It is a solution where the molecules of the different species are so similar to each other that the molecules of one component of the solution can replace molecules of another component without changing the spatial structure or the energy of the intermolecular interactions of the solution.

A solution of two components A and B is ideal if the interactions between molecules AA, AB and BB are equal. This situation occurs approximately in isotopic species. $^{12}CH_3Cl$ and $^{13}CH_3Cl$.

This model allows us to approach the study of real solutions. The deviations from the ideal behavior are due to different intermolecular interactions for AA, AB and BB and to differences in size and shape between the A and B molecules.