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 solution (or "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 vapour pressure of the solution obeys Raoult''s law, and the activity coefficients (which measure deviation from ideality) are equal to one.
The concept of an ideal solution is fundamental to chemical thermodynamics and its applications, such as the use of colligative properties.