Passage:
Solubility of gases in liquids is greatly affected by pressure and temperature. Henry gave the quantitative relationship between pressure and solubility of gas in a solvent. According to Henry’s law, partial pressure of a gas above a liquid is directly proportional to its mole fraction in solution and is expressed as \(P = K_H.x\), where \(K_H\) is Henry’s constant and x is mole fraction of gas. \(K_H\) is a function of nature of gas.
F M Raoult gave a quantitative relationship between partial pressures and mole fractions in the binary solution of volatile liquids. Raoult’s law states that for a binary solution of volatile liquids, the partial pressure of each component in the solution is directly proportional to its mole fraction. Thus for a solution of component \(1\) and \(2\), partial pressure of each component \(P_1 = p_1^0x_1\), where \(p_1^0\) is the vapour pressure of pure component \(1\) at the same temperature. Similarly \(P_2 = p_2^0x_2\)
Which of the following show negative deviation from Raoult’s law?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Acetone and Chloroform
In a solution of two liquids A and B, Raoult’s law holds when the A–B interactions are of the same strength as the A–A and B–B interactions. The deviations come from differences in these forces:
Positive deviation: A–B forces are weaker; molecules escape more easily, so the vapour pressure is higher than predicted.
Negative deviation: A–B forces are stronger; molecules escape less easily, so the vapour pressure is lower than predicted.
In a mixture of chloroform and acetone, the hydrogen of \( \ce{CHCl3} \) (made acidic by the three Cl atoms) forms a hydrogen bond with the oxygen of acetone:
\( \ce{Cl3C-H\cdots O=C(CH3)2} \)
These new attractions are stronger than those in either pure liquid, so the escaping tendency falls and the mixture shows negative deviation.
Why the other options are incorrect
Option (1) Water and Ethanol — Incorrect. Ethanol breaks some of the hydrogen bonds of water and vice versa; this mixture shows positive deviation (it forms a minimum boiling azeotrope at about 95% ethanol).
Option (2) Ethanol and Acetone — Incorrect. Acetone breaks the hydrogen bonds between ethanol molecules; positive deviation.
Option (4) Carbondisulphide and Acetone — Incorrect. The dipolar interactions between the two are weaker than those in pure acetone; positive deviation.
NCERT (2026-27 edition):
Non-ideal Solutions – …
