Different related definitions of an ideal solution have been proposed. The simplest definition is that an ideal solution is a solution for which each component obeys Raoult's law p i = x i p i ∗ ∗ for all compositions. Here p i is the vapor pressure of component i above the solution, x i is its mole fraction and p i ∗ ∗ is the vapor pressure of the pure substance i at the same temperature.