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Real gases
Intermediate
Molecular volume and attraction make real gases deviate from ideal.
Molecules have finite volume and attract each other, so real gases deviate from PV = nRT at high pressure and low temperature, and can liquefy.
CO₂ liquefies at 5 atm and 20 °C.
Attraction lowers the pressure; volume raises it.
In more detail
Real gases differ from ideal gases because molecules have finite size and attract each other. These effects become important at high pressure and low temperature.
Real-world uses
- Industrial processes involving high-pressure gases require real-gas equations.
- Liquefaction of gases depends on intermolecular attractions.
- Engineers designing high-pressure gas cylinders must use real gas equations, since the ideal model is inaccurate at high pressure.
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Frequently asked questions
When is a gas close to ideal?
At low pressure and high temperature, where molecules are far apart.
Why do real gases liquefy?
Attractions between molecules can dominate at low temperature, pulling them together.
What makes a real gas non-ideal?
Molecules have finite volume and attract each other, which changes the pressure and volume compared with the ideal model.
Related topics in Kinetic Theory & Statistical Physics
Diffusion · Fick's law · Graham's law of effusion · Van der Waals equation · Compressibility factor · Liquefaction of gases