Change of State

# Fusion

The change of a body from its solid state into liquid state is known as fusion. During the change of state, the volume of the substance changes.
On heating, the temperature of a solid begins to rise. At a certain temperature, it begins to melt. This temperature is known as the melting point of the solid. During the process of melting, the temperature of the solid remains constant. Unit mass of a substance requires a fixed amount of heat to change its state. Some solids like wax, lead, etc. expand on heating while other solids like ice, bismuth, cast iron, etc. contract on heating.

## Effect of Pressure on Melting Point

Suppose a substance which expands on heating. If pressure is applied on the substance while heating, then the applied pressure will prevent it from expanding i.e. it prevents the substance from melting. Thus, the substance will need more amount of heat to melt.
Thus, when pressure is applied on those substances which expand on heating, then the melting point of the substances increases.
Now, if we take another substance which contracts on heating, then the applied pressure causes the substance to contract sooner i.e. melting point decreases.
Hence, when pressure is applied on those substances which contract on heating, then the melting point of the substances decreases.

## Latent Heat of Fusion

When a substance is melting, its temperature does not increase. The amount of heat supplied causes only change of state from solid to liquid. This amount of heat is known as latent heat of fusion.
Latent heat of fusion of a substance is defined as the amount of heat required to change the unit mass of the substance from solid to liquid state at a constant temperature.
Let $Q$ be the amount of heat required to change $m$ kg of solid to liquid. Then, it is found that, $Q ∝m$ $Q=mL_f$ where, the proportionality constant $L_f$ is called latent heat of fusion of the substance. Its value is different for different substances depending upon their nature.

## Some Substances with their Latent Heat of Fusion

 Substance Latent Heat of Fusion (cal/g) Mercury 2.72 Lead 5.49 Gold 15 Silver 25.1 Ethanol 25.8 Paraffin 35.1 Glycerol 47.95 Copper 49.2 Iron 49.9 Ice, Water 79.72 Aluminium 94.6