Solvent–solute interactions are the same as solute–solute and solvent–solvent interactions, on average. Consequently, the enthalpy of mixing (solution) is zero and the change in Gibbs free energy on mixing is determined solely by the entropy of mixing. Hence the molar Gibbs free energy of mixing is
or for a two-component ideal solution 
where m denotes molar, i.e., change in Gibbs free energy per mole of solution, and  is the mole fraction of component . Note that this free energy of mixing is always negative (since each , each  or its limit for  must be negative (infinite)), i.e., ideal solutions are miscible at any composition and no phase separation will occur.
The equation above can be expressed in terms of chemical potentials of the individual components
where  is the change in chemical potential of  on mixing. If the chemical potential of pure liquid  is denoted , then the chemical potential of  in an ideal solution is
Any component  of an ideal solution obeys Raoult's Law over the entire composition range:
where  is the equilibrium vapor pressure of pure component  and is the mole fraction of component  in solution.