1. Unit 61: Engineering Thermodynamics Lesson 4: Properties of Pure Substances

2. Objective The purpose of this lesson is to consider the relationships between pressure, specific volume and temperature for a pure substance.

3. The p - V - T Surface A pure substance is homogeneous i.e. it has the same chemical composition throughout in the various combinations of its three phases (solid, liquid and gas) Consider an experiment in which a solid is contained in a piston-cylinder arrangement such that the pressure is maintained at a constant value. Heat is applied to the cylinder causing the substance to pass through all three phases.

4. The p - V - T Surface SolidLiquidVapour

5. The p - V - T Surface T V Solid Liquid Vapour Saturated Vapour Saturated Liquid Boiling Melting

6. The p - V - T Surface If the experiment is repeated a number of times using different pressures a T – v diagram results

7. The p - V - T Surface solid S - L Liquid L - V Vapour S - V T v Critical Point P = constant

8. The p - V - T Surface A pressures that exceed the pressure of the critical point, there is no longer a distinction between a liquid and a vapour: the substance is referred to as a supercritical fluid. The data obtained in an actual experiment could be presented as a three-dimensional surface with P = P(v,T)

9. The p - V - T Surface

10. The P- v – T surface may be projected onto the P – v, T – v and the P – T plane