns.

    • Collect samples of sodium, potassium, calcium, aluminium, iron, magnesium, zinc and copper.

    • Put small pieces of samples separately in beakers half filled with cold water.

    • Observe which metals reacted with cold water.

    Did any metal produce fire in water?

    Did any metal start floating after sometime?

    • Put the metals that do not react with cold water in beakers filled with hot water.

    • Observe which metals reacted with hot water.

    • For those metals which did not react with hot water arrange the apparatus as shown in figure.

                             

    • Observe which metals react with steam.

    Did any metal not react even with steam?

    • Make an appropriate report on reaction of metals with water.

    In Activity 2.11, you have observed that all metals are not equally reactive. Metals react with water to form a metal hydroxide and hydrogen gas. Some metals react with cold water. For example, sodium, potassium and calcium. Some other metals such as magnesium react with hot water. It does not react with cold water. Metals such as aluminium, iron and zinc do not react with either cold or hot water. They react with steama to form metal oxide and hydrogen. There are some metals that do not react even with steam. For example, copper, gold, silver and mercury. When a metal reacts with cold water or hot water, the products formed are metal hydroxide and hydrogen gas.

                               

    a Steam is a gaseous form of water. It is very hot.

    When metals such as magnesium, aluminium, zinc and iron react with steam, the products formed are metal oxide and hydrogen gas.
    Metal + Steam → Metal oxide + Hydrogen
    Note: Metal oxides are basic in nature. Their solutions in water turn red litmus into blue. Some metal oxides react with water to form alkali.

    Example 1 Sodium reacts vigorously with cold water to form sodium hydroxide and hydrogen gas.

    The reaction of sodium metal with water is also highly exothermic (heat producing). This is the reason why hydrogen gas formed during the reaction catches fire and burns causing little explosions. Thus, sodium is a very reactive metal.


    Example 2 Potassium reacts violently with cold water to form potassium hydroxide and hydrogen gas.

    The reaction of potassium metal with water is highly exothermic (heat producing). This is the reason why hydrogen gas formed during the reaction catches fire immediately. Thus, potassium is also a very reactive metal.

    Note: Potassium is more reactive than sodium.

    Example 3

    Calcium reacts with cold water to form calcium hydroxide and hydrogen gas.

    The reaction of calcium with water is less violent. The heat produced is not sufficient for the hydrogen to catch fire. Calcium starts floating in water because bubbles of hydrogen formed during the reaction stick to surface of metal.

                                  

    Note: Calcium is less reactive than sodium.

    Example 4

    Magnesium metal does not react with cold water. It reacts with both hot water and steam. Magnesium reacts with hot water to form magnesium hydroxide and water.

                                    

    In this reaction, the piece of magnesium metal starts floating on water due to the bubbles of hydrogen gas sticking to its surface.

    Note: Calcium reacts with cold water but magnesium reacts only with hot water. This shows that magnesium is less reactive than calcium.
    Magnesium reacts very rapidly with steam to form magnesium oxide and hydrogen.

                                           

    When magnesium reacts with hot water, it forms magnesium hydroxide and hydrogen. In this reaction, magnesium reacts with steam to form magnesium oxide and hydrogen.

    Example 5 (i) Aluminium reacts with steam to form aluminium oxide and hydrogen gas.

                                           

     (ii) Zinc reacts with steam to form zinc oxide and hydrogen gas.

                                         
     (iii) Iron reacts with steam to form iron oxide and hydrogen gas.

                                        
    Example 6

    Copper does not react with water (or steam)

                                            

    2.5.2 Reaction of Metals with Acids


    ACTIVITY 2.12: Illustrating Reaction of Metals with Dilute Acids


    Caution:
    Do not touch dilute hydrochloric and sulphuric acids with your bare hands.

    • Collect small pieces of magnesium, aluminium, zinc, copper and iron.

    • Clean the sample metals with sand paper.

    • Put these metal pieces in separate test tubes.

    • Add 10 ml dilute hydrochloric acid to each test tube.

    • Observe carefully the rate of formation of hydrogen gas bubbles. Did any metal not react with dilute hydrochloric acid?

    • Repeat the activity again with dilute sulphuric acid. Did any metal not react with dilute sulphuric acid?

    • Make an appropriate report on reaction of metals with dilute acids.

    In Activity 2.12, you must have observed that the rate of formation of hydrogen bubbles was the fastest in magnesium. The reactivity decreases in the order Mg > Al > Zn > Fe. In case of copper, no bubbles were seen. This shows that copper does not react with dilute hydrochloric acid.

