Question

At which of the temperatures, the value of surface tension of water is minimum?
A. ${{4}^{o}}C$
B. ${{25}^{o}}C$
C. ${{50}^{o}}C$
D. ${{75}^{o}}C$

Answer 1

Hint: The cohesive forces of molecules in liquids act on other molecules so that the surface area of the liquid is minimum. This also allows insects usually denser than water to float on the surface of water. The surface tension is proportional to the length of the boundary of the surface of interest.

Complete answer:
Surface tension is the name given to the cohesive forces of the molecules of a liquid on its surface. Due to these cohesive forces, the shape of water changes at the surface, for example when a droplet of water is on the floor it takes a curved shape. That is done to reduce the surface area of the liquid as much as possible, this is also the reason drops of liquid falling have kind of a spherical shape, there are few disturbances due to other external forces too. It can be most distinctly observed in small masses of mercury, they form a perfect sphere when a very small quantity is present. Now as the temperature increases, the molecules of water gain more energy and the bonds become weaker, therefore the surface tension decreases with the increase in temperature. Here we are given four different options, we will choose the one with the highest temperature, which will be ${{75}^{o}}C$.

Hence, the correct option is D, i.e. ${{75}^{o}}C$

Note:
Surface tension also allows insects to “walk” on the surface of water. The units of surface tension are of force per unit length or energy per unit area. The second one gives an understanding for the effects we observe, the shape of the drops is spherical which reduces the surface area and this in turn reduces the surface-energy to minimum.