Question

Define Latent Heat of vaporisation, $L_v$
A. The specific latent heat of vaporisation of a substance is the quantity of heat required to convert a unit mass of the substance from liquid state to vapour state at its boiling point.
B. The specific heat of vaporisation of a substance is the quantity of heat required to convert a unit mass of the substance from solid state to liquid state at its melting point.
C. The specific latent heat of vaporisation of a substance is the quantity of heat required to raise the temperature of unit mass of the substance by $1^{0}C$ .
D. The specific latent heat of vaporisation of a substance is the quantity of heat required to lower the temperature of unit mass of the substance by $1^{0}C$ .

Answer 1

Hint: Latent heat of vaporisation is a physical quantity related with the phase change from liquid to gas. Define the process of latent heat and give the answer to this question in terms of the phase change from the liquid to vapour state.

Complete step by step answer:
When a substance changes phase, the temperature of the system does not change. Only the arrangement of the molecule of the substance changes. The latent heat of vaporisation is related to the phase change of a substance of liquid state to vapour state.
When the phase of the substance changes, if the new arrangement of molecule has more thermal energy then the old arrangement, the substance absorbs thermal energy from the environment to make the phase change. If the new arrangement has lower thermal energy then the older arrangement, then the substance releases the thermal energy to the environment to make the phase change.
Latent heat of vaporisation of a substance can be defined as the amount of heat required to apply to the liquid at its boiling point under standard atmospheric pressure to change its phase to vapour. The heat energy given to the substance or the system is used as latent heat or used to change the phase of the system and the temperature of the system remains constant.
So, we can say that the specific latent heat of vaporisation of a substance is the quantity of heat required to convert a unit mass of the substance from liquid state to vapour state at its boiling point.
The correct option is (A).

Note:
We have two types of latent heat. The latent heat of fusion and the latent heat of vaporisation. When a phase change of solid to liquid occurs, the latent heat of fusion is used. When a phase change of liquid to gases or vapor occurred the latent heat of vaporization comes into play