Which of the following is not an example of ideal solution?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Solution of ethanol and acetone
An ideal solution obeys Raoult’s law over the whole range of concentration, with \( \Delta_{mix}H = 0 \) and \( \Delta_{mix}V = 0 \). This happens when the A–B attractions are nearly equal to the A–A and B–B attractions, as in mixtures of very similar liquids.
Option (1) Solution of n-hexane and n-heptane — A nearly ideal solution. Both are non-polar alkanes held by similar dispersion forces.
Option (2) Solution of ethanol and acetone — Not an ideal solution. Ethanol molecules are hydrogen bonded. Acetone molecules get in between them and break some of the hydrogen bonds, so the A–B attractions are weaker than in pure ethanol. Molecules escape more easily, and the solution shows positive deviation from Raoult’s law.
Option (3) Solution of bromoethane and chloroethane — A nearly ideal solution. The two haloalkanes are similar in size and polarity.
Option (4) Solution of benzene and toluene — A nearly ideal solution. Both are aromatic hydrocarbons with similar intermolecular forces.
NCERT (2026-27 edition):
Ideal Solutions – “… If the intermolecular attractive forces between the A-A and B-B are nearly equal to those between A-B, this leads to the formation of ideal solution. A perfectly ideal solution is rare but some solutions are nearly ideal in behaviour. Solution of n-hexane and n-heptane, bromoethane and chloroethane, benzene and toluene, etc. fall into this category.”
Non-ideal Solutions – “… In case of positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules. This means that in such solutions, molecules of A (or B) will find it easier to escape than in pure state. This will increase the vapour pressure and result in positive deviation. Mixtures of ethanol and acetone behave in this manner.”