(a) State two factors on which surface tension depends. (b) How can mosquito larvae be made to sink in stagnant water?

Answer Details

(a) Two factors on which surface tension depends

• Temperature of the liquid: surface tension decreases as the temperature rises (the molecules gain more energy and their mutual attraction weakens).
• Nature of the liquid and any dissolved substances (impurities): different liquids have different surface tensions, and adding impurities such as detergent or soap lowers it, while some dissolved salts raise it.

(b) Making mosquito larvae sink

Mosquito larvae hang from the underside of the water surface and breathe there, being supported by the surface tension of the water. If a thin film of oil (or kerosene, or a little detergent) is spread over the stagnant water, the surface tension is greatly reduced. The surface can then no longer support the larvae, so they lose their hold on the surface film and sink. The oil film also blocks their breathing tubes, so they are unable to take in air.