Ideal Solutions

A solution that obeys Raoult's law over the whole range of concentration — from pure component A right through to pure component B — is called an ideal solution. Alongside this defining feature, an ideal solution shows two further characteristic properties when it is formed by mixing its pure components:

What these two conditions mean physically

• $\Delta_{mix}H = 0$ — no heat is absorbed and no heat is released when the two liquids are combined. Forming the solution is thermally "silent".
• $\Delta_{mix}V = 0$ — the volume of the solution is exactly the sum of the volumes of the two liquids taken separately. Nothing shrinks or expands on mixing.

The molecular reason

Consider two components A and B. In each pure liquid there are attractive interactions of only one type — A–A forces in pure A, and B–B forces in pure B. When the liquids are mixed, a new kind of interaction appears: A–B forces between unlike molecules.

An ideal solution results when the A–B attractive forces are nearly equal in strength to the A–A and B–B forces:

Because a molecule feels essentially the same pull whether its neighbours are like or unlike itself, mixing changes neither the total energy of interaction (so $\Delta_{mix}H = 0$) nor the packing of the molecules (so $\Delta_{mix}V = 0$), and each component escapes into the vapour exactly as Raoult's law demands. …