The basic concepts of classical thermodynamics can be summarized by invoking the following four postulates (Callen, 1985):

1. There exist particular states (called equilibrium states) of simple compressible systems that, macroscopically, are characterized completely by the internal energy, U, the volume, V, and the mole or particle numbers, N ₁, N ₂ , , N _k, of the chemical components.

2. There exists a function called the entropy, S, of the extensive parameters ( U, V, N _i) of any composite system, defined for all equilibrium states and having the following property: The values assumed by the extensive parameters in the absence of an internal constraint are those which maximize the entropy for the composite isolated system.

3. The entropy of a composite system is additive over the constituent subsystems. Moreover, the entropy is a continuous, differentiable, and monotonically increasing function of the internal energy.

4. The entropy of any system vanishes in the state for which ( ?U/ ?S) _V,N = 0 (i.e., at the zero of temperature).

Recall that a simple compressible system is defined as one that is macroscopically homogeneous, uncharged, and chemically inert, that is sufficiently large that surface effects can be neglected, and that is not acted on by electric, magnetic, or gravitational fields. Although these four basic postulates are restricted to simple compressible systems, they can readily be extended to more complex systems (Lewis and Randall, 1961).

The first and...