Ideal gas law
For an ideal gas, pressure, volume, temperature and the amount of substance are linked by a single equation: pV equals nRT.
Practise this conceptThe ideal gas law ties four properties of a gas together into one equation: pressure times volume equals the amount of substance times a constant times the absolute temperature. Change any one of them, and at least one of the others has to respond.
Because it's one equation linking four quantities, holding one of them fixed turns it into a familiar, simpler relationship. Keep temperature constant and it becomes Boyle's law — pressure and volume trade off against each other. Keep pressure constant and it becomes Charles's law — volume and temperature move together. The ideal gas law is really the parent of all these special cases.
Temperature here must be measured on an absolute scale — kelvin, not Celsius — because the law is really about the average kinetic energy of the gas particles, and that energy is naturally zero at absolute zero, not at the arbitrary zero point of the Celsius scale.
Key ideas
Requires: Kinetic theory of gases
Unlocks: The laws of thermodynamics
Formulas
| Symbol | Name | Unit |
|---|---|---|
| pressure | Pa (pascal) | |
| volume | m^3 (cubic metre) | |
| amount of substance | mol (mole) | |
| temperature | K (kelvin) |
Common misconceptions
- If the pressure of a gas increases at constant temperature, its volume must also increase.
- The ideal gas law only applies to gases at very low temperatures.
- Heating a gas at constant volume decreases its pressure.