Idealia

Meet Idealia, the ideal gas, who links pressure, volume and temperature in one tidy rule.

The ideal gas law, PV = nRT, joins several older findings: Boyle's law (at fixed temperature, pressure times volume is constant), Charles's law (at fixed pressure, volume grows with temperature) and Avogadro's insight that equal volumes of gas at the same conditions hold equal numbers of molecules. It treats a gas as tiny particles that only bounce off each other and the walls.

Example: one mole of gas at 0 °C (273.15 K) and one atmosphere (101,325 Pa) fills about 22.4 litres. Halve the volume at constant temperature and the pressure doubles; heat it from 300 K to 600 K at fixed volume and the pressure doubles too.

The law is used constantly: tyre pressures, weather balloons, breathing and engines all follow it closely.

Real gases deviate at high pressure and low temperature, where molecules are close together and attract each other. Near condensation the ideal law fails, and models such as the van der Waals equation do better.