Real Gas Definition and Examples

Real Gas vs Ideal Gas

A real gas does not behave as an ideal gas because the gas molecules interact with each other.
A real gas does not behave as an ideal gas because the gas molecules interact with each other. INDIGO MOLECULAR IMAGES, Getty Images

A real gas is a gas that does not behave as an ideal gas due to interactions between gas molecules. A real gas is also known as a nonideal gas because the behavior of a real gas in only approximated by the ideal gas law.

When Real Gases Differ From Ideal Gases

Usually, it's fine to use the ideal gas law to make calculations for gases. However, the approximation gives considerable error at very high pressure, near the critical point, or near the condensation point of a gas. Unlike ideal gases, a real gas is subject to:

  • Van der Waals forces;
  • Compressibility effects;
  • Non-equilibrium thermodynamic effects;
  • Variable specific heat capacity; and
  • Variable composition, including molecular dissociation and other chemical reactions.

Real Gas Example

While cool air at ordinary pressure behaves like an ideal gas, increasing its pressure or temperature increases the interactions between molecules, resulting in real gas behavior that cannot be predicted reliably using the ideal gas law.

Sources

  • Cengel, Yunus A. and Michael A. Boles (2010). Thermodynamics: An Engineering Approach (7th Ed.). McGraw-Hill. ISBN 007-352932-X.
  • Xiang, H. W. (2005). The Corresponding-States Principle and its Practice: Thermodynamic, Transport and Surface Properties of Fluids. Elsevier. ISBN 978-0-08-045904-2.