text: An ideal solution would follow Raoult's law, but most solutions deviate from ideality. Interactions between gas molecules are typically quite small, especially if the vapor pressures are low. However, the interactions in a liquid are very strong. For a solution to be ideal, the interactions between unlike molecules must be of the same magnitude as those between like molecules. This approximation is only true when the different species are almost chemically identical. One can see that from considering the Gibbs free energy change of mixing:
  This is always negative, so mixing is spontaneous. However, the expression is, apart from a factor , equal to the entropy of mixing. This leaves no room at all for an enthalpy effect and implies that  must be equal to zero, and this can only be true if the interactions between the molecules are indifferent.
  It can be shown using the Gibbs–Duhem equation that if Raoult's law holds over the entire concentration range  in a binary solution then, for the second component, the same must also hold.
  If deviations from the ideal are not too large, Raoults law is still valid in a narrow concentration range when approaching  for the majority phase (the solvent). The solute also shows a linear limiting law, but with a different coefficient. This relationship is known as Henrys law.
  The presence of these limited linear regimes has been experimentally verified in a great number of cases, though large deviations occur in a variety of cases.  Consequently, both its pedagogical value and utility have been questioned at the introductory college level. In a perfectly ideal system, where ideal liquid and ideal vapor are assumed, a very useful equation emerges if Raoults law is combined with Daltons Law:
  where  is the mole fraction of component  in the solution, and  is its mole fraction in the gas phase. This equation shows that, for an ideal solution where each pure component has a different vapor pressure, the gas phase is enriched in the component with the higher vapor pressure when pure, and the solution is enriched in the component with the lower pure vapor pressure. This phenomenon is the basis for distillation.