Question

The SI unit of specific latent heat is
$A. cal \quad {g}^{-1}$
$B. cal \quad {g}^{-1}\quad {K}^{-1}$
$C. J \quad {kg}^{-1}$
$D. J \quad {kg}^{-1}\quad {K}^{-1}$

Answer 1

When a body undergoes a phase change, it either absorbs heat from the surroundings or releases it. This amount of heat required depends upon the specific latent heat of the phase change. To answer this question, use the above-mentioned definition of specific latent heat. Then, write the formula for specific latent heat which is in terms of energy and mass. Now, substitute the SI base units of energy and mass in the formula and find the SI unit of specific latent heat.

Complete answer:
Specific latent heat is defined as the amount of heat released or absorbed by the substance per unit mass for changing the state of the substance. We can write this as,
$Specific \quad latent \quad heat= \dfrac { Amount\quad of\quad heat\quad absorbed\quad or\quad released }{ Mass\quad of\quad the\quad substance }$ …(1)
Thus, the unit of specific latent heat should be of $\dfrac {Energy}{Mass}$.
SI unit of energy is Joule (J) and that of mass is kilogram (kg).
Substituting these SI units in the equation. (1) we get,
$Specific \quad latent \quad heat= \dfrac {J}{kg}$
$\Rightarrow Specific \quad latent \quad heat= J \quad {kg}^{-1}$.
Thus, the SI unit of specific latent heat is $J \quad {kg}^{-1}$.

So, the correct answer is option C i.e. $J \quad {kg}^{-1}$.

Note:
Students usually get confused between specific latent heat and specific heat capacity. Specific latent is the amount of energy required to change the state of 1 kg of substance with no temperature change. Specific heat capacity is the energy transferred to 1 kg of substance to raise its temperature by 1°C. Specific heat capacity and the specific latent heat both are properties of the specific material.