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Vapor Pressures of Binary Solutions

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.

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Snapshot 1: an ideal solution
Snapshot 2: acetone-carbon disulfide solution, showing strong positive deviation from ideality
Snapshot 3: acetone-chloroform solution, showing negative deviation from ideality
Reference: P. Atkins and J. de Paula, Physical Chemistry 7th ed., New York: W. H. Freeman and Co., 2002, pp. 168–172.

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