Phase equilibrium

A phase is a homogenous physical state into which a substance can exist; for example, solid, liquid and vapor/gas

Factors affecting the phase of a substance

These include temperature and pressure. By varying temperature and pressure, it is possible to convert solid into a liquid or gas and vice versa or a liquid into a gas or a gas into a liquid.

A plot of vapor pressure against the temperature of a substance in the liquid, gas or solid-state yield graphs called phase diagrams.

There are three forms of phase diagrams:

(a) Form A is shown by a substance such as water which is accompanied with a decrease in volume when they melt or accompanied with increase in volume when they solidify.

(b) Form B is shown by a substance such as benzene that is accompanied with an increase in volume when they melt

(c) Phase diagram form C is shown by a substance such as sulphur that exists in different solid form (allotropy)

At point O, all the three phases are in equilibrium and is called triple point. I.e. triple point is a point of temperature and pressure at which all three phases (solid, liquid, and vapor) are in equilibrium. The temperature and pressure corresponding to the triple point are triple point temperature and triple point pressure respectively.

The triple point temperature Tt⁰C and pressure (Pt) for water and carbon dioxide are given below

Substances                                          Tt⁰C                        Pt

Water                                               0.0098                   0.0060

Carbon dioxide                                 -56.1                      5.0

With the further rise in temperature the curve OA is obtained and this represents the conditions of temperature and pressure under which the liquid and vapor are in equilibrium.

This curve ceases at A and beyond this point the liquid and vapor are indistinguishable. This point is called the critical point and the corresponding temperature and pressure are called critical temperature (Tc⁰C) and critical pressure (Pc) respectively.

Example                               Tc⁰C       Pc

Water                                   374         218

CO₂                                        37           72.8

OB represents conditions of temperature and pressure at which solid and vapor are in equilibrium.

OC represents conditions of temperature and pressure at which liquid and solid are in equilibrium

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Phase equilibrium

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Compiled by Dr. Bbosa Science