Question

What is the effect of impurities on surface tension?

Answer 1

Hint: As we all know that insects usually denser than water can easily float on water and that is due to surface tension. After adding impurities in water like oil then the insects are not able to float as they sink.

Complete step by step answer:
We know that when the liquid surface sinks to the minimum surface area possible, this tendency is called surface tension. This tension on the surface film caused by the attraction of particles on the surface by the whole mass of the liquid is defined by surface tension. The cohesive force between the molecules at the surface and the bulk of the liquid and the liquid bulk is trying to attract the molecules and making the molecules to acquire the least possible area.
Yes, by adding impurities the surface tension of water increases or decreases is determined by the cohesion between the molecules or the adhesion among the water molecules and impurity. We also know that by adding salt the surface tension of water increases but not by a great amount. For example, if we add salt in water i.e $NaCl$, the salt immediately breaks into $Na^{+}$ and $Cl^{-}$ ions which tends to make much stronger attraction among water molecules around these ions, and hence the surface tension of water increases.
We can say that by adding oil to water, the surface tension of water decreases. This is because the balancing force between the water molecules and oil molecules reduce the surface tension at the water surface.

Note:
From the above points, it can be concluded that by dissolving highly soluble impurities in water, the surface tension of water increases, and by adding insoluble impurities like oil, etc. the surface tension of water decreases. So, we can say that impurities greatly affect surface tension.