Question

The SI unit of specific latent heat is
(A) $cal\,{g^{ - 1}}$
(B) $cal\,{g^{ - 1}}{K^{ - 1}}$
(C) $J\,k{g^{ - 1}}$
(D) $J\,k{g^{ - 1}}{K^{ - 1}}$

Answer 1

Hint : The SI units is the International System of Units, abbreviated SI. The unit of mass in SI is kilogram $(kg)$ and that of temperature is Kelvin $(K)$ . The unit of energy or heat is Joule $(J)$ . Specific latent heat is the amount of heat required per unit mass at a specific temperature.

Complete step by step answer
Specific latent heats are defined for temperatures at which the state of matter is changing from one state to another at the melting or boiling point. It is the extra heat required per unit mass for the matter to change its state keeping the temperature constant. The average kinetic energy does not change during this heat addition, therefore temperature increase of decrease does not occur.
We know that during phase change $Q$ amount of latent heat is required by the matter for a phase transition.
$\Rightarrow Q = mL$ ,
where $m$ is the mass of the substance, and $L$ is the specific latent heat.
$\Rightarrow L = \dfrac{Q}{m}$ .
Therefore the specific latent heat is equal to the amount of thermal energy absorbed divided by the mass of the substance.
Unit of $L = \dfrac{J}{{kg}}$ ,
where $J$ is the unit of $Q$ , and $kg$ is the unit of $m$ . $$
Thus the unit of specific latent heat $(L)$ is $J\,k{g^{ - 1}}$ .
Therefore the correct answer is option (C) $J\,k{g^{ - 1}}$ .

Note
Specific latent heat can be at various temperatures depending upon the state transformation. If the change in the phase of matter is from solid to liquid, the specific latent heat is termed as the specific latent heat of fusion. If the change in the phase of matter is from liquid to gas, the specific latent heat is termed as the specific latent heat of vaporization.