What is the effect of impurities on surface tension of a liquid?

Answer: Highly soluble impurities increase the surface tension; sparingly soluble impurities decrease it.

• Highly soluble substances (e.g. common salt, NaCl, dissolved in water) increase the surface tension of water, because the dissolved ions strengthen the attractive forces in the surface layer.
• Sparingly soluble substances (e.g. soap, detergent, phenol in water) decrease the surface tension, because their molecules collect at the surface and weaken the cohesive forces there. This is why soap and detergents help in washing: the lower surface tension lets water wet and penetrate fabric.