An (imaginary) gas which obeys the gas laws perfectly for all temperatures and pressures is called an ideal (or perfect) gas.

In order for a gas to be considered ideal

1. there must be negligible forces of attraction between its molecules
    

2. the total volume of its molecules must be negligible compared with the volume occupied by the gas.
    

Real Gases

Real gases near s.t.p. (760mmHg and 0°C) behave like an ideal gas.

Real gas molecules attract each other and do not occupy negligible volume when the gas is at high pressure. If we decrease the temperature and increase the pressure of a real gas it will eventually change its state. At this stage the gas laws no longer apply (obvious really…since you don’t have a gas any more!). Some melting and boiling points of elements which are gaseous at room temperature are shown in the table below.

These temperatures are for normal atmospheric pressure.

• The Equation of State of an Ideal Gas

• Thermal Physics Chapters Index
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