The gases are mixable with each other in all proportions. Since the ideal gas law

is valid for any ideal gas, one may think that it’s insignificant whether the mole number n concerns one single gas or several gases. It is true, which can be shown experimentally.

Let’s think that we mix the volumes V ₁, V ₂, ..., V _k of different gases having an equal pressure p and an equal temperature T. If one measures the volume V of the mixture in the same pressure and temperature, one notices that

Each of the gases satisfies an equation pV _i = n_iRT, and thus

This is similar as the general equation (1). If we think that the same volume V would be filled by any of the gases alone, we had an equation

for each gas; here the pressure p_i, i.e. n_i, is called the partial pressure of the gas i. By (2), we have

Accordingly we have obtained the

Dalton’s law. The pressure of a gas mixture is equal to the sum of the partial pressures of the component gases.

This law was invented by J. Dalton in 1801.