At constant pressure, the vapour of a dissociating substance has an anomalously high heat capacity
C
g
p
. The anomalous part is always positive and may be many times the ‘normal’ value for an otherwise similar, inert vapour. The
saturated
vapour of the same substance, however, with a heat capacity
C
g
sat
may show either a smaller anomaly in the same sense, or be entirely free from any anomaly, or show an anomaly in the opposite sense to, and even larger than, that in
C
g
p
. The factors governing the occurrence, the size and the sign of this anomaly are the degree of dissociation, and the relative magnitudes of the enthalpies of dissociation and vaporization. Expressions are derived for the heat capacities of saturated vapours for the general dissociation (
Am
^
mA
) and the particularly common case of dimerization (
A
2
^2
A
) and their application to real substances illustrated. The influence of gas imperfections is discussed. Except in the region of the critical point (where valid, explicit equations of state are not available) the existence of non-ideality modifies the conclusions only in degree. Like the heat capacity of saturated vapour, the latent heat of vaporization and its temperature coefficient also contain anomalous, reactive contributions. The influence of these anomalies on the form of the relationship between vapour pressure and temperature is examined. It is found that, despite the existence of such a mobile equilibrium, and even in the presence of an abnormally large heat capacity of saturated vapour, the saturation line (in the form of a graph of In
P
against
T
-1
) may show no marked curvature. Conversely, linearity of a graph of In
P
against
T
-1
is no guarantee that reactions of dimerization or polymerization are absent from a vapour phase.
Raoult’s law revisited: accurately predicting equilibrium relative humidity points for humidity control experiments
