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)11OSO₃Na+ at room temperature. The correct assignment of the sketches is:

Impurities affect surface tension appreciably. It is observed that impurities which tend to concentrate on the surface of liquids, compared to its bulk lower the surface tension. Substances like detergents and soaps (CH3(CH2)11SO₃Na+) 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 a smaller dielectric constant. The dielectric constant is directly proportional to surface tension. So, on adding CH3OH in water, overall dielectric constant decreases and surface tension decreases. In inorganic impurities present in the bulk of a liquid such as KCl tend to increase the surface tension of water.