S5: Chemistry

         Key unit competency:

To be able to interpret the phase diagrams for different compounds.
• Define a phase
• Explain the term phase equilibrium
• Explain the effect of change of state on changing pressure and temperature
• Define heterogeneous and homogeneous equilibria
• Define triple point, critical point, normal boiling and melting points of 
   substances
• Relate the physical properties of compounds to their phase diagrams.
• Locate triple point, critical point, normal boiling and melting points on the 
   phase
• diagrams
• Compare the phase diagrams for water with that carbon dioxide 
• Develop analysis skills, team work, and attentiveness in interpreting the 

   phase diagrams and in practical activities

Introductory Activity 

In our daily life, we use varied materials which have distinct properties and they 
can exist in different physical states of the matter. In this regard, we sometimes 
need to keep a given substance in a certain state because we know that it will 
serve better. Explain two important conditions that should be dealt with to 
maintain stable some physical states of the matter as we need them.

Explain why ice flots on water.

10.1. Phase equilibrium

Activity 10.1

1. Observe the diagrams that represent different systems and assess the 

    number of phases and components involved in each system.

             

      2. Differentiate a phase and a physical state of matter. Explain your answer.

      10.1.1. Definition of key terms

A phase is a homogeneous portion of a system which has uniform physical 
characteristics. It can be separated from other parts of the system by a clear boundary 
(limit). A phase can be a solid, liquid, vapor (gas) or aqueous solution which is uniform 

in both chemical composition and physical state.

Examples

i. A mixture of gases (air) consists of one phase only
ii. A mixture of oil and water consists of two different liquid phases

iii. A mixture of solids, each solid is regarded as having one phase 

A component: it is a chemical species which may be used to specify the 
composition of a system. For example;

• A three-phase system of water (i.e. water, ice, and vapor) is a one component 
system. The constituent substance of the three phases is water only.

• A mixture of water and ethanol is a one phase, two components system 
because there are two different chemical compositions.

• An equilibrium: it is the state of a reaction or physical change in which the 
rates of the forward and reverse processes are the same and there is no net 
change on the amount of the equilibrium components

• A phase equilibrium: it is a balance between phases, that is, the coexistence 
of two or more phases in a state of dynamic equilibrium. The forward process 
is taking place at the same rate as the backward process and therefore the 
relative quantity of each phase remains unchanged unless the external 

condition is altered.

            

Checking up 10.1

1. Which of the following is not an example of phase equilibrium?

              

b) Carbon dioxide in a stoppered fizzy drink:

c) Vapours above the surface of liquid water in a closed container, at a given 
    temperature.
2. At 0.001°C and 0.00603 atm water, ice and vapor can coexist in a closed 
    container.
a) Explain the number of phases that this equilibrium has.

b) How many components does this system have? Explain.

10.2. Homogeneous and heterogeneous equilibria

Activity 10.2

Differentiate homogeneous mixture from heterogeneous mixture. mixture
of solids, each solid is regarded as having one phase 

1. Homogeneous equilibrium
A system with one phase only is described as a homogeneous system and when 
this system is at equilibrium, it is said to be a homogenous equilibrium.
 
In general, a homogeneous equilibrium is one in which all components are present 
in a single phase. In a case of a chemical reaction, both reactants and products exist 
in one phase (gaseous phase, liquid phase or aqueous solution and solid phase).
 
For example, in the esterification of acetic acid and ethanol the equilibrium is 
homogeneous because all involved substances are in the same liquid phase.

All the reactants and products are liquids

2. Heterogeneous equilibrium
A system consisting of more than one distinct phases is described as heterogeneous 
system. A heterogeneous equilibrium is a system in which the constituents are 
found in two or more phases. The phases may be any combination of solid, liquid, 
gas, or solutions. 
For example, in the manufacture of quick lime from lime
stone the following equilibrium is involved:

                                
 It is a heterogeneous equilibrium because some of the components are solids (lime 

stone and quick lime) and another is a gas (carbon dioxide).

Checking up 10.2

Classify the following reactions as homogeneous equilibrium and heterogeneous 

equilibrium

              

10.3. Phase diagrams

Activity 10.3

1. When ice cream trucks drive through towns on hot season days, they 
    keep their products from melting by using dry ice (solid carbon dioxide) 

   as shown in the image below. Why is dry ice used instead of ice?

