UPSC Combined Geo-Scientist, Mains · 2017 · Mains Paper II

Q1(j). Surface tension of isomers

Question

Why does liquid ethanol have greater surface tension than liquid dimethyl ether although their molecular masses are equal?

Ethanol forms extensive intermolecular O–H hydrogen bonds; dimethyl ether lacks an O–H donor and cannot form the same self-associated network.

Both molecules have formula C₂H₆O and the same molar mass, but their intermolecular interactions are different.
Ethanol is both a hydrogen-bond donor and acceptor, producing stronger cohesive interactions.
Creating a surface breaks or weakens cohesive interactions, so stronger cohesion usually entails a larger surface free energy per unit area, hence larger surface tension.

The full model answer

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CoversSurface tension of isomersIntermolecular forces