Chemistry

LEARNING OBJECTIVES

After reading this unit, you will be able to:

•categorize different oxides.

•explain how different oxides are formed.

•state the uses of different oxides.

•describe the reaction of oxides with other substances.

•describe the process of producing slaked lime.

KNOWLEDGE GAIN

                   

Corrosion is a natural process which converts a refined metal to a more stable form such as its oxide or hydroxide.

ACTIVITY 8.1: Illustrating Types of Oxides

•Take a sample of magnesium, iron filings, sulphur and carbon.

•Burn each sample one by one.

•Observe the product formed and gas produced.

Was any residue left in case of non‑metals?

Can you name the gas produced when carbon burns in air?

Write the chemical equation for each reaction.

8.1 PREPARATION OF OXIDES

We have already studied in Unit 2 that elements (metals and non‑metals) react with oxygen to produce oxide. Oxides can be prepared by the following methods:

•Direct combination of an element with oxygen.

•Thermal decomposition of hydroxides, carbonates and nitrates.

Let us discuss these methods one by one.

8.1.1 Direct Combination of an Element with Oxygen

Combination of Metals with Oxygen

ACTIVITY 8.2: Illustrating Nature of Metal Oxide

Repeat Activity 8.1 with Calcium.Make a report on the products obtained from the reaction of metals with oxygen.

Metals react with oxygen to produce metal oxide. Metal oxides are generally basic in nature.

Metal + Oxygen → Metal Oxide

Some elements react with oxygen at room temperature, some react on heating whereas others react only on strong heating. The reaction of oxygen with some elements is given below:

(i) Sodium combines with oxygen at room temperature with yellow flame to form sodium oxide.

  

Potassium also combines with oxygen at room temperature to produce potassium oxide. Try to write equation for this reaction.

(ii) Magnesium reacts with oxygen on heating to form magnesium oxide. It does not react with oxygen at room temperature.

  

(iii) Aluminium reacts with oxygen on heating to form aluminium oxide

                  

(iv) Zinc reacts with oxygen on strong heating to form zinc oxide

                    

(v) Copper does not burn in air (oxygen) even on strong heating. Copper reacts with oxygen on prolonged heating to form a black substance (copper oxide).

(vi) Iron reacts with oxygen on heating to form iron oxide.

      

Note:Iron metal does not burn in air but iron filings (small particles of iron) burn vigorously when sprinkled on flame.

(vii) Calcium reacts with oxygen to form calcium oxide

            

Calcium is a silvery white metal. The surface of calcium metal is covered with a thin layer of oxide that helps protect the metal from attack by air.

            

Calcium is quite reluctant to start burning, but once ignited, calcium burns in (reacts with) oxygen to give white calcium oxide.

EXERCISE 8.1

1. Metals react with oxygen to produce _______ .

2. Sodium combines with oxygen at room temperature to form sodium oxide.

                                                                               (True or False)

3. Write the molecular formula of iron oxide.

4. Name the metal which does not react with oxygen at room temperature?

5. Which of these formula are correct:

(a) Al₂O₃                  (b) Na₂O                 (c) Only (a)                   (d) both (a) and (b)

Combination of a Non-metal with Oxygen

ACTIVITY 8.3: Illustrating Nature of Non-metal Oxide

Take about 1 g in powdered sulphur in a deflagrating (combustion) spoon.

Heat the sulphur over the Bunsen burner, till it catches fire and starts burning with a blue flame. Lower the deflagrating spoon in a glass gas jar. Wait till the sulphur stops burning. Take out the deflagrating spoon from the gas jar and cover it with a glass disc.

   

With the help of a dropper pour about 4–5 ml water in the gas jar and shake well. Test the water with the strips of red litmus paper and blue litmus paper.

Note your observations.

Make a report on the products obtained from the reaction of non‑metals with oxygen.

In Activity 8.3, you will observe that blue litmus turns red, but there is no change in color in case of red litmus paper. From this observation, it is evident that the product formed by the burning of sulphur dissolves in water and is acidic in nature.

