Pressure and temperature changes lead to large changes in gas volumes as compared with either liquids or solids.

All gases that exist in our physical world are real gases. We refer to them for convenience as real gases strictly to distinguish that no assumptions have been made concerning their behavior. Real gas behavior is more complex than is "ideal" gas behavior.

Applying the ideal gas assumptions simplifies the gas's behavior and is appropriate for most conditions. The characteristics of real and ideal gases are contrasted in the table below [local].

Ideal gases are assumed to follow the gas laws at all conditions of pressure and temperature. (This can't be so; all known substances can form a condensed phase.)

Ideal gases are good approximations for real gases at typical temperatures and pressures. The ideal gas model exists to simplify the description of real gas behavior. Ideal assumptions fail at low temperatures and high pressures.

There are several so-called non-ideal gas equations (equations of state). The van der Waals equation adds two compensating variables to the ideal gas equation to account for real gas characteristics at the extreme.

Remember, ideal gases do not really exist. The assumptions are quantifiable and under certain situations.