CONCEPT: Surface tension: • The property by virtue of which liquid tries to minimize its free surface area is called surface tension. • In spherical shape the surface area is minimum and for this reason, the raindrops are spherical. • Two factors can affect the surface tension of a liquid. The factors are; • Temperature: If temperature increases then the surface tension of a liquid decreases. • Soluble Impurities: In the case of less soluble impurities, the surface tension decreases. But, for highly soluble impurities in the liquid the surface tension increases. • Examples: • When kerosene oil is sprinkled on the water the surface decreases and for this reason, the mosquito larvas die due to sinking in the water. • Warm soup is tasty because for high temperatures the surface tension becomes low and spreads equally over the tongue. • The surface tension of antiseptics is low that the reason they reach the small cracks of the wound and clean the germs. • The surface tension of soap solution is less than pure water and for this reason, the soap solution cleans clothes better than pure water. EXPLANATION: • Due to the exertion of surface tension by the molecules, water droplets always take a round shape when dropped onto plane surfaces. So option 4 is correct.