Oxides of sulphur and nitrogen react with water to form acids. Sulphur forms sulphuric acid and nitrogen forms nitric acid. These acids can cause severe burns. It is thus important to handle them with care.

Non‑metals react with oxygen to produce non‑metal oxide. Non‑metal oxides are generally acidic in nature.

Non‑metal + Oxygen → Non‑metal oxide

Similarly, other non‑metals are also burnt in air to produce acidic oxides.

The equations for reaction of some non‑metals with oxygen are:

(i) Sulphur combines with oxygen to give sulphur dioxide.

  S(s) + O₂(g) → SO₂(g)

(ii) Carbon combines with oxygen to give carbon dioxide.

   C(s) + O₂(g) → CO₂(g)

(iii) Hydrogen combines with oxygen to form neutral oxide water.

             

(iv) Phosphorus burns with dazzling white flame in oxygen forming phosphorus pentoxide.

             4P + 5O₂→ 2P₂O₅

EXERCISE 8.2

1. Write the molecular formula of phosphorus pentoxide.

2. Complete the following chemical equation.

   S (s) + O₂ (g) → ?

3. Non‑metal oxides are generally ______ in nature.

4. Oxides of sulphur and nitrogen react with water to form acids.

                                                                      (True or False)

5. Which of these is/are not an example of non‑metal oxides?

         (a) CO₂            (b) H₂O             (c) SO₃                (d) none of these

8.1.2 Thermal Decomposition of Hydroxide, Carbonates and Nitrate

ACTIVITY 8.4: Illustrating Thermal Decomposition

Make a report on the products obtained from thermal decomposition of hydroxides, carbonates and nitrates.

When a decomposition reaction is carried out by heating, it is called thermal decomposition. Thermal means related to heat. In the following sections, we will study the formation of oxide by thermal decomposition.

Thermal Decomposition of Hydroxide

Metal hydroxide decomposes to form metal oxide and water. For example,

(i) Magnesium hydroxide on heating at a high temperature decomposes into magnesium oxide and water.

                       

(ii) Calcium hydroxide on heating decomposes into calcium oxide and water.

                      

(iii) Aluminium hydroxide decomposes into aluminium oxide and water.

                

Thermal Decomposition of Carbonates

ACTIVITY 8.5: Thermal Decomposition of Copper Carbonate

Materials Required

Copper carbonate, Bunsen burner, boiling test tube.Procedure

•Take a little amount of copper carbonate in a boiling test tube.

•Arrange all apparatuses as shown in the following figure.

                

•Heat the test tube on Bunsen burner.

•Observe the change.

In Activity 8.5, you will notice that copper carbonate (greenish) decomposes into copper oxide (black) and carbon dioxide. The carbon dioxide gas can be detected using lime water. Lime water turns cloudy white when carbon dioxide is bubbled through it.

Reaction

    

All the metal carbonates undergo thermal decomposition to give metal oxide and carbon dioxide. For example,

(i) When calcium carbonate is heated, it decomposes (breaks) to give calcium oxide and carbon dioxide.

             

(ii) Magnesium carbonate decomposes into magnesium oxide and carbon dioxide.

       

Note: The process of breaking up of magnesium carbonate at high temperature is known as calcining.

(iii) Zinc carbonate decomposes into zinc oxide and carbon dioxide.

     

(iv) Lithium carbonate decomposes into lithium oxide and carbon dioxide.

        

Note: Carbonates of sodium and potassium decompose at very high temperatures. They do not decompose at Bunsen temperature.

Thermal Decomposition of Nitrates

ACTIVITY 8.6: Thermal Decomposition of Lead Nitrate

•Take about 0.5 g lead nitrate powder in a boiling tube.

•Hold the boiling tube with a pair of tongs.

•Heat it over a flame, as shown in figure.

                   

Safety

•Wear goggles, do not look directly into the boiling test tube.

•Do not touch the boiling test tube.

•Hold the boiling test tube with pair of tongs properly.

In Activity 8.6, you will observe the release of brown fumes. These fumes are of nitrogen dioxide (NO₂).

