Reading Comprehension Topics

Liquid molecules have the property of cohesion. Cohesion is the tendency of a liquid to cling together. Cohesive forces between water molecules are due to hydrogen bonding. The molecules at the surface of water are attracted by many molecules below it but not from above. The molecules at the surface, therefore feel a net attraction inward. This attraction creates surface tension. The water surface behaves like a stretched piece of rubber. A razor blade if kept carefully on the surface of water can be made to float although it is made of steel.

For a molecule to come to the surface it must overcome the attraction directed towards the interior of the liquid. In other words, its potential energy must increase i.e. Work must be done to pull it to the surface. Making the surface of a liquid larger, therefore, requires an input of energy, and the amount of energy required is the liquid’s surface tension. The surface tension of a liquid decreases with the increase in temperature because the increased kinetic energy of the molecules decreases the effect of intermolecular forces of attraction. Water has high surface tension because of hydrogen bonding. Raindrops have a spherical shape because a sphere has the least surface-to-volume ratio which it acquires due to surface tension.