Question

The qualitative sketches I, II and III given below show the variation of surface tension with molar
concentration of three different aqueous solutions of KCl, CH₃OH and CH₃ (CH₂)₁₁ OSO₃⁻ Na⁺ at room
temperature. The correct assignment of the sketches is Surface tension
(A) I : KCl II : CH₃OH III : CH₃ (CH₂)₁₁ OSO₃⁻ Na⁺
(B) I : CH₃ (CH₂)₁₁ OSO₃⁻ Na⁺ II : CH₃OH III : KCl
(C) I : KCl II : CH₃ (CH₂)₁₁ OSO₃⁻ Na⁺ III : CH₃OH
(D) I : CH₃OH II : KCl III : CH₃ (CH₂)₁₁ OSO₃⁻ Na⁺

Answer 1

Answer:

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Answer 2

(D) is the correct answer.

Impurities affect surface tension appreciably. It is observed that impurities which tend to concentrate on surface of liquids, compared to its bulk lower the surface tension.

Substances like detergents and soaps

(CH3(CH2)11SO3-Na+) decrease the surface tension sharply.

Those like alcohol (eg.CH3OH,C2H5OH) lower the surface tension slightly. This can also be related to the fact that CH3OH has smaller dielectric constant. Dielectric constant is directly proportional to surface tension. So, on adding CH3OH in water, overall dielectric constant decreases and surface tension decreases.

Inorganic impurities present in bulk of a liquid such as KCl tend to increase the surface tension of water.