When lead nitrate is heated strongly, it breaks down to give lead monoxide, nitrogen dioxide and oxygen.

                     

Most metal nitrates decompose on heating to give the metal oxide, brown fumes of nitrogen dioxide, and oxygen. Group I nitrates except LiNO₃ decompose to give metal nitrates and oxygen. For example,

(i) Copper nitrate decomposes on gentle heating to give copper oxide, nitrogen dioxide and oxygen.

               

(ii) Lithium nitrate decomposes on heating to produce lithium oxide, nitrogen dioxide and oxygen.

                      

Note:All nitrates of group 1 (from sodium to caesium) decompose to give metal nitrites and oxygen. As you go down the group you have to use higher temperature.

EXERCISE 8.3

1. Metal hydroxide decomposes to form ______ and _______.

2. Complete the following reaction:

       

3. Lithium nitrate decomposes on heating to produce ______, _______ and _______ .

4. What do you mean by thermal decomposition?

5. Carbonates of sodium and potassium decompose at very low temperature.

                                                                                     (True or False)

8.2 REACTION OF OXIDES WITH WATER

ACTIVITY 8.7: Illustrating Combination of Oxides with Water

•Take a little quicklime (CaO) in a hard beaker.

•Add water to it slowly as shown in the figure.

•Observe the change in temperature by touching the beaker.

              

Can you tell whether the reaction is exothermic or endothermic?

In Activity 8.7, you will observe that calcium oxide reacts vigorously with water to form calcium hydroxide.

CaO(s) + H₂O(l) → Ca(OH)₂

The reaction between quicklime (calcium oxide) and water to form slaked lime is an exothermic reaction because it releases a lot of heat.

Most of the metal oxides are insoluble in water. Only a few metal oxides dissolve (react) in water to form alkalis. Sodium oxide and potassium oxide are two metal oxides which are soluble in water. They dissolve in water to form alkali. The equations for reaction of metal oxides with water are:

(i) Sodium oxide reacts with water to form sodium hydroxide.

Na2O(s) + H2O(l) → 2NaOH(aq)

(ii) Potassium oxide reacts with water to form potassium hydroxide.

K2O(s) + H2O(l) → 2KOH(aq)

(iii) Magnesium oxide dissolves partially in water to form magnesium hydroxide.

MgO(s) + H2O(l) → Mg(OH)2(aq)

Alkali: Water soluble bases are called alkalis. For example, sodium hydroxide and potassium hydroxide.

ACTIVITY 8.8: Non-metal Oxides Dissolve in Water to form Acid

•Collect rainwater in a plastic container.

•Measure the pH of rainwater.

•Compare the color of pH paper with the standard pH‑color chart.

What is the pH of collected rainwater?Make a note on your observation.

In Activity 8.8, you will observe that the pH of rainwater is between 4.5 and 5.5 which shows it is slightly acidic. This is because of non‑metal oxides. The non‑metal oxides such as carbon dioxide, nitrogen dioxide, and sulphur dioxide are released in environment by industrial activities. These gases (non‑metal oxides) combine with rainwater to form dilute carbonic acid, nitric acid, and sulphuric acid.

The non‑metal oxides dissolve in water to form acids. For example,

(i) Carbon dioxide dissolves in water to form carbonic acid.

             

A solution of carbon dioxide gas in water turns blue litmus to red showing it is acidic in nature. (ii) Sulphur dioxide reacts with water to give sulfurous acid.

                     

A solution of sulphur dioxide and water turns blue litmus to red showing it is acidic.

EXERCISE 8.4

1. Name the product formed, when calcium oxide reacts with water.

2. Write the molecular formula of magnesium hydroxide.

3. Which of the following is correct?

(a) H₂SO₄            (b) H₂SO₃                (c) H₂CO₃                 (d) all of these

4. Sulphur dioxide is acidic in nature.

                           (True or False)

5. Name two metal oxides which are soluble in water.

8.3 REACTION OF OXIDES WITH ACIDS AND BASES

ACTIVITY 8.9: Illustrating Combination of Oxides with Acid and Bases

•Take a small amount of copper oxide in a beaker.

