In chemistry, an ideal solution or ideal mixture is a solution with thermodynamic properties analogous to those of a mixture of ideal gases. The enthalpy of mixing is zero as is the volume change on mixing; the closer to zero the enthalpy of solution is, the more "ideal" the behavior of the solution becomes. The vapor pressure of the solution obeys Raoult's law, and the activity coefficient of each component (which measures deviation from ideality) is equal to one.