Thermodynamic Properties of Pure Substance

L# 1

Pure substance refer to: substance with chemicals homogeneity and constant chemicals composition.

H2O is pure substance as it meets both the above requirements. Any substance which under goes a chemical reaction,can not be pure substance.

Some important definitions:-

Sensible heating:-

Heating of substance in single phase. It can rise in temperature of substance,’’In case of cooling in above condition it shell be called sensible cooling’’

Latent heating:-

It is the heating of substance for causing its phase change without any change in its temperature .it will be called latent heat.

Normal boiling point:-

It is the temperature at which vapour pressure equals to atmospheric pressure. And phase change from liquid to gas begins.

Melting point:-

It is the temperature at which phase change from solid to liquid takes place upon supplying latent heat.

Saturation state:-

The state at which its phase transformation takes place without any change in pressure and temperature. These can be saturated solid state,liquid state,vapour state.

Saturation pressure:-

It is pressure at at which substance change its phase for any given temperature. Such as at any given temperature water shell get convert into steam at a definite pressure the pressure is called saturation pressure corresponding to given temperature. For water at 1000 C the saturation pressure is 1atm pressure.

Saturation temperature:-

The temperature at which substance change its phase for any given pressure. For water at 1 atm pressure the saturation temperature 1000C

Triple point:-.

The state at which substance can coexist in solid,liquid and gaseous phase in equilibrium.for water is 0.01oC I.e at this temperature ice water and steam can coexist in equilibrium  

Critical state:-

It that state of substance at which liquid and vapour coexist in equilibrium.

Dryness fraction:-

It is mass of vapour in mixture of liquid and vapour at any point in liquid -vapour mixture region

It generally denoted by  ‘x’


Dryness friction = x

= mass of vapour/(mass of liquid + mass of vapour)


                     X= mv / (ml + mv)   

Compressed liquid or Sub cooled liquid:-

Liquid temperature less than saturation temperature corresponding to given pressure.

‘Degree of sub cooling in given temperature difference b/w liquid temperature and saturation temp. Of liquid at given pressure'


Degree of sub cooling = saturation temp. At given pressure - temp.of liquid.

Super heated steam:-

Steam having more than the saturation temp. Corresponding to given pressure

Degree of super heat = temp. Of steam - saturation temp. At given pressure.

P-V Diagram for water:-


p-v diagram for water

Saturated liquid line meets saturated Vapour line is called critical point.

p-T Diagram for water:-

p-T diagram for water

Steam table and Mollier chart:-

Steam table is a tabular presentation of properties such as specific enthalpy ,entropy,internal energy, and specific volume at different saturation pressure or temperature. Steam table may be on pressure basis and on temperature basis.

 1. Pressure basis:-
pressure basis steam table

2. Temperature basis:-
temperature basis steam table

 Mollier chart:-

It is the enthalpy-entropy (h-s) diagrams of steam .this diagrams is obtained on the basis of following eqn depending upon the phase transformation.


Tds = dh - vdp  (1st and 2nd law combination)  


For constant pressure (dh / ds)p = T


h-s diagram for all phases:-1

h-s diagram
h-s diagram:-2