Question

Choose the correct option concerning the definition of the normal boiling point of a liquid?
A.The temperature at which the liquid boils
B.The pressure at which the liquid boils at ${100^\circ }C$
C.The temperature at which the liquid vapour pressure at 1 atm ${100^\circ }C$
D.Dependent upon the pressure above the liquid

Answer 1

We can define the boiling point of a liquid as the temperature at which its vapour pressure is equal to the pressure of the gas above it and we define the normal boiling point of a liquid as the temperature at which its vapour pressure is equal to one atmosphere i.e. 1 atm (760 torr).

Complete step by step answer:
The temperature at which the pressure exerted by the surroundings upon a liquid is equalled by the pressure exerted by the vapour of the liquid; under this condition, addition of heat results in the transformation of the liquid into its vapour without raising the temperature, is known as Boiling point.
A liquid partly vaporizes into the space above it until the pressure exerted by the vapour reaches a characteristic value, at any temperature is called the vapour pressure of the liquid at that temperature. As the temperature increases, the vapour pressure also increases; at the boiling point.
So, normal boiling point of a liquid is the temperature at which the vapour pressure of liquid is equal to the atmospheric pressure at sea level, which is equal to 1 atm

Therefore, the correct answer is option (C).

Note: When a liquid is heated then its vapour pressure increases until the vapour pressure becomes equal to the pressure of the gas above it. The bubbles of vaporized liquid (i.e., gas) form within the bulk liquid and they rise to the surface and there they burst and release the gas. The vapour inside a bubble has enough pressure to keep the bubble from collapsing at the boiling temperature. The molecules of the liquid must overcome the forces of attraction between them to form vapour.