    Metals usually react with dilute acids to give a metal salt and hydrogen gas. Some metals react violently (explosively) with dilute acids whereas some react rapidly. Sodium reacts violently with dilute acids and magnesium reacts rapidly. Some metals react slowly with dilute acids whereas a few metals do not react with acids at all. Aluminium, iron, zinc and copper react slowly with dilute acids; whereas gold and silver do not react at all.

                                                  DO YOU KNOW?

    Aqua regia, (Latin for ‘royal water’) is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3 : 1. It can dissolve gold, even though neither of these acids can do so alone. Aqua regia is a highly corrosive, fuming liquid. It is one of the few reagents that is able to dissolve gold and platinum.

    When a metal reacts with dilute hydrochloric acid, the products formed are metal chlorides and hydrogen gas.

                              

    When a metal reacts with dilute sulphuric acid, the products formed are metal sulphate and hydrogen.

                             

    Example 1

    (i) Sodium metal reacts violently with dilute hydrochloric acid to give sodium chloride and hydrogen gas

                                         

     (ii) Sodium metal reacts with dilute sulphuric acid to give sodium sulphate and hydrogen gas.

                                         

    Example 2

    (i) Magnesium reacts with dilute hydrochloric acid to give magnesium chloride and hydrogen gas.

                                            

     (ii) Magnesium reacts with dilute sulphuric acid to form magnesium sulphate and hydrogen gas.

                                                

    Example 3

    (i) Calcium reacts with dilute hydrochloric acid to form calcium chloride and hydrogen gas.

                                                      

     (ii) Calcium reacts with dilute sulphuric acid to give calcium sulphate and hydrogen gas.

                                                           

    Example 4

    (i) Aluminium reacts with dilute hydrochloric acid to give aluminium chloride and hydrogen gas

                                                            

    (ii) Aluminium reacts with dilute sulphuric acid to form aluminium sulphate and hydrogen gas.

                                                          

    Example 5

    (i) Zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.

                                                         

     (ii) Zinc metal reacts with dilute sulphuric acid to give zinc sulphate and hydrogen gas.

                                                   

    Example 6

    (i) Iron reacts slowly with cold dilute hydrochloric acid to form iron chloride and hydrogen gas.

                                                       

     (ii) Iron reacts with sulphuric acid (dilute) to give iron sulphate and hydrogen gas.

                                                   

    Example 7

    (i) Copper does not react with dilute hydrochloric acid.

                                                       Cu(s) + HCl(aq) → No reaction

    (ii) Copper does not react with dilute sulphuric acid.

                                            

    Hydrogen gas is not evolved when a metal (e.g., Cu) reacts with nitric acid (HNO3).

    It is because nitric acid is a strong oxidising agent. It oxidises the hydrogen produced to water and nitric acid itself is reduced to any of the nitrogen oxides. The examples of nitrogen oxide are nitrogen monoxide (NO), nitrogen dioxide  and dinitrogen monoxide.Only magnesium and manganese react with very dilute nitric acid to evolve hydrogen gas. The reaction of magnesium and manganese metals with very dilute nitric acid are:

    • Magnesium reacts with very dilute nitric acid to form magnesium nitrate and hydrogen gas

                                             

    • Manganese reacts with very dilute nitric acid to form manganese nitrate and hydrogen gas.

                                                 

    2.5.3 Reaction of Metals with Halogens


    The elements of group 17 in the periodic table are called halogens. Fluorine (F), Chlorine (Cl), bromine (Br), Iodine (I) and Astatine (At) are halogens.

    Metals react with halogens to form ionic halide.

    Metal halides are usually solid and conduct electricity in solution. Let us see some equations for reaction of metals with chlorine.

    (i) Sodium reacts with chlorine to form sodium chloride

                                                      

     (ii) Calcium reacts with chlorine to form calcium chloride

                                                        

     (iii) Aluminium reacts with chlorine to form aluminium chloride.

                                                             

     (iv) Iron reacts with chlorine to form iron chloride.

                                                           

     (v) Copper reacts with chlorine to form copper chloride.

                                                           

     (vi) Zinc reacts with chlorine to form zinc chloride.

                                                             

    Note: All the metal chlorides are ionic compounds.

    2.5.4 Reaction of Metals with Oxygen

    ACTIVITY 2.13: Illustrating Reaction of Metals with Oxygen


    Safety: The activity needs the teacher’s assistance. Students should wear eye protection.

