Phase Diagram of Single Component System (Water)

Definition

A phase diagram of a single component system is a plot showing the equilibrium relationships between different phases of one pure substance at various combinations of pressure and temperature.

For water, the phase diagram indicates the regions where solid ice, liquid water, and water vapour are stable, along with the lines of equilibrium separating them. It also includes the triple point, where all three phases coexist in equilibrium, and the critical point, beyond which liquid and vapour become indistinguishable.

Main Content

1. Phase Rule and Degree of Freedom in Water System

• The phase diagram of water is based on Gibbs phase rule, which is given by:
  F = C - P + 2
  where F = degrees of freedom, C = number of components, and P = number of phases.

• Since water is a single-component system, C = 1, so the phase rule becomes:
  F = 3 - P

This means:

• If one phase is present, F = 2, so both pressure and temperature can be varied independently.
• If two phases coexist, F = 1, meaning only one variable can be changed freely while the other is fixed.
• If three phases coexist, F = 0, meaning the system is invariant and neither temperature nor pressure can be changed without disturbing equilibrium.

This explains why the triple point is a fixed point and why each equilibrium line on the diagram represents one degree of freedom.

2. Equilibrium Curves in the Water Phase Diagram

• The water phase diagram contains three main equilibrium curves:
• Fusion curve: equilibrium between solid and liquid phases
• Vapourization curve: equilibrium between liquid and vapour phases
• Sublimation curve: equilibrium between solid and vapour phases
• These curves divide the diagram into three phase regions:
• Solid region: ice is stable at low temperature
• Liquid region: water is stable at moderate temperature and suitable pressure
• Vapour region: water vapour is stable at high temperature and/or low pressure

Important characteristics of these curves:

• The fusion curve of water slopes negatively, unlike most substances. This means that increasing pressure lowers the melting point of ice.
• The vapourization curve ends at the critical point.
• The sublimation curve shows direct transition from solid to vapour without becoming liquid, which occurs at low pressure.

The negative slope of the fusion curve is due to the fact that ice is less dense than liquid water. When pressure increases, the system favours the phase with smaller volume, i.e., liquid water.

3. Triple Point, Critical Point, and Anomalous Behaviour of Water

• The triple point of water is the unique temperature and pressure at which ice, water, and water vapour coexist in equilibrium.
• Temperature: 0.01°C
• Pressure: 4.58 mm Hg (approximately 611.7 Pa)
• At the triple point:
• all three phases are simultaneously stable
• the system is invariant
• even a slight change in temperature or pressure causes one phase to disappear
• The critical point is the upper limit of the liquid–vapour equilibrium curve.
• At temperatures and pressures above the critical point, water exists as a supercritical fluid
• For water, the critical temperature is about 374°C and critical pressure is about 218 atm

Anomalous behaviour of water:

• Ice occupies more volume than liquid water, so it is less dense
• This unusual density difference causes the fusion curve to slope backward
• Due to this property, ice floats on water, which is vital for aquatic life in cold climates

These special points make water distinct from most other substances and highly important in thermodynamics.

Working / Process

1. Establish the pressure and temperature conditions

Determine the external pressure and temperature acting on the water system. These variables decide which phase will be stable.

2. Locate the point on the phase diagram

Using the values of pressure and temperature, identify the corresponding region or line on the phase diagram:

• solid region for ice
• liquid region for water
• vapour region for steam
• boundary line for phase equilibrium

3. Interpret phase changes

• If the point lies on a boundary line, two phases coexist in equilibrium.
• If it lies at the triple point, all three phases coexist.
• If it lies beyond the critical point, liquid and vapour cannot be distinguished.
• Movement across the diagram indicates phase transitions such as melting, freezing, boiling, condensation, sublimation, or deposition.

Advantages / Applications

Helps understand phase transitions of water

• such as melting, freezing, boiling, condensation, sublimation, and deposition under different conditions.

Useful in scientific and industrial applications

• including refrigeration, distillation, steam generation, and high-pressure processing.

Explains natural phenomena and environmental behaviour

• such as ice floating on water, formation of frost, cloud processes, glacier behaviour, and survival of aquatic organisms in winter.

Summary

The phase diagram of water shows the stability regions of ice, liquid water, and vapour under different pressure and temperature conditions. It is a very important single-component system because of its unique negative fusion curve and well-defined triple and critical points. The diagram provides a clear understanding of phase equilibrium and the conditions under which water changes state.