Which of the following statements are correct:
(A). In ideal solutions, the enthalpy of mixing of the pure components to form the solution is zero.
(B). The volume of the solution is equal to the sum of the volumes of the two components.
(C). In binary ideal solutions, attractive forces between A-A and B-B are nearly equal to those between A-B.
(D). In negative Raoult's law deviation, attractions between A-B are weaker than those of A-A and B-B attractions.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (B), and (C) only
An ideal solution obeys Raoult’s law over the whole range of concentration. This happens when the attractions between unlike molecules (A–B) are nearly the same as those between like molecules (A–A and B–B). Mixing then changes neither the energy nor the packing of the molecules, so:
\( \displaystyle \Delta_{\text{mix}}H = 0, \qquad \Delta_{\text{mix}}V = 0 \)
In a non-ideal solution the A–B attractions differ. If they are weaker than A–A and B–B, molecules escape more easily and the solution shows a positive deviation; if they are stronger, the solution shows a negative deviation.
(A) In ideal solutions, the enthalpy of mixing of the pure components to form the solution is zero. — Correct. No heat is absorbed or given out on mixing, because the attractions are unchanged.
(B) The volume of the solution is equal to the sum of the volumes of the two components. — Correct. For an ideal solution \( \Delta_{\text{mix}}V = 0 \), so the volumes are additive.
(C) In binary ideal solutions, attractive forces between A-A and B-B are nearly equal to those between A-B. — Correct. This is the molecular reason for ideal behaviour.
(D) In negative Raoult’s law deviation, attractions between A-B are weaker than those of A-A and B-B attractions. — Incorrect. In a negative deviation the A–B attractions are stronger, so the vapour pressure is lower than Raoult’s law predicts (e.g. chloroform and acetone, which hydrogen bond with each other). Weaker A–B attractions give a positive deviation.
NCERT (2026-27 edition):
Ideal Solutions
Ideal Solutions – “… At molecular level, ideal behaviour of the solutions can be explained by considering two components A and B. In pure components, the intermolecular attractive interactions will be of types A-A and B-B, whereas in the binary solutions in addition to these two interactions, A-B type of interactions will also be present. 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.”
Non-ideal Solutions – “… In case of negative deviations from Raoult’s law, the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure resulting in negative deviations.”