Question

What is latent heat of fusion?

Answer 1

Hint: Latent heat of fusion is involved during the phase change of a solid into a liquid at constant pressure. It literally means ‘hidden’ heat which means that it does not cause a rise in the temperature of the body and gets hidden in the body.

Complete step by step answer:
Latent heat of fusion is the heat required per unit mass for a body to undergo the phase change from solid to liquid under constant pressure at the melting point of the substance. This heat is absorbed by the body to break the forces of attraction between the molecules in the solid to go further apart to be in the liquid state. Therefore, this heat does not cause a rise in the temperature of the body and it seems that the heat energy has become ‘hidden’ in the body. Therefore it is named as latent heat as the word ‘latent’ means ‘hidden’.
Therefore, the latent heat of fusion ${{L}_{fusion}}$ can be mathematically expressed as
${{L}_{fusion}}=\dfrac{H}{m}$ --(1)
where $H$ is the total heat absorbed by a body of mass $m$ to undergo the phase change from solid to liquid at the melting point of the substance at constant atmospheric pressure.
Hence, from (1), we can see that the SI unit of latent heat of fusion is Joule per kilogram.
$J.k{{g}^{-1}}$
Since the SI unit of heat is Joule $\left( J \right)$ and the SI unit of mass is kilogram $\left( Kg \right)$.

Note: It is a very important point to remember that the latent heat of fusion is defined for a constant pressure and this pressure is $1atm=1.01325\times {{10}^{5}}Pa$, which is the normal atmospheric pressure at sea level. This is because melting points and the heat required changes upon the pressure of the surroundings and since the latent heat of fusion is a characteristic property of a substance, it must be defined as a constant and therefore, be defined at a specific pressure.