Assertion (A): A solution of n-hexane and n-heptane is nearly ideal in behaviour.
Reason (R): In a solution of n-hexane and n-heptane, the A-B interactions are weaker than those between A-A or B-B.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is true but (R) is false.
Whether two liquids A and B behave ideally depends on how the A–B attractions compare with the A–A and B–B attractions. Nearly equal forces give an ideal solution; weaker A–B forces give positive deviation from Raoult’s law.
Assertion (A): A solution of n-hexane and n-heptane is nearly ideal in behaviour. — True. This pair is one of the examples of nearly ideal solutions, with bromoethane and chloroethane, and benzene and toluene.
In it the A–A and B–B attractive forces are nearly equal to the A–B forces.
Reason (R): In a solution of n-hexane and n-heptane, the A-B interactions are weaker than those between A-A or B-B. — False. Weaker A–B interactions are the cause of positive deviation; such a solution would not be nearly ideal.
For n-hexane and n-heptane the A–B forces are nearly equal to the A–A and B–B forces.
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 – “…In case of positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B …”