In a liquid mixture, the fugacity of each component is equal to that of a vapor component in equilibrium with the liquid. In an ideal solution, the fugacities obey the Lewis-Randall rule:
where  is the mole fraction in the liquid and  is the fugacity of the pure liquid phase. This is a good approximation when the component molecules have similar size, shape and polarity.
In a dilute solution with two components, the component with the larger molar fraction (the solvent) may still obey Raoults law even if the other component (the solute) has different properties. That is because its molecules experience essentially the same environment that they do in the absence of the solute. By contrast, each solute molecule is surrounded by solvent molecules, so it obeys a different law known as Henrys law. By Henrys law, the fugacity of the solute is proportional to its concentration. The constant of proportionality (a measured Henrys constant) depends on whether the concentration is represented by the mole fraction, molality or molarity.