Question

At the boiling point of water, its surface tension
A. Is infinite
B. Is zero
C. Is the same as that room temperature
D. Is maximum

Answer 1

The reason why the phenomena of surface tension occurs is due to the cohesiveness of the liquid molecules. The cohesive nature of the molecules starts decreasing, resulting in the decrease of the surface tension.Think what happens to the molecules if the temperature of water is changed.

Complete answer:
Consider the molecules at the surface of water. The molecules experience two types of forces, one from the like water molecules just below the surface and the other from the unlike molecules above the surface. The main point here can be seen that the surface tension depends on how the molecules at the surface interact with other molecules. So, what happens when the temperature of water is increased?
When the temperature of water is raised, the molecules gain energy and start to move with more kinetic energy. This results in less interaction with the neighbour molecules. The cohesive nature of the molecules starts decreasing, resulting in the decrease of the surface tension. When the temperature reaches the boiling point of water, the molecules move from bottom to the surface continuously – known as convection, and eventually the surface tension approaches to zero.
Therefore, at the boiling point of water, its surface tension is zero.

So, the correct answer is “Option B”.

Note:
The surface tension approaches to zero and not equals to zero at the boiling point. Here the most appropriate answer will be zero. The surface separating the water and the water vapor gets destroyed when it reaches a certain temperature called critical temperature. At that point the surface tension becomes equal to zero.