An ideal solution of two liquids

and

obeys Raoult's law, which states that the partial vapor pressure of each component is proportional to its mole fraction:

and

, where

and

are the vapor pressures of the pure components at a given temperature (very often 25 °C). The total vapor pressure above the solution is then given by

, assuming Dalton's law. Ideal solutions are fairly uncommon but serve as a convenient reference system to describe nonideal solutions. Pairs of liquids that are well approximated by Raoult's law usually contain molecules of similar size, shape, and chemical structure. Some well-known examples are benzene and toluene, chlorobenzene and bromobenzene, and carbon tetrachloride and silicon tetrachloride.

Most real solutions exhibit deviations from Raoult's law. A positive deviation is characterized by

and

and indicates that the attractive interactions between like molecules is greater than that between

and

molecules. A negative deviation has

and

, implying stronger mutual interactions between unlike molecules. The curves shown in the graphic are qualitative approximations to the actual dependence of vapor pressures on composition. The blue and red curves represent the partial pressures of

and

, respectively, while the black curve shows the total vapor pressure. The dashed lines refer to the hypothetical ideal behavior of the corresponding vapor pressures.

Even for nonideal solutions, Raoult's law is asymptotically approached for

or

. Dilute solutions, on the other hand, are approximated by Henry's law: the linear relations

for

and

for

, which can be displayed using the "show Henry's law" checkbox.