    Collect small pieces of potassium, sodium, magnesium, aluminium, zinc, copper and iron. Also collect some iron filings.

    • Hold any of the samples taken above with a pair of tongs and try burning over a flame. Repeat with other metal samples.

    • Collect the product if formed.

    • Let the products and the metal surface cool down. Which metals burn easily? How does the metal surface appear after burning? Are the products soluble in water?

    • Make an appropriate report on reaction of metals with oxygen.

    You will observe in Activity 2.13 that almost all metals combine with oxygen to form metal oxides, but all metals do not react with oxygen at the same rate. Different metals show different reactivity towards oxygen.

    Metals such as potassium and sodium react so vigorously that they catch fire if kept in the open. Hence, to protect them and to prevent accidental fires, they are kept immersed in kerosene oil. At ordinary temperature, the surfaces of metals such as magnesium, aluminium, zinc, lead, etc., are covered with a thin layer of oxide. The protective oxide layer prevents the metal from further oxidation. Iron does not burn on heating but iron filings burn vigorously when sprinkled in the flame of the burner. Copper does not burn, but the hot metal is coated with a black colored layer of copper(II) oxide. Silver and gold do not react with oxygen even at high temperatures.

    Let us see some equations for reaction of metals with oxygen.

     (i) Sodium reacts with oxygen to form sodium oxide.

                                             

     (ii) Potassium reacts with oxygen to form potassium oxide.

                                              

     (iii) Magnesium reacts with oxygen to form magnesium oxide

                                              

    EXERCISE 2.9


     1. ______ is a gaseous form of water.

    2. Metals react with water to form metal oxide and hydrogen gas.  (True or False)

    3. Metals react with steam to form metal hydroxide and hydrogen gas.  (True or False)

    4. Complete and balance the following equations:

                                          

     5. Gold and Silver ______ react with dilute acids.

    6. Hydrogen gas is not evolved when a metal reacts with ______ .

    7. All metal chlorides are ionic in nature. (True or False)

    2.6 CHEMICAL PROPERTIES OF NON-METALS


    Non-metals neither react with water nor with dilute acids. In other words, non-metals do not displace hydrogen gas from acids and water.


    2.6.1  Reaction of Non-metals with Halogen (Chlorine)


    Non-metals react with chlorine to form covalent chlorides. Non-metal chlorides are usually liquids or gases. They do not conduct electricity.

    Example 1

                  


    2.6.2  Reaction of Non-metals with Oxygen

    Non-metals react with oxygen to form acidic oxides or neutral oxides.

     (i) Carbon reacts with oxygen to form carbon dioxide

                     

     (ii) Hydrogen reacts with oxygen to form water.
                           
    EXERCISE 2.10


     1. Non-metals do not displace hydrogen gas from acids. (True or False)

    2. Non-metals react with ______ to form covalent chlorides.

    3. Non-metals react with oxygen to form 

    (a) acidic oxides              (b) neutral oxides            (c) both (a) and (b)             (d) none of these

    4. Complete the following equations:

                 

     5. Non-metal chlorides do not conduct electricity. (True or False)

    2.7 COMPARISON AMONG THE PHYSICAL AND CHEMICAL PROPERTIES OF METALS AND NON-METALS


    We have studied the physical and chemical properties of metals and non-metals. Let us see the main points of difference between the metals and non-metals.

              

               

                 2.8 USES OF METALS AND NON-METALS

                   

    EXERCISE 2.11


     1. State True or False 

    (a) Mercury is used in thermometers. 

    (b) Gold and Silver are used for making electric wires. 

    (c) Helium is used in balloons. 

    (d) Phosphorus is used in fertilizers. 

    (e) Chlorine is not used in the disinfection of drinking water.

    2. Gap filling: 

    (a) Sulphur is used for making ______. 

    (b) Silicon is used for making ______. 

    (c) Sodium is used in ______ reactors.

    3. Name the Noble gas used in electric bulbs.

    4. Which element is used for coating iron containers for packaging food?

    5. Name two metals used for making machines.

    2.9 SUMMARY

    • Elements can be classified as metals, metalloids and non-metals.

    • On moving from left to right in a period, the metallic character of elements decreases whereas non-metallic character increases.

    • On going down in a group of the periodic table, the metallic character of elements increases whereas non-metallic character decreases.

    • Metals are lustrous, malleable, ductile and good conductors of heat and electricity. They are solids at room temperature, except mercury which is a liquid.

    • Non-metals have properties opposite to that of metals. They are neither malleable nor ductile. They are bad conductors of heat and electricity, except for graphite, which conducts electricity.