•Note the color of copper oxide.

•Add dilute hydrochloric acid slowly while stirring with a glass rod.

•Observe the change:

              – Did copper oxide dissolve in dilute hydrochloric acid?

              – Did the color of mixture change?

In Activity 8.9, you will observe that black color of copper oxide changes to blue‑green due to the formation of copper chloride.

Most of the metal oxides are basic in nature but some metal oxides show both acidic and basic nature. The metal oxides which show both acidic and basic nature are called amphoteric oxides. Amphoteric oxides react with both acids and bases to form salt and water. Examples of amphoteric oxides are: aluminium oxide and zinc oxide.

The chemical equations for these oxides with acid and bases are:

(i) Aluminium oxide reacts with hydrochloric acid to form aluminium chloride (salt) and water.

             

In this reaction, aluminium oxide behaves as a basic oxide because it reacts with an acid to form salt and water.

(ii) Aluminium oxide reacts with sodium hydroxide to form sodium aluminate (salt) and water.

                 

In this reaction, aluminium oxide behaves as an acidic oxide because it reacts with a base to form salt and water.

(iii) Zinc oxide reacts with hydrochloric acid to form zinc chloride (salt) and water

                        

In this reaction, zinc oxide behaves as a basic oxide. Do you know why?

Basic: It turns red litmus solution into blue.

Acidic: It turns blue litmus solution to red.

(iv) Zinc oxide reacts with sodium hydroxide to form sodium zincate (salt) and water

             

In this reaction, zinc oxide behaves as an acidic oxide. Do you know why?

EXERCISE 8.5

1. The color of copper oxide is

(a) blue (b) black (c) green (d) yellow

2. Most of the metal oxides are ______ in nature.

3. Complete the following reactions:

(i) Al2O3 (s) + HCl (aq) → ? + ?

(ii) ZnO (s) + NaOH (aq) → ? + ?  Also balance the both reactions.

4. Write balanced chemical equation for the following:

(i) When aluminium oxide reacts with sodium hydroxide.

(ii) When zinc oxide reacts with hydrochloric acid.

8.4 CLASSIFICATION OF OXIDES

ACTIVITY 8.10: Illustrating Acidity/Basicity of Oxides

Analyze the position of non‑metals in the Periodic Table and observe the relationship between position of elements in the Periodic Table and acidity/basicity of oxides.

 

ACTIVITY 8.11: Illustrating Chemical Properties of MgO

•Take 2 g magnesium oxide in a test tube.

•Add 10 ml hydrochloric acid.

•Gently shake the test tube.

•Observe the change.

                   – Did magnesium oxide dissolve in hydrochloric acid?

                  – Did the temperature of test tube change?

Reaction

In Activity 8.11, you will notice that magnesium oxide dissolves to form a clear solution. Also the test tube becomes warm.This implies that magnesium oxide reacts with hydrochloric acid. Therefore, magnesium oxide is basic in nature.

MgO(s) + HCl(aq) → MgCl₂ + H₂O

                      

ACTIVITY 8.12: Illustrating Effect of Litmus Solution to Different Oxides

Safety: Wear goggles.

•Make a solution of different oxides such as sodium oxide, potassium oxide, magnesium oxide, etc. with water.

•Add 2–3 ml of each sample in a test tube.

•Put all test tubes in a test tube stand as shown in figure (see right).

•Add 2–3 drops of red litmus solution to each test tube.

•Observe and note if there is change in color.

•If red litmus does not change to blue, add 2–3 drops of blue litmus solution.

Did not any oxide change one of the litmus solutions?

•Record your result in tabular form.

             

Oxides are compounds of oxygen with another element. For example, magnesium oxide, sodium oxide, aluminium oxide, nitrogen oxide and calcium oxide. Depending upon the nature and the properties exhibited by these oxides, they are classified into:

•Acidic oxides

•Basic oxides

•Neutral oxides

•Amphoteric oxides

                      

Looking across a period on the periodic table, metals give way to non‑metals. Similarly, there is a trend from basic oxides (that are ionically bonded) through a few amphoteric oxides to acidic oxides (that are covalently bonded). The noble gases, of course, do not form oxides.

