Phase Change of a Pure Substance

A pure substance is a substance that has a fixed chemical composition throughout. It can exist in more than one phase, but the chemical composition must be the same in all phases. An example of this is ice-liquid water mixture and liquid water-steam mixture.

Scenario: Let us consider a system consisting 1 Kg of liquid water in a piston cylinder apparatus as shown below. Assume that the ambient pressure and the piston weight maintain the cylinders pressure at 0.125MPa with the initial temperature at 25°C.

At the initial conditions (P = 0.125MPa, T = 25°C), water is a called subcooled liquid i.e., it will not vaporize if heat is transferred to the system. 

When heat is transferred to the water, its temperature increases significantly and the specific volume increases slightly while pressure remains constant. 

When T = 105.99°C, the additional heat transfer results in the water boiling and a phase change occurs. A liquid that is about to boil is referred to as a saturated liquid. 

When the liquid is vaporizing, its pressure and temperature remain constant but its specific volume increases. At this point it is called a saturated liquid-vapour mixture where both liquid and vapour phases coexist in equilibrium. 

When all liquid vapourizes, only vapour exists in the cylinder and is called saturated vapour. Any heat loss from a saturated vapour leads to condensation. 

An increase in heat into a saturated vapour leads to an increase in both temperature and specific volume and its called a superheated vapour.

  

This whole water heating process at a constant pressure can be represented on a T - Ṽ diagram as shown below:

• State 1 is the initial subcooled liquid state

• State 2 is the saturated liquid state. A saturated liquid is ready to boil with the addition of heat, and T = 105.99°C represents the boiling point temperature.

• State 4 represents a saturated vapor. A saturated vapor is ready to condense with the removal of heat, and T = 105.99°C represents the dew point temperature.

• The horizontal line joining states 2 and 4 represents an isobaric and isothermal process where phase change from liquid to vapor, or vice versa, occurs. During phase change the liquid and vapor phases are in equilibrium with each other and, as a result, both temperature and pressure remain constant.

• The line joining the states 4 and 5 represents the process in which the steam is superheated at constant pressure.

At any given pressure, the temperature at which a pure substance boils is referred to as the saturation temperature (T^sat). While at any given temperature, the pressure at which a pure substance boils is referred to as the vapour or saturation pressure (P^vap). For a pure substance, there is a definite relation between the vapour pressure and the saturation temperature which results in a vapour pressure curve shown below. The boiling of a pure component starts when the P^vap = Ambient Pressure. This explains why water boils at a temperature less than 100°C at a mountain top.