How many of the following pairs of liquids form nearly ideal solutions?
n-Hexane and n-heptane, Ethanol and acetone, Bromoethane and chloroethane, Phenol and aniline, Benzene and toluene, Carbon disulphide and acetone, Chloroform and acetone
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3
A solution is nearly ideal when the A–A and B–B attractive forces are nearly equal to the A–B forces. If the A–B forces are weaker, the solution shows positive deviation from Raoult’s law; if they are stronger, it shows negative deviation.
Nearly ideal:
n-Hexane and n-heptane
Bromoethane and chloroethane
Benzene and toluene
Not nearly ideal:
Ethanol and acetone — positive deviation; acetone breaks some of the hydrogen bonds between ethanol molecules.
Phenol and aniline — negative deviation; the phenolic proton forms a hydrogen bond with the lone pair on nitrogen of aniline, stronger than that between like molecules.
Carbon disulphide and acetone — positive deviation; the dipolar interactions between unlike molecules are weaker.
Chloroform and acetone — negative deviation; a chloroform molecule forms a hydrogen bond with an acetone molecule.
Nearly ideal: 3 pairs.
NCERT (2026-27 edition):
1.5.1 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.”
1.5.2 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 …. In a solution formed by adding carbon disulphide to acetone, the dipolar interactions between solute-solvent molecules are weaker than the respective interactions among the solute-solute and solvent-solvent molecules. This solution also shows positive deviation.”
“…An example of this type is a mixture of phenol and aniline. … Similarly, a mixture of chloroform and acetone forms a solution with negative deviation from Raoult’s law.”