Question

The SI unit of specific latent heat is:
A. $J.k{{g}^{-1}}{{K}^{-1}}$
B. ${{J}^{0}}{{K}^{-1}}$
C. $J.k{{g}^{-1}}$
D. $J.kg.{{K}^{-1}}$

Answer 1

Hint: The specific latent heat of phase change is the amount of thermal energy absorbed or released per unit mass by the substance for performing the phase change. Therefore, it has a unit that represents energy per unit mass.

Complete step-by-step answer:
Whenever a body undergoes a phase change, it either absorbs heat from the surroundings or releases it. This total amount of thermal energy required depends upon the specific latent heat of the phase change.
The specific latent heat of the phase change for a substance is defined as the amount of heat released or absorbed by a substance per unit mass for undergoing the phase change. Hence, essentially,
$\text{Specific Latent heat = }\dfrac{\text{Total amount of heat absorbed or released}}{\text{Mass of the body}}$
Therefore, the units for specific latent heat should be that of $\dfrac{\text{Energy}}{\text{Mass}}$.
The SI unit of energy is Joule $\left( J \right)$. The SI unit of mass is Kilogram $\left( kg \right)$.
Therefore, the SI unit of specific latent heat will be $\dfrac{\text{Joule}\left( J \right)}{\text{Kilogram}\left( kg \right)}=\dfrac{J}{kg}=J.k{{g}^{-1}}$
Hence, the correct option is C) $J.k{{g}^{-1}}$.

Additional information:
The specific latent heat can be considered a measure of how spontaneously a substance undergoes a phase change. A substance that has a lower specific latent heat requires less heat per unit mass to undergo the phase change and hence will do so faster and easily. On the other hand, it will take more energy and time for a substance with a high specific latent heat to undergo the same phase change.
Water has a high specific latent heat. That is why when sweat evaporates from our body, we feel cool. The sweat absorbs a lot of heat from our body and cools our body down.

Note: Students can often get confused between specific latent heat and specific heat capacity. However, an easy way to remember is to understand the meaning of the word ‘latent’. The word ‘latent’ means ‘hidden’. Thus, the heat required in a phase change is hidden in the sense that it is completely absorbed or released by the body without any change in its temperature. Thus, it seems like the thermal energy vanishes or gets ‘hidden’ in the body without any change in it. Hence, the specific latent heat should not have the unit of temperature $\left( K \right)$in any form in it.
Students can remember in this way and not get confused between the two. If they would have got confused, they could have marked the unit of specific heat capacity that is option A) as the correct answer.