Pure substance and state postulate

Pure substance

A pure substance is one with a homogeneous and invariable chemical composition throughout its mass, whatever the phase. Examples: water (liquid, steam or ice-water mixture), nitrogen, air in a single gas phase.

State postulate

The state of a simple compressible system (a system with no electrical, magnetic, gravitational, motion or surface-tension effects) is completely specified by two independent intensive properties.

In general, the number of independent intensive properties needed equals the number of relevant quasi-equilibrium work modes plus one (the one is for the heat-transfer mode). A simple compressible system has only the P dV work mode, so 1 + 1 = 2 properties are needed.

Explanation

• Properties are independent if one can be varied while the other is held fixed. For example, in a single-phase system, T and v are independent, so (T, v) fixes P, u and h.
• In the two-phase region, T and P are not independent: P = P_sat(T). So a third property, such as quality x or v, is needed together with T (or P) to fix the state.
• For an ideal gas, P v = R T gives the third property from any two.

Example: for steam at 1 MPa, specifying T = 300 °C fixes v, u, h and s from the tables.