3
Properties of Pure Substances
3.1. Definition of the pure substance. 3.2. Phase change of a pure substance. 3.3. p-T (pressure-
temperature) diagram for a pure substance. 3.4. p-V-T (pressure-volume-temperature) surface.
3.5. Phase change terminology and definitions. 3.6. Property diagrams in common use. 3.7. Forma-
tion of steam. 3.8. Important terms relating steam formation. 3.9. Thermodynamic properties of
steam and steam tables. 3.10. External work done during evaporation. 3.11. Internal latent heat.
3.12. Internal energy of steam. 3.13. Entropy of water. 3.14. Entropy of evaporation. 3.15. Entropy
of wet steam. 3.16. Entropy of superheated steam. 3.17. Enthalpy-entropy (h-s) chart or Mollier
diagram. 3.18. Determination of dryness fraction of steam—Tank or bucket calorimeter—throttling
calorimeter—separating and throttling calorimeter—Highlights—Objective Type Questions—
Theoretical Questions—Unsolved Examples.3.1. Definition of the Pure Substance
A pure substance is a system which is (i) homogeneous in composition, (ii) homogeneous in
chemical aggregation, and (iii) invariable in chemical aggregation.
—“Homogeneous in composition” means that the composition of each part of the system is
the same as the composition of every other part. “Composition means the relative pro-
portions of the chemical elements into which the sample can be analysed. It does not
matter how these elements are combined.
For example in Fig. 3.1 system (a), comprising steam and water, is homogeneous in compo-
sition, since chemical analysis would reveal that hydrogen and oxygen atoms are presents in the
ratio 2 : 1 whether the sample be taken from the steam or from the water. The same is true of
system (b) containing uncombined hydrogen and oxygen gas in the atomic ratio 2 : 1 in the upper
part, and water in the lower part. System (c) however, is not homogeneous in composition, for the
hydrogen and oxygen are present in the ratio 1 : 1 in the upper part, but in the ratio 2 : 1 (as water)
in the lower part.
Steam H+ O2 2^12 H+O(Gas)^22(a) (b) (c)
Satisfies condition (i)
Satisfies condition (ii)Satisfies condition (i)
Does not satisfy
condition (ii)Does not satisfy
condition (i)Fig. 3.1. Illustrating the definition of a pure substance.63