Factors Responsible for Deviation from Raoult's Law

Six distinct factors can drive a solution away from ideal behaviour:

1. Solute-solvent interactions. An ideal solution requires the A-A (solvent-solvent), B-B (solute-solute), and A-B (solute-solvent) intermolecular interactions all to be roughly similar in strength. Whenever these interactions are genuinely dissimilar, the solution deviates from ideal behaviour -- this is really the root cause behind both the positive- and negative-deviation examples of section 9.8.2.
2. Dissociation of solute. When a dissolved solute splits apart into its constituent ions, those ions interact strongly with the solvent (via ion-dipole forces), and this strong new interaction causes deviation from Raoult's law. Example: potassium chloride in water,

   $\mathrm{KCl(s) + H_2O(l) \rightarrow K^+(aq) + Cl^-(aq)}$

   the resulting K$^+$ and Cl$^-$ ions form strong ion-dipole interactions with water molecules, causing deviation.
3. Association of solute. Conversely, when solute molecules associate with each other (rather than splitting apart), that too causes deviation from ideal behaviour. Example: acetic acid in solution forms a dimer through intermolecular hydrogen bonding (Figure 9.9) -- two CH$_3$COOH molecules link via two O-H$\cdots$O hydrogen bonds in a cyclic structure. (This same association is revisited quantitatively for abnormal molar mass in section 9.11.1.)
4. Temperature. Raising a solution's temperature raises the average kinetic energy of its molecules, which weakens the attractive forces holding them together -- pushing the solution further from ideal behaviour.
5. Pressure. At high pressure, molecules are forced closer together, which strengthens their mutual intermolecular attraction and thereby also causes the solution to deviate from Raoult's law. …