CHEMISTRY · GASES

The Gas Laws: Boyle, Charles and Gay-Lussac

Squeeze a syringe and the gas pressure rises; put a balloon in the fridge and it shrinks. Hold one quantity fixed, change another, and the links between a gas’s pressure, volume and temperature appear.

Further reading: Gas laws (Wikipedia)

🔗 Explore more: Gas MoleculesChoose “1 Gas, Random Speeds” and change volume and temperature | Designed by Paul Falstad | Internet required

Pressure P
Volume V
Temperature

The container holds 1 mol of ideal gas: 22.4 L at 0 °C and 1 atm. Molecule speeds follow the temperature qualitatively; their number is not the real count. Real gases depart from ideal behaviour at low temperature or high pressure, and may even liquefy.

How to Use

  1. Choose a law to explore: Boyle (constant temperature), Charles (constant pressure) or Gay-Lussac (constant volume).
  2. Boyle: drag the volume slider to move the piston and watch the pressure; each point is plotted and they trace a hyperbola.
  3. Charles: keep the outside pressure fixed and change the temperature; the piston moves out or in and the V–T graph is a straight line.
  4. Gay-Lussac: the piston is held fixed; change the temperature and watch the gauge.
  5. Check P×V, V/T or P/T in the panel: each stays constant under its own condition.

About This Lesson

Gas pressure comes from molecules constantly striking the walls. In 1662 Boyle found that at constant temperature the pressure of a fixed amount of gas is inversely proportional to its volume: PV is constant. Squeeze the gas into less room and the molecules hit the walls more often, so the pressure rises.

Charles and Gay-Lussac studied temperature. At constant pressure a gas’s volume changes linearly with temperature; extend the line and the volume reaches zero near −273 °C. That is absolute zero, which defines the absolute scale T(K) = t(°C) + 273. In absolute temperature V ∝ T, and at constant volume P ∝ T.

Combining these with Avogadro’s law gives the ideal gas equation PV = nRT. Bicycle pumps, hot-air balloons, pressure cookers and tyres that burst on hot days can all be explained with these laws.