Cleansing action of detergents: Oil stains and grease on dirty clothes cannot be removed by simply washing the clothes with water because water does not wet them. By adding detergent or soap to water, the greasy dirt can be easily removed. The cleansing action of detergents can be explained as follows:
i. Soap or detergent molecules have the shape of a hairpin.
ii. When detergent is dissolved in water, the heads of its hairpin shape molecules get attracted to water surface.
iii. When clothes with greasy stains are dipped in water containing detergent, the pointed ends of detergent molecules get attached to the molecules of grease. So a water-grease interface is formed. Thus surface tension is greatly reduced. The greasy dirt is held suspended.
iv. When the clothes are rinsed in water, the greasy dirt is washed away by running water.
So when detergent is added to water, the surface tension of water is reduced, its area of contact with grease is increased and hence its cleansing ability is increased.