                      

     2. Why most of the time very high mountains are covered by ice?    

                        

      3. Explain the conditions that are required to be changed so that the pure 

          substance change from one state of matter to another. 

 Aphase diagram is a graph illustrating the conditions of temperature and pressure 
under which equilibrium exists between the distinct phases (states of matter) of a 
substance. Phase diagrams are divided into three single phase regions that cover 
the pressure-temperature space over which the matter being evaluated exists: 
liquid, gaseous, and solid states. The lines that separate these single-phase regions 
are known as phase boundaries. Along the phase boundaries, the matter being 
evaluated exists simultaneously in equilibrium between the two states that border 

the phase boundary.

The general form of a phase diagram for a substance that exhibits three phases 

is shown below in the Figure 10.1.                      

             

 Under appropriate conditions of temperature and pressure of a solid can be in 
equilibrium with its liquid state or even with its gaseous state. The phase diagram 
allows to predict the phase of substance that is stable at any given temperature and 
pressure. It contains three important curves, each of which represents the conditions 

of temperature and pressure at which the various phases can coexist at equilibrium.

i. Boiling point 

The line TC is the vapor pressure curve of the liquid. It represents the equilibrium 
between the liquid and the gas phases. The temperature on this curve where the 
vapor pressure is equal to 1atm and it is the normal boiling point of the substance. 
The vapor pressure curve ends at the critical point (C) which is the critical 
temperature corresponding to the critical pressure of the substance which is the 

pressure required to bring about liquefaction at critical temperature.

ii. Critical point

Critical point consists of the temperature and pressure beyond which the liquid and 
gas phases cannot be distinguished. Every substance has a critical temperature 

above which the gas cannot be liquefied, regardless the applied pressure.

iii. Sublimation point

The line AT is the sublimation curve which represent the variation in the vapor 
pressure of the solid as it sublimes into gas at different temperatures. The reverse 
process is deposition of the gas as a solid. Sublimation point is the temperature at 

which the solid turns to gas at a constant pressure.

iv. Melting point

The line TB is the melting point curve which represent the change in melting point 
of the solid with increasing pressure.The line usually slopes slightly to the right as 
pressure increases. For most substances, the solid is denser than the liquid, therefore, 
an increase in pressure favors the more compact solid. Thus, higher temperatures 
are required to melt the solid at higher pressures. The temperature at which the 

solid melts at a pressure of 1atm is the “normal melting point”.

v. Triple point

The triple point T is a point where the three curves intersect. All the three phases 
exist at equilibrium at this temperature and pressure. The triple point is unique for 

each substance.

vi. Supercritical fluid

Supercritical fluid of a substance is the temperature and pressure above its own 
thermodynamic critical point that can diffuse through solids like a gas and dissolved 

materials like a liquid.

Any point on the diagram that does not fall on a line corresponds to conditions 
under which one phase is present. Any other point on the three curves represents 
equilibrium between two phases.

The gas phase is stable phase at low pressures and elevated temperatures. The 
conditions under which the solid phase is stable extend to low temperatures and 
high pressures. The stability range for liquids lie between the other two regions. That 

is between solid and liquid regions.

10.3. 1. Phase diagram of water

Water is a unique substance in many ways due to its properties. One of these special 
properties is the fact that solid water (ice) is less dense than liquid water just above 

the freezing point. The phase diagram for water is shown in the Figure 10.2.

                   

Water can turn into vapor at any temperature that falls on the vapor pressure curve 
depending on the conditions of pressure, but the temperature at which water liquid 
turns into vapor at normal pressure (1atm) is called the normal boiling point of 

water, 100 °C (Figure 10.2)

Point E in the Figure 10.2 is the critical point of water where the pressure is equal 
to 218 atm and the temperature is about 374 °C. At 374°C, particles of water in the 
gas phase are moving rapidly.At any other temperature above the critical point of 
water, the physical nature of water liquid and steam cannot be distinguished; the 
gas phase cannot be made to liquefy, no matter how much pressure is applied to 

the gas.

