Chemistry

UNIT 10. PHASE DIAGRAMS

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

10.1. Phase equilibrium

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.

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.

In homogeneous equilibrium, all substances are in the same phase while in

heterogeneous equilibrium, substances are in distinct phases.

10.2. Homogeneous and heterogeneous equilibria

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).

10.3. Phase diagrams

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

CO2 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 CO2.

10.4. Comparison of phase diagrams of substances that expand and those that contract on freezing

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.

10.5. Applied aspect of phase diagrams

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.

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

processing, and performance.