Avoga

Meet Avoga, a pair of balloons who show that more gas needs more room.

In 1811 Amedeo Avogadro proposed that equal volumes of different gases, at the same temperature and pressure, contain the same number of molecules. It follows that the volume of a gas is proportional to the amount of gas n, so V₁/n₁ = V₂/n₂ when the temperature and pressure stay fixed.

At 0 °C and normal atmospheric pressure, one mole of an ideal gas takes up about 22.4 litres, whether it is helium, oxygen or nitrogen. One mole is about 6.02 × 10²³ molecules. Put in two moles and the balloon swells to about 44.8 litres.

Real gases follow the law closely at ordinary pressures, but drift away from it when they are squeezed hard or cooled close to turning into liquids.

Avogadro's idea helped chemists count molecules they could not see and is built into the ideal gas law, pV = nRT.