    • Metals form positive ions by losing electrons.

    • Non-metals form negative ions by accepting electrons.

    • Elements in-between the metals and non-metals are known as metalloids.

    • The chemical reactivity of metals increases on going down in a group of the periodic table.

    • The chemical reactivity of non-metals decreases on going down in a group of the periodic table.

    • Metals react with water to form metal hydroxide or metal oxide and hydrogen gas.

    • Metals usually react with dilute acids to give a metal salt and hydrogen gas.

                Metal + Hydrochloric acid → Metal chloride + Hydrogen  

                Metal + Sulphuric acid → Metal sulphate + Hydrogen.

     • Copper does not react with dilute acids.

    • Metals react with halogen to give ionic halides.

    • Metals react with oxygen to form metal oxides (basic oxides).

    • Different metals have different reactivities with water and dilute acids.

    • Arrangement of common metals in order of their decreasing reactivity is known as an activity series.

    • Metals above hydrogen in the Activity series can displace hydrogen from dilute acids.

    • A more reactive metal displaces a less reactive metal from its salt solution.

    • Non-metals neither react with water nor with dilute acids. They do not displace hydrogen gas from water and acids.

    • Non-metals react with chlorine to form covalent chlorides

    . • Non-metals react with oxygen to give acidic oxides or neutral oxides.

    2.10 GLOSSARY

    Antiseptic: a substance that prevents the growth of disease-causing micro-organisms.

    Carbohydrates: a large group of organic compounds occurring in foods including sugars, starch, and cellulose.

    Fertilizers: a chemical or natural substance added to soil or land to increase its fertility.

    Fluorescent: emitting light.

    Fungicides: a chemical that destroys fungus.

    Metal: a material that is hard, opaque, shiny and has good electrical and heating conductivity.

    Metalloid: a chemical element with properties in between those of metals and nonmetals.

    Non-metal: a material that does not have properties of metals.

    Nuclear reactor: a device used at nuclear power plants for electricity generation.

    Ornaments: a thing used or serving to make something look more attractive.

    Thermometer: an instrument for measuring temperature.

    Weapon: something designed or used for inflicting physical injury or damage.

    2.11 UNIT ASSESSMENT


    I. Multiple Choice Questions

    1. Which of the following metal exists in the liquid state? 

    (a) Sodium        (b) Silver          (c) Mercury              (d) Neon

    2. On going down in a group of the periodic table, the metallic character of
     elements .................. 

    (a) increases             (b) decreases             (c) both (a) and (b)                 (d) neither (a) nor (b)

     3. Silicon and germanium are examples of .................. 

    (a) metals (b) metalloids (c) non-metals (d) none of these

    4. All non-metals are solids and gases except: 

    (a) fluorine (b) chlorine (c) bromine (d) iodine

    5. Which metal is the best conductor of electricity? 

    (a) Silver (b) Copper (c) Aluminium (d) Iron

    6. Which of the following statement(s) is/are not correct. 

    (a) The hardest substance known on earth is diamond.  (b) Graphite – a non-metal – conducts electricity.  (c) The melting points of gallium and caesium are very high.  (d) Brittleness is the characteristic property of non-metal.

    7. On moving from left to right in a period, the chemical reactivity of elements .................. 

      (a) increases   (b) decreases  (c) first decreases and then increases (d)  first increases and then decreases

    8. Name the gas produced when metals react with dilute acids.

      (a) Oxygen (b) Hydrogen (c) Nitrogen (d) Chlorine

    9. The molecular formula of manganese nitrate is 

         

    10. Carbon reacts with oxygen to give 

    (a)  carbon dioxide   (b) carbon monoxide  (c) both (a) and (b)   (d) none of these

    II. Open Ended Questions

    1. What is meant by saying that the metals are malleable and ductile? Explain with examples.

    2. With the help of example, describe how metals differ from non-metals.

    3. Name one metal and one non-metal which exist in liquid state at room temperature.

    4. (a) Name the most abundant metal in the earth’s crust. 

        (b) Name the most abundant non-metal in the earth’s crust. 

        (c) Name one metal which has low melting point. 

        (d) Name one non-metal which is kept under water. 

        (e) Name one metal which is stored in kerosene oil.