The phase diagram of water is not a typical example of a one component system 
because the line AD (melting point curve) slopes upward from right to left. It has a 
negative slope and its melting point decreases as the pressure increases. This occurs 
only for substances that expand on freezing. Therefore, liquid water is denser than 

solid water (ice), the reason why ice floats on water.

10.3.2. Phase diagram of carbon dioxide

Compared to the phase diagram of water, in the phase diagram of carbon dioxide 
the solid-liquid curve exhibits a positive slope, indicating that the melting point for 
increases with pressure as it does for most substances. The increase of pressure 
causes the equilibrium between dry ice and carbon dioxide liquid to shift in the 
direction of formation of dry ice that is freezing. Carbon dioxide contracts on freezing 
and this implies that dry ice has higher density than that of liquid carbon dioxide. 

The Figure 10.3 shows the phase diagram of carbon dioxide.

                      

              

The triple point is observed at the pressure above 1atm, indicating that carbon 
dioxide cannot exist as a liquid under normal conditions of pressure. Instead, 
cooling gaseous carbon dioxide at 1atm results in its deposition into the solid state. 
Likewise, solid carbon dioxide does not melt at 1atm pressure but instead sublimes 

to yield gaseous 

Checking up 10.3

1. If a piece of dry ice is left on the lab counter, you will see it get smaller 
    until it disappears, with no liquid left around it. Explain why.

2. Describe what conditions of pressure and temperature will carbon 
    dioxide exist as a liquid?

3. What is the meaning of the term “critical temperature”, and what is the 
     value of the critical temperature of ?

4. Why does  make an excellent fire extinguisher?

5. Explain the following observations:
    a) When a closed glass container full of water is put in fridge, it directly 
         breaks when the water freezes.
   b) The water of oceans at the poles of the Earth are normally covered by 

     ice and ice does not submerge in water.

                 

10.4. Comparison of phase diagrams of substances that 

          expand and those that contract on freezing

Activity 10.4

1. Analyze the phase diagrams of water and carbon dioxide previously 
     discussed to assess their similarities and differences.
2. The glacier easily slides on ice as shown in the photo below. Explain how 

      the property of water facilitates this movement.

                                    

For the phase diagrams, some materials contract on freezing while others expand 
on freezing. The main differences between substances that expand and those 
that contract on freezing can be highlighted by comparing the phase diagrams 
of carbon dioxide and that of water. In the phase diagram of carbon dioxide, the 

substance contracts on freezing and that of water expands on freezing.

Both phase diagrams for water and carbon dioxide have the same general Y-shape, 
just shifted relative to one another. This shift occurs because the liquid phase in 
the dry ice can only occur at higher temperatures and pressures, whereas, in ice 
the liquid phase occurs at lower temperatures and pressures. There are two more 
significant differences between the phase diagram of carbon dioxide and that of 

water:

10.4.1. Melting point curve

The melting point curve of carbon dioxide slopes upwards to right (Figure 10.3) 
whereas that of water slopes upward to left (Figure 10.2). This means that for carbon 
dioxide the melting point increases as the pressure increases, a characteristic 
behavior of substances that contract on freezing. Further, water expands on freezing 
(Figure 1.4) and this unusual behavior is caused by the open structure of the regular 
packing of water molecules in ice due to the network of hydrogen bonding in ice 

which is more extensive than in liquid.

                        

Ice floats on liquid water (Figure 10.5), this unusual behavior is caused by the open
structure of the regular packing of water molecules in ice due to the network of 
hydrogen bonding in ice which is more extensive than in liquid. The ice is less dense 

than water reason why it floats in water.

                               

10.4.2. Triple point 

The triple point of carbon dioxide is above atmospheric pressure. This means that 
the state of liquid carbon dioxide does not exist at ordinary atmospheric pressure. 
Dry ice remains as a solid below -78ºC and changes to fog (gas) above -78ºC. It 
sublimes without forming liquid at normal atmospheric pressure (Figure 10.6). The 
sublimation of carbon dioxide results in a low temperature which causes water 

vapors in the air to form moist. 

                   

 Ice is stable below 0 ºC and water is stable between 0ºC and 100 ºC while water 
vapor is stable above 100 ºC. At normal atmospheric pressure, ice can first melts and 

ultimately boils as the temperature increases.

