Give any two illustrations of surface tension.

Explanation

(i) Needle floats on water surface: A greased needle of steel and a piece of blotting paper are take and gently placed over the water surface. Blotting paper soaks water and soon sinks down but the needle keeps floatinng. The floating needle causes a litte depression, the forces F, due to the surface tension of the curved surface are inclined as shown in Figure. The vertical components of these two forces support the weight of the needle. Now add liquid soap to the water and stir it. It is found that the needle sinks.

(ii) Take a plastic sheet and cut out a piece in the shape of a boat. A tapering and smooth front with a notch at the back is suggested. A piece of camphor is put into the notch of the boat. Gently release the boat on the surface of the water and we find that the boat is propelled forward when the camphor dissolves. The surface tension is lowered, as the camphor dissolves and produces a difference in surface tension in the water nearby the notch. This causes the water to flow away from the back of the boat, which moves the boat forward.