    5. Complete and balance the following equations:
           

                                      
     6. Describe the trends and patterns in the properties of elements in groups and periods.

     7. Illustrate electrical conductivity of metals and non-metals.

    8. Distinguish between the chemical properties of metals and non-metals.

    9. Can you explain why copper does not react with water?

    10. Give three uses of non-metals in daily life.


    III. Practical-based Questions

    1. Which of the following is an example of non-metals?

                       
     

     2. Which is the most reactive metal in the given table?

                         

                        (a) Li                   (b) Na                        (c) K                   (d) Rb

    3. Which is the least reactive metal in the given table?

                       

                       (a)  Na               (b) Cl                      (c) Si                         (d) P

    4. How many non-metals are there in the following figure?

                                        

                  (a) Five          (b) Eleven              (c) Six (                 d) Seven

     5. In the following periodic table, the yellow color represents ......................

         
               (a) Metals                   (b) Non-metals                (c) Metalloids                   (d) Liquid metals



  • Unit 3: Water Pollution

    LEARNING OBJECTIVES

    After reading this unit, you will be able to:

    • define water pollution.

    • identify the main water pollutants.

    • describe the dangers of polluted water.

    • suggest the ways of preventing water pollution.

    KNOWLEDGE GAIN

             

    Tap water which is considered safe for drinking sometimes includes harmful microbes. These microbes do not alter color and odor of water but are very harmful

        3.1 WATER POLLUTION

    ACTIVITY 3.1: Showing Awareness about Water Pollution

    Investigate the level of awareness about water pollution in your area. Collect data on the sources of drinking water and polluted water from newspaper and magazines.

    What are the common water-borne diseases in the community? You can consult your local doctor/health worker for this.

    Which are the governmental and non-governmental organizations working in this field? What are the measures being taken by them for generating awareness?

    Prepare an illustrative presentation on “water pollution” from the data collected. Present it in class.

    Water is essential for life. Without water there would be no life. We usually take water as granted for its purity, but we must ensure the quality of water. Most of the water which we use comes from rivers and lakes.

    Everyday, many unwanted and harmful substances are thrown (or discharged) into the rivers and lakes. They make the water of rivers and lakes impure (or contaminated). So, we say that the water has been polluted.

    The contamination of water of rivers, lakes and ponds, etc., with unwanted and harmful substances is called water pollution.

     Water is said to be polluted when it becomes unfit for drinking or bathing. Pollution of water originates from human activities. Through different paths, pollution reaches surface or ground water. Easily identified source or place of pollution is called point source. For example, municipal and industrial discharge pipes where pollutants enter the water-source. Non-point sources of pollution are those where a source of pollution cannot be easily identified. For example, agricultural run off (from farm, animals and crop-lands), acid rain, storm-water drainage (from streets, parking lots and lawns), etc.

    3.1.1 Causes of Water Pollution

    (i) Pathogens:

    The most serious water pollutants are the disease causing agents called pathogens. Pathogens include bacteria and other organisms that enter water from domestic sewage and animal excreta. Human excreta contains bacteria such as Escherichia coli and Streptococcus faecalis which cause gastrointestinal diseases.

    (ii) Organic wastes:

    The other major water pollutant is organic matter such as leaves, grass, trash, etc. They pollute water as a consequence of run off. Excessive phytoplankton growth within water is also a cause of water pollution. These wastes are biodegradable.

                       

                

    ACTIVITY 3.2: Illustrating Effects of Quantities


    Pour a cup of black ink into a river. What do you observe? Are you able to see it? Now do the same in a bucket. Do you find any change?

    When you poured a cup of black ink into a river, the ink quickly disappeared into the river’s much larger volume of clean water. The ink would still be there in the river, but in such a low concentration that you would not be able to see it. At such low levels, the chemicals in the ink probably would not present any real problem. However, when you poured a cup of ink into a bucket, the bucket quickly turned black. The chemicals in the ink could very quickly have an effect on the quality of the water.

    Thus, water pollution is all about quantities of pollutants. It depends on how much of a polluting substance is released and how big a volume of water it is released into.

    EXERCISE 3.1


     1. Most of the water which we use comes from ______ and ______ .

    2. What do you mean by water pollution?

    3. Name two pathogens which cause gastrointestinal disease.

    4. Pollution of water originates from human activities. (True or False)

    5. What are the causes of water pollution?

    3.2 MAIN WATER POLLUTANTS


    ACTIVITY 3.3: Illustrating Major Pollutants of Water


    Visit your nearby water body. Collect information on the major pollutants added to the water body. Illustrate with pictures the main idea of water pollution in the water body.