8.4.1 Acidic Oxides

Acidic oxides are oxides of non‑metals. They turn blue litmus solution to red. Most acidic oxides are soluble in water. They react with water to produce an acid. All acidic oxides react with alkali to give salt and water.

Example of acidic oxides are sulphur dioxide, sulphur trioxide, carbon dioxide, phosphorus pentoxide.

Properties of Acidic Oxides

1. Acidic oxides do not react with acids.

2. They react with bases and alkalis to form salt and water.

3. They dissolve in water to form acidic solutions.

4. They are usually gases at room temperature.

8.4.2 Basic Oxides

Basic oxides are oxides of metals. They turn red litmus solution to blue. Most metal oxides are insoluble in water but some of these dissolve in water to form alkalis. They react with water to produce a base. All basic oxides react with acid to form salt and water.

Example of basic oxides are sodium oxide, magnesium oxide, potassium oxide, copper oxide and calcium oxide.

Properties of Basic Oxides

1. Basic oxides do not react with bases.

2. They react with acids to form salt and water.

3. The basic oxides are usually insoluble in water. Those that dissolve in water form alkaline solutions.

8.4.3 Neutral Oxide

Some non‑metals react with oxygen to form neutral oxides. Neutral oxides do not show acidic nor basic characteristics. For example, carbon monoxide, nitric oxide, and nitrous oxide. These compounds are also called neutral compounds.

Properties of Neutral Oxides

1. Neutral oxides do not react with either acids or bases.

2. They have no effect on litmus solution.

8.4.4 Amphoteric Oxides

Some metals react with oxygen to produce amphoteric oxides. Amphoteric oxides exhibit both acidic and basic characteristics.These oxides react with acids as well as bases to form salt and water. Example of amphoteric oxides are aluminium oxide and zinc oxide.

Properties of Amphoteric Oxides

1. Amphoteric oxides react with both acids and bases.

2. They change red litmus to blue and blue litmus to red.

3. They undergo neutralization reaction to give salt and water.

Note: Amphoteric oxides show basic as well as acidic behavior. Amphoteric oxides react with both acids and bases to form salt and water.

Classification of oxides as basic, acidic, amphoteric and neutral is given in Table 8.1.

  

Basic oxides are oxides mostly of metals, especially alkali and alkaline earth metals.

Other elements which form amphoteric oxides are gallium, indium, scandium, titanium, zirconium, silver, gold, germanium, tin, antimony, bismuth, and tellurium.

EXERCISE 8.6

1. Name the types of oxides. Give one example of each.

2.  (i) ________ are oxides of non‑metals.

     (ii) ________ are oxides of metals.

     (iii) Some metals react with oxygen to produce ________.

     (iv) Some non‑metals react with oxygen to produce ________.

3. Make a list of oxides categorizing into their types.

8.5 USES AND PRODUCTION OF SLAKED LIME (ISHWAGARA)

Calcium oxide (lime or quicklime) reacts vigorously with water to form calcium hydroxide (slaked lime):

This is a combination reaction in which two compounds, calcium oxide and water combine to form single compound calcium hydroxide. A large amount of heat is released when calcium oxide reacts with water to form calcium hydroxide. Solid calcium hydroxide is also known as slaked lime (ishwagara). Slaked lime is a white powder.

A solution of slaked lime produced by the reaction of quicklime and water is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO₃.

             

EXERCISE 8.7

1. Write two uses of ishwagara.

2. Chemical formula of marble is:

(a) Ca(OH)₂       (b) CaO          (c) CaCO₃              (d) CaCl₂

3. A large amount of heat is absorbed when lime reacts with water.

                                           (True or False)

4. Slaked lime is a ________ powder.

5. Solid calcium hydroxide is also known as:

(a) Marble      (b) Talcum         (c) Granite          (d) None of these

8.6 SUMMARY

•Oxides are compounds of oxygen with another element.

