Question

The normal boiling point of water is ${100^o}C$ . At ${100^o}C$ ,:
A.The average kinetic energy of molecules in vapour is greater than that of liquid water
B.The average potential energy of molecules in vapour is greater than that of liquid water
C.The molar internal energy is the same for vapor and liquid water
D.Water boils at any pressure

Answer 1

Water boils at ${100^o}C$ because at this temperature the vapour pressure of the liquid reaches the surrounding pressure from the atmosphere.

Complete step by step answer:
As a liquid is heated, its temperature rises and as its temperature increases, the molecules of the liquid gain energy which results in an increase of kinetic energy. With an increase in kinetic energy, the molecular motion also rises and the molecules of the liquid thrash the force of attraction between them.
On heating consistently and continuously, a specific temperature is reached where the molecules of the liquid leave the surface in the form of vapour. This creates a pressure above the liquid which is equal to the atmospheric pressure and leads to boiling of liquid.
At this point, the temperature of the liquid stays stationary even on heating it further. This stationary temperature at which the vapour pressure of the liquid is the same as the atmospheric pressure can be termed as the boiling point of that liquid. At this temperature, bubbles start to form and rise in the liquid. The bubbles do not form before this temperature is attained because the atmospheric pressure is greater than the pressure in the bubbles and they collapse.
At an atmospheric pressure of precisely 760mm Hg (1 atm), the temperature at which a liquid boils is termed as the normal boiling point of the liquid. In case of water, the vapour pressure approaches the standard atmospheric pressure of 1 atmosphere at ${100^o}C$ . Hence, the normal boiling point of water is ${100^o}C$ ( $212^\circ F$ or 373K). Thus, at ${100^o}C$ (normal boiling point), the average kinetic energy of molecules in vapour is greater than that of liquid water.

Hence, the correct option is (A).

Note:
It must be noted that the boiling point of pure water is not always ${100^o}C$. It rises on the addition of soluble substances such as common salt or sugar. The boiling point of pure water increases with increase in pressure.