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  for all compositions. Here  is the vapor pressure of component  above the solution,  is its mole fraction and  is the vapor pressure of the pure substance  at the same temperature.
This definition depends on vapor pressure, which is a directly measurable property, at least for volatile components. The thermodynamic properties may then be obtained from the chemical potential μ (which is the partial molar Gibbs energy g) of each component. If the vapor is an ideal gas,
The reference pressure  may be taken as  = 1 bar, or as the pressure of the mix, whichever is simpler.
On substituting the value of  from Raoult's law, 
This equation for the chemical potential can be used as an alternate definition for an ideal solution.
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  . Here  is the fugacity of component  in solution and  is the fugacity of  as a pure substance. Since the fugacity is defined by the equation 
this definition leads to ideal values of the chemical potential and other thermodynamic properties even when the component vapors above the solution are not ideal gases. An equivalent statement uses thermodynamic activity instead of fugacity.