Amagat's law or the law of partial volumes describes the behaviour and properties of mixtures of ideal (as well as some cases of non-ideal) gases.
Amagat's law states that the extensive volume V = Nv of a gas mixture is equal to the sum of volumes Vi of the K component gases, if the temperature T and the pressure p remain the same:[1][2]
This is the experimental expression of volume as an extensive quantity.
According to Amagat's law of partial volume, the total volume of a non-reacting mixture of gases at constant temperature and pressure should be equal to the sum of the individual partial volumes of the constituent gases.
are considered to be the partial volumes of components in the gaseous mixture, then the total volume V would be represented as Both Amagat's and Dalton's law predict the properties of gas mixtures.
Amagat's law assumes that the volumes of the component gases (again at the same temperature and pressure) are additive; the interactions of the different gases are the same as the average interactions of the components.
The interactions can be interpreted in terms of a second virial coefficient B(T) for the mixture.
where the subscripts refer to components 1 and 2, the Xi are the mole fractions, and the Bi are the second virial coefficients.
The cross term B1,2 of the mixture is given by and When the volumes of each component gas (same temperature and pressure) are very similar, then Amagat's law becomes mathematically equivalent to Vegard's law for solid mixtures.