Ideal Solutions

An ideal solution is one in which every component -- both the solute and the solvent -- obeys Raoult's law across the entire range of concentration, not just when dilute. In strict practice, no real solution is perfectly ideal over its whole composition range; but when the solute concentration is very low, a dilute solution's behaviour approaches ideal closely enough to use the ideal-solution equations. Two components A and B tend to form a solution close to ideal when they are similar in molecular size and structure, and when the intermolecular attractive forces between A-A, B-B, and A-B are all roughly equal in strength.

Three defining conditions of an ideal solution:

1. No volume change on mixing: $\Delta V_{mixing} = 0$ -- the volume of the mixture equals the sum of the separate volumes of solute and solvent.
2. No heat exchange on mixing: $\Delta H_{mixing} = 0$ -- mixing is neither exothermic nor endothermic.
3. Unchanged escaping tendency: each component's tendency to escape into the vapour phase, when present in the mixture, is the same as its escaping tendency in the pure liquid. …