Question

When the temperature is increased, surface tension of water:
(A) Increases
(B) Decreases
(C) Remains constant
(D) Shows irregular behavior

Answer 1

Surface tension of water is due to attraction between the molecules which are on the surface of the water with the ones which are in the bulk. With increase in the temperature, kinetic energy of molecules increases.

Complete step by step solution:
Let’s see what is meant by surface tension of water.
- We know that there are many molecules of ${{H}_{2}}O$ in water. Some of them will be on the surface and majority of them will be surrounded by other molecules of water. Now, every molecule in water of any liquid experiences a special attractive force with each other.
- So, as a result the molecules of liquid which are surrounded by other molecules, experience a net force of zero magnitude because all the force vectors are in different directions and they cancel out each other. The molecules on the surface of the liquid do not have other molecules of liquid on one side. So, there is a net downward force on those molecules. So, they have a tendency to get attracted to them. Thus, the liquid tries to obtain the lowest surface area possible. This is due to surface tension of the liquid.
- Thus, surface tension is defined as the force acting per unit length which is perpendicular to the surface of the liquid.
- As we increase the temperature of the liquid, the molecules have more kinetic energy. That means that they have more velocity. This decreases the attraction between the molecules of the liquid. As a result there is a net decrease in the surface tension of water as we increase its temperature.

So, the correct answer is (B).

Note: Remember that in all liquids, increase in temperature brings increase in their velocity of molecules and so that those molecules are not able to interact with other molecules easily and as a result their surface tension decreases.