A perfectly ideal solution is rare but some solution behave nearly ideal. Which of the following does not fall in this category ?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Ethanol and acetone
An ideal solution obeys Raoult’s law over the whole range of composition, with ΔmixH = 0 and ΔmixV = 0. This happens when the attractions A–B are nearly equal to the attractions A–A and B–B.
In ethanol, the molecules are held together by hydrogen bonds. When acetone is added, its molecules get in between the ethanol molecules and break some of these hydrogen bonds, so the A–B attractions are weaker than the A–A attractions. The molecules escape more easily, the vapour pressure is higher than Raoult’s law predicts, and the solution shows a positive deviation. So ethanol and acetone do not form a nearly ideal solution.
Option (1) n-hexane and n-heptane — A nearly ideal solution. Two non-polar alkanes of similar size.
Option (2) Ethanol and acetone — Not a nearly ideal solution. It shows positive deviation.
Option (3) Benzene and toluene — A nearly ideal solution. Two aromatic hydrocarbons of similar structure.
Option (4) Bromoethane and chloroethane — A nearly ideal solution. Two haloalkanes of similar polarity and size.
NCERT (2026-27 edition):
Ideal Solutions – …
NCERT (2026-27 edition):
Non-ideal Solutions – “… Mixtures of ethanol and acetone behave in this manner. In pure ethanol, molecules are hydrogen bonded. On adding acetone, its molecules get in between the host molecules and break some of the hydrogen bonds between them. Due to weakening of interactions, the solution shows positive deviation from Raoult’s law [Fig. 1.6 (a)].”