Question

Soap helps is cleansing clothes, because
(A) It attracts the dirt particles
(B) It decreases the surface tension of water
(C) It increases the cohesive force between water molecules
(D) It increases the angle of contact.

Answer 1

Hint : The surface tension of water tends to allow water molecules to stick to other water molecules than other elements molecules. Cohesive forces cause surface tension. Higher surface tension creates a larger angle of contact.

Complete step by step answer
Generally, soap as it is known, is an object which helps in washing our clothes, and utensils. Soaps are usually made of chemicals called surfactants. These surfactants are molecules which have two ends. One of its ends is highly sticky to water and this allows the reduction of the surface tension of the water. We can recall that surface tension allows water to stick to itself more than other molecules, which is why water forms droplets on glass surfaces or dirty wood and all. Since, the molecules stick to water, this tends to reduce the tendency for the water to stick to itself. However, what completes the action is the other end of the molecule of the surfactants. This end sticks to grease and dirt, and other unwanted stains. Hence, this forms a link between the water and the dirt, and allows the water to easily permeate the fabric of the clothing. As the water molecules are washed away, they take along the surfactant which takes along the dirt.
Hence, the correct answer is B.

Note
For clarity, it is the adhesion of the end of the surfactants to the water molecule, and the other end to the dirt molecules, that pulls the water molecules and dirt molecules together. If the link is broken, the water molecules attract themselves again, and would not permeate the fabric.