Ideal and real gases

Pure Substance

A pure substance is one that has a homogeneous and invariable chemical composition. It may exist in more than one phase but chemical composition is the same in all phases.

Some times the mixture of gases, such as air is considered a pure substance as long as there is no change of phase. Further our emphasis will be on simple compressible substances

Early experiments on the variables of state (such as T, P, V, and n) showed that only two of these variables of state need to be known to know the state of a sample of matter.

1. Extensive variables:

depend on the amount of substance present. Examples include the volume, energy, enthalpy, and heat capacity.

2. Intensive variables:

do not depend on the amount of substance present. Examples include the temperature and pressure.