    The substances which cause water pollution are known as water pollutants. Most water pollution does not begin in the water itself. For example, in oceans around 80% of pollution enters from the land. The main water pollutants are:

    3.2.1 Sewage


    ACTIVITY 3.4: Illustrating Disposal of Sewage


    Visit in groups, the sewage disposal system of your locality. Try to find out the answer to the following questions:

    • How is sewage collected from your home?

    • Where does it go thereafter?

    Sewage is a water carrying waste. It generally consists of faeces, urine and laundry waste. It also contains harmful micro-organisms such as bacteria, protozoa, fungi, viruses and parasites.

                      

    Disposal of sewage is a big problem in developing countries. Most of the people do not have access to proper sanitation facilities. It affects people’s immediate environment causing various water-borne diseases such as diarrhea. Even if there are flush toilets, the problem still continues. When you flush the toilet, the waste has to go somewhere. Even after it leaves sewage treatment works, there is still waste to dispose off. Sometimes sewage waste is pumped untreated into the sea.

         EXERCISE 3.2


     1. Sewage generally consists of ______, ______ and ______.

    2. Disposal of sewage is not a problem in developing countries. (True or False)

    3. Name three harmful micro-organisms sewage contains.

    3.2.2 Nutrient-rich Waste Water

    The farmers use large amounts of fertilizers in the fields to increase the crop yields. These are rich in nitrates and phosphates. The excess fertilizers dissolve in water and run into rivers, lakes and ponds. Fertilizers are plant nutrients. They cause rapid growth of tiny, green, water plants called algae in the water body. Algae cover the entire water body like a green sheet. Algae compete with other organisms in the water for dissolved oxygen. As a result, there is a threat to the aquatic life.

                                    

        EXERCISE 3.3


     1. Nutrient-rich waste water causes rapid growth of green plants called ______ in the water body.

    2. Is nutrient-rich waste water a threat to aquatic life?

    3. Name the organism which covers the entire water body like a green sheet.

      3.2.3 Chemical Waste

    Almost all the industries produce poisonous chemicals as their waste products. These are called chemical waste or industrial wastes. These wastes are discharged untreated in nearby water bodies. In this way, the water bodies get polluted with chemicals. The chemicals present are the compounds of harmful metals such as mercury, cadmium, lead, arsenic and nickel. These may also include detergents and polychlorinated biphenyls (PCBs). These chemicals can kill aquatic animals and plants. They also cause severe disorders in humans such as cancer and nervous disorders.

                               

    EXERCISE 3.4


     1. What do you mean by chemical waste?

    2. Chemical waste can cause ______ and ______ in humans.

    3. Compounds of which elements are present in chemical waste?

     3.2.4 Radioactive Waste


    ACTIVITY 3.5: Showing Radioactive Waste


    Look at the figure. Observe the sign on the dustbin. Answer the questions raised.

                      

    Describe the picture. Make a report and submit it to your teacher.

    Radioactive waste is a waste that contains radioactive substance. A radioactive substance is unstable and produces dangerous kinds of radiation. People view radioactive waste with great alarm – and for good reason. At high concentration it can kill, at lower concentrations it causes diseases like cancer. They are carried into water from nuclear power plants, wastes of uranium and thorium during their mining and refining processes and also from medical and scientific institutions.


    EXERCISE 3.5

    1. What do you mean by radioactive substance?

    2. Radioactive waste are generated from ______ and _______ .

    3. How are radioactive wastes harmful?

      3.2.5 Oil Pollution

    Oil and oil wastes enter water bodies from different sources such as oil refineries, storage tanks, automobile waste oil, and industries. Spillage of oil from ships also results in pollution. The pollution caused by oil and oil wastes is termed as oil pollution. Oil is insoluble in water; it floats and spreads rapidly into a thin layer. This layer prevents oxygen transfer from atmosphere. As a result of this, less oxygen is available for aquatic life.

                                   

                        

    At sea, oil layer is responsible for the death of birds. The oil penetrates the bird feathers thereby affecting their floating and flying abilities.

    EXERCISE 3.6


     1. From where do oil wastes enter into water bodies?

    2. The pollution caused by oil and oil wastes is termed ______ .

    3. How is oil pollution responsible for death of aquatic animals and plants?


    3.2.6 Plastic


    ACTIVITY 3.6:
    Say No to Plastics
        

    Look at the banner below. Have you seen this before? What is meant by polythene? Why and when was it banned in Rwanda? How did Rwanda accomplish it? Make a report to be presented in the class.

                  

    Note: Polythene bags have been banned in Rwanda since 2008.

