The Velocity of Gases and Graham’s Law of Effusion

The word ‘effusion’ comes from the latin for ‘pour out’ and, in a modern scientific context, is defined as the process by which a gas passes through a hole or orifice smaller than the mean free path of the molecules.

In 1848, Scottish chemist Thomas Graham formulated what he labelled Graham’s law of effusion. The law states that the rate of effusion of a gas is inversely proportional to the square root of the mass of its particles. Therefore, gases with a lower molecular weight effuse faster than those with a higher molecular weight.

Lee is a freelance writer with a keen interest in history and science.