•Metals combine with oxygen to give basic oxides or amphoteric oxides.

•Non‑metals combine with oxygen to form acidic oxides or neutral oxides.

•Amphoteric oxides exhibit both acidic and basic characteristics.

•Neutral oxides show neither acidic nor basic characteristics.

•Basic oxide turns red litmus solution into blue.

•Acidic oxide turns blue litmus solution into red.

•Oxides can be prepared by thermal decomposition of carbonates, hydroxides and nitrates.

•Metal oxides react with acid to give salt and water.

•A few metal oxides react with water to form alkalis.

•Non‑metal oxides react with water to produce acid.

•Slaked lime (ishwagara) is a white powder produced by reaction of quicklime and water.

•Slaked lime is used for white washing and formation of calcium carbonate.

8.7 GLOSSARY

•Alkaline: having a ph greater than 7

•Dazzling: extremely bright

•Covalent: the sharing of electrons between atoms.

•Deflagrate: sharp combustion, burning with flame.

•Deflagrating spoon: a long vertical handled spoon with cover used in deflagration experiments.

•Endothermic: a reaction or process accompanied by or requiring the absorption of heat.

•Exothermic: a reaction or process accompanied by the release of heat.

•Ignition: the action of setting something on fire or starting to burn.

•Prolonged: continuing for a long time.

•Reluctant: resistant

•Sprinkled: spray

•Thermal decomposition: a chemical decomposition caused by heat

8.8 UNIT ASSESSMENT

I. Multiple Choice Questions

1. Metals react with oxygen to produce

(a) basic oxides                         (b) amphoteric oxides       

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

2. Non‑metals react with oxygen to give

(a) acidic oxides                          (b) neutral oxides

(c) both (a) and (b)                      (d) none of these

3. Metal which reacts with oxygen only on strong heating is

(a) zinc     (b) sodium        (c) potassium           (d) iron

4. Metal hydroxide decomposes to form

(a) metal oxide and water         (b) metal oxide and carbon dioxide

(c) both (a) and (b)                    (d) none of these

5. The color of nitrogen dioxide gas is

(a) red        (b) brown         (c) blue         (d) white

6. All metal nitrates decompose to give

(a) metal oxide      (b) nitrogen oxide         (c) oxygen               (d) all of these

7. The reaction between quicklime and water is

(a) an exothermic reaction                 (b) an endothermic reaction

(c) both (a) and (b)                             (d) none of these

8. Aluminium oxide and zinc oxide are examples of

(a) amphoteric oxides              (b) neutral oxides          (c) basic oxides       (d) none of these

9. Nitric oxide and nitrous oxide are examples of

(a) amphoteric oxides (b) basic oxides (c) neutral oxides (d) none of these

10. Production of slaked lime from calcium oxide and water is a

(a) combination reaction (b) combustion reaction (c) single replacement reaction (d) double replacement

reaction

II. Open Ended Questions

1. Compare between acidic oxide and basic oxide.

2. Distinguish between amphoteric and neutral oxides.

3. List two examples of each

(i) Acidic oxide (ii) Basic oxide (iii) Amphoteric oxide (iv) Neutral oxide

4. Explain the preparation of oxide by thermal decomposition of carbonate.

5. State the properties of acidic oxides and basic oxides.

6. What happens when

(i) calcium oxide reacts with water (ii) magnesium combines with oxygen

(iii) sulphur burns in oxygen   (iv) lithium carbonate decomposes (v) carbon dioxide dissolves in water

7. Write balanced chemical equation for each reaction of Q6.

8. Make a table showing a list of acidic and basic oxides.

9. Can you name some compounds which decompose to produce metal oxide and carbon dioxide.

10. Carry out an experiment to show the product obtained when magnesium reacts with oxygen. Write a balanced equation

                                                 PROJECT

Collect some corroded and some fresh metals. Observe and make a presentation distinguishing corroded and fresh metal. Show your presentation in the class.