Checking up 10. 4 

1. Explain three ways that dry ice is different to the normal ice.
2. Explain why the liquid phase is not observed in the dry ice as it sublimes, 
    whereas all three phases are observed in the ice?
3. At temperature and pressure of 5ºC and 1atm (refer to both phase 
     diagram of  and  are normal ice and dry ice at the same phase? 
      Explain your reasoning. 
4. Draw and label a phase diagram for water and carbon dioxide and 
     explain why they are different? 

5. Explain the reason why a glass container breaks when water freezes.

 10.5. Applied aspect of phase diagrams

Activity 10.5

Engineers use diverse materials in construction of houses, bridges, etc. and in 
making different other products such as cars, airplanes, computers, etc. Explain 
if the knowledge of the phase diagrams of those materials the engineers use is 

important to them.

The applications of phase diagrams are useful for engineer’s materials and material 
applications. The scientists and engineers understand the behavior of a system 
which may contain more than one component. Multicomponent phase’s diagrams 

show the conditions for the formation of solutions and new compounds. 

The phase diagrams are applied in solidification and casting problems. Many materials 
and alloy system exist in more than one phase depending on the conditions of 
temperature, pressure and compositions. In the area of alloy development, phase 
diagrams have proved invaluable for tailoring existing alloys to avoid over design 
in current applications, each phase has different microstructure which is related 
to mechanical properties. The development of microstructure is related to the 
characteristics of phase diagrams. Proper knowledge and understanding of phase 
diagrams lead to the design and control of heating procedures for developing the 

required microstructure and properties.

Phase diagrams are consulted when materials are attacked by corrosion. They 
predict the temperature at which freezing or melting begins or ends. Phase 
diagrams differentiate the critical point, triple point, normal boiling point, etc of 

some substances.

Examples

• Zn-Fe based high-order phase diagrams have found a wide range of 
applications in continuous galvanizing. 
• The Zn-rich corner of the Zn-Fe-Al phase diagram is being used daily for 

scientific interpretation of bath assays.

In general the industrial applications of phase diagrams include alloy design, 

processing, and performance.

Checking up 10.5

Do research and explain different applications of phase diagrams.

10.6. End unit assessment

1. At pressures lower than triple point, water cannot exist as liquid, 
       regardless of the temperature.
      a. True                                    b) False

2. The melting point of water decreases as the pressure is augmented 
       because water contracts on freezing.
     a. True                                  b) False

3. The melting point of carbon dioxide increases as the pressure is raised 
     because carbon dioxide expands on freezing.
     a). True                                                  b). False

4. Use the following phase diagram of water to answer the questions 
     related:
a. At a pressure of 1atmosphere, what is the normal freezing point of 
    water?
b. What is the normal boiling point of water, at 1atmosphere of water?

c. In Karisimbi, we live approximately 5,500 feet above sea level, which 
     means the normal atmospheric pressure is less than 1atm. In Karisimbi, 
     will water freeze at a lower temperature or a higher temperature than 
    at 1atmosphere?
d. Will water boil at a higher or lower temperature, than at 1atmosphere?

5. If we shake a carbon dioxide fire extinguisher on a cool day , we 
    can hear liquid  sloshing around inside the cylinder. However, the 
    cylinder appears to contain no liquid on a sweltering day,      
    Explain these observations.

6. Observe the diagram below and answer the related question.

                     

a. Explain what is labeled in the parts X, Y, Z, C and T
b. Would the substance represented on this graph contract or expand when 
    it was frozen? Explain your answer.
c. Describe what will happen to Y if the temperature is increased at constant 
     pressure.
d. Explain what will happen to X if the pressure is much lowered at constant 
     temperature.

7. The diagram below shows the variation of vapor pressure with temperature 

    for pure substance.

                

a. What sections represent liquid, gas, solid phases?
b. What letter represents the triple point? Give the definition of the triple 
    point.
c. What is the substance‘s normal boiling and melting point?
d. Above which temperature it is not possible to liquefy the gas of the 
     substance, no matter how much pressure is applied?
e. At a constant temperature, what would you do to cause this substance 

    to change from the liquid phase to the solid phase?