Question

It is easy to wash clothes in hot water, because, its
(A) Surface tension is less
(B) Surface tension is more
(C) Surface tension is the same
(D) None of the above

Answer 1

Hint The molecules on the surface of a liquid have potential energy. That potential energy is equal to the work done to overcome the downward pull. Therefore the energy required to increase the unit area of the surface of a liquid is called the surface energy. The surface tension is defined as the surface energy per unit area.

Complete Step by step solution
The surface tension between the water and the dirt molecule hinders the removal of dirt from the clothes. In order to remove the dirt completely and effectively, we have to reduce the surface tension of water. We know that the value of surface tension depends on temperature. For liquids, the value of surface tension usually decreases with the increase in temperature. Hence we know that it is easy to wash clothes in hot water because its surface tension is less.

Option (A): Surface tension is less

Additional information
A liquid drop tries to reduce its surface area to a minimum. A sphere has the minimum surface area for any given volume. Hence liquid drop takes a spherical shape. The reason why liquids tend to rise through narrow tubes is also surface tension.

Note
The reason why we use detergents to wash clothes is also the same. Detergents help in reducing the surface area and remove dirt effectively. The molecules of the detergent have the shape of a hairpin. One end of the molecule attaches to the water molecule and the other end is attracted to the dirt molecule. This reduces the surface tension between water and the dirt molecule.