However, the vapor above the solution may not actually behave as a mixture of ideal gases. Some authors therefore define an ideal solution as one for which each component obeys the fugacity analogue of Raoult's law f i = x i f i ∗ ∗ . Here f i is the fugacity of component i in solution and f i ∗ ∗ is the fugacity of i as a pure substance. Since the fugacity is defined by the equation