    Polythenes or polyethylenes are the most common plastics. Plastic is far and away the most common substance that washes up with the waves. There are three reasons for this: plastic is one of the most common

    materials, used for packaging, and making any kind of manufactured object from clothing to automobile parts; plastic is light and floats easily so it can travel enormous distances across the oceans; most plastics are not biodegradable (they do not break down naturally in the environment). Once in a water body they amass in landfills, litter streets, obstruct sewers and hurt aquatic life.

            
    A plastic bottle can survive an estimated 450 years in the ocean and plastic fishing line can last up to 600 years.

    EXERCISE 3.7


     1. ______ is the most common plastics.

    2. Plastics are biodegradable.  (True or False)

    3. Why is use of plastics prohibited?


    3.2.7 Alien Species

    Alien species (sometimes known as invasive species) are animals or plants from one region that have been introduced into a different ecosystem where they do not belong. The water hyacinth which was introduced as an ornamental plant has since invaded lakes in Rwanda.

    It has invaded from Muhazi to Rweru from the river Nyabarongo, and even reached Lake Victoria through Akagera river. The water hyacinth is a major biodiversity problem in the ecosystem of the Lake Victoria Basin.

    Rampant growth of water hyacinth can destroy native wetlands and waterways, killing native fish and other wildlife. Water hyacinth can form dense mats that spread out across water surfaces eventually choking the entire water body. Heavy weed cover also prevents the exchange of air, which normally occurs on an open water surface. This stagnation affects water quality and may result in the death of aquatic animals.

                          

    EXERCISE 3.8


     1. What do you mean by alien species?

    2. ______ is a major biodiversity problem of the Lake Victoria Basin.

    3. Growth of ______ can destroy aquatic life.

    3.2.8 Other Forms of Pollution

    This category includes the most common forms of pollution – but by no means the only ones. Heat or Thermal pollution from factories and power plants also causes problems in the river. By adding hot water into the water body it raises the temperature. The rise in the temperature has an adverse effect on the animals and plants living in it.

                                   


    3.3 DANGERS OF POLLUTED WATER


    ACTIVITY 3.7: Illustrating Effects of Polluted Water


    Learners plan a field visit to nearby areas and collect various samples of water. Pour each into separate glass containers. Compare the samples for smell, acidity and color. Complete the following table in your exercise book.

                       

    What is the possible cause of smell, acidity and color in the water? What is the possible cause of this difference? Make a comparative report of your observations.

    Acidity of water is measured as pH (power of hydrogen) of the water body. It is a figure between 0 and 14 defining how acidic or basic a body of water is along a scale. The lower the number, the more acidic the water is. The higher the number, the more basic it is. A pH of 7 is considered neutral. The pH of pure water is 7. You can use a pH strip to measure PH. The color of the strip after dipping in water will give its PH.

                                 

    Addition of pollutants to water changes its physical, chemical and biological properties. Water from different sources is likely to have different pollutants. For example, a river situated near an industry is more likely to be affected by its discharge. Water pollution is very harmful to humans, animals and water life. The effects can be catastrophic, depending on the kind of chemicals, concentrations of the pollutants and where they are polluting. Dangers of polluted water include:


    3.3.1 Eutrophication

    The entry of nutrient-rich water results in a thick growth of algae (tiny plant) called algal bloom, and many other weeds. Rapid growth of these plants covers the entire surface of water. This is called eutrophication. Eutrophication may be defined as the process of nutrient enrichment of water bodies and the subsequent overgrowth of plants on the surface of water. The algae use up a lot of oxygen that other aquatic animals die due to lack of it. It also blocks light to reach under water affecting aquatic plants. Eutrophication hence results in loss of aquatic life. Slowly, it results in the death of “lake or river”.

                          

    3.3.2 Acidification


    The oceans are normally a natural carbon sink, absorbing carbon dioxide from the atmosphere. Carbon dioxide (CO2) is released into the oceans as a result of water pollution by nutrients. It enhances the unwanted changes in ocean acidity due to atmospheric increases in CO2. It impacts primarily the ecosystems and fish communities that live in the ocean. In particular, the rising levels of CO2 acidify the ocean. Even though the ocean can absorb carbon dioxide that originates from the atmosphere, the carbon dioxide levels are steadily increasing. The ocean’s absorbing mechanisms, due to the rising of the ocean’s temperatures, are unable to keep up with the pace. This results in acidification of oceans. Due to this, there are concerns that structures made of calcium carbonate may become vulnerable to dissolution, affecting corals and the ability of shellfish to form shells.

