
(steam, water, ice) P ¼ 1, and C ¼ 1, Eqn (2.31) gives D ¼ 2. Th is means that it is necessary to have
two independent properties, e.g. p and T, to define the state of the substance.
If the substance is changing phase, i.e. from water to steam, on one of the boundary lines then
P ¼ 2, and D ¼ 1: hence, the state of the substance can be defined by a single property during the
phase change because p and T are not independent properties in this region. Another characteristic of
phase change which comes from Gibbs energy (or, more correctly, chemical potential ) is that the Gibbs
energy of both phases is equal during the phase change process. This means ...