                                  

    3.3.3 Health Hazards


    ACTIVITY 3.8: Diseases Caused by Polluted Water


    Design a questionnaire to find out how many students in your class have been affected by one of the following diseases:

               

    Include in your questionnaire the cause of their disease as diagnosed by their doctors.

    Virtually all types of water pollution are harmful to the health of humans and animals. Water pollution may not damage our health immediately but can be harmful after long term exposure. People cannot survive without drinking water, and if their freshwater resources are polluted, they can fall ill by drinking them. Different types of pollutants affect human health in different ways:

    • Heavy metals from industrial processes can accumulate in nearby lakes and rivers. These are toxic to aquatic life such as fish and shellfish, and subsequently to the humans who eat them. Heavy metals can slow development; result in birth defects and some are carcinogenic, i.e., can cause cancer.

    • Industrial waste often contains many toxic compounds that damage the health of aquatic animals and those who eat them. Some of the toxins in industrial waste may only have a mild effect whereas others can be fatal. They can affect immune system, reproductive system or cause poisoning.

    • Microbial pollutants from sewage often result in water-borne diseases that infect aquatic life and terrestrial life through drinking water. Microbial pollutants include bacteria, virus and protozoa.

                                       

    DO YOU KNOW?

    Microbial water pollution is a major problem in the developing world. These illnesses are particularly dangerous for young children; in fact, they account for almost 60 per cent of early childhood deaths worldwide.

                      

    EXERCISE 3.9

    1. Addition of pollutants to water changes its physical and chemical properties. (True or False)

    2. The pH of pure water is ______ .

    3. Water pollution is harmful to _______ and ______ .

    4. What are the dangers of water pollution? 5. Name two water-borne diseases.

    3.4 PREVENTION OF WATER POLLUTION

    ACTIVITY 3.9: Illustrating Prevention of Water Pollution

    ‘Water water everywhere but not a drop to drink’.

    Comment on the statement given above. Make a poster/PowerPoint presentation on how you can save water. Display it in class.

    There is no easy way to solve water pollution; if there were, it would not be so much of a problem. Broadly speaking, there are three different things that can help to tackle the problem—education, laws, and economics—and they work together as a team.

    3.4.1 Education

    Making people aware of the problem is the first step towards solving it. Education can help people determine their best strategies to avoid contaminating local water sources: avoiding urinating or defecating in or near the water; building toilets/sites for waste

    downhill from wells to reduce risks of contaminating groundwater; employing household water treatment and safe storage techniques are examples. Greater public awareness can make a positive difference. Awareness helps to prevent disposal of solid and human waste and chemical and industrial waste into waterways as much as possible. It also includes treating wastes before they go into waterways.This can be achieved by setting up of educational camps.

            

    3.4.2 Laws

    One of the biggest problems with water pollution is its trans boundary nature. Many rivers cross countries, while seas span whole continents. Pollution discharged by factories in one country can cause problems in neighboring nations, even when they have tougher laws and higher standards. Environmental laws can make it tougher for people to pollute, but to be really effective they have to operate across national and international borders. Proper implementations of national and international laws is another issue faced by the government. Without tougher implementation it is difficult to solve the problem of water pollution. As in Rwanda, the ban of polythene is successfully implemented with inspection officers.

                         

    3.4.3 Economics

    Most environmental experts agree that the best way to tackle pollution is through something called the polluter pays principle. This means that whoever causes pollution should have to pay to clean it up, one way or another. Polluter pays can operate in all kinds of ways. It could mean that shoppers should have to pay for their plastic grocery bags, as is now common in Ireland, to encourage recycling and minimize waste. Or it could mean that factories that use rivers must have their water inlet pipes downstream of their effluent outflow pipes, so if they cause pollution, they themselves are the first people to suffer. Ultimately, the polluter pays principle is designed to hinder people from polluting. It makes it less expensive for them to behave in an environmentally responsible way.

                            

    3.5 OUR CLEAN FUTURE

    Life is ultimately about choices—and so is pollution. We can live with dirty surroundings, dead rivers, and fish that are too poisonous to eat. Or we can work together to keep the environment clean so the plants, animals, and people who depend on it remain healthy. We can take individual action to help reduce water pollution. These actions are:

    1. Use less water:This might sound simplistic, but decreasing your water consumption is one of the keys to minimize water pollution. By reducing the amount of water you use, you will reduce the amount of water that flows into sewage treatment systems.

                  

    2. Use environment-friendly house-hold products: Don’t use household products that contain chemicals. Instead, use green products, like biodegradable soap and all-natural toiletries.