Understanding Latent Heat Of Fusion: Formula And Ice Value

Think about what happens when you hold a piece of ice at 0°C and keep supplying heat to it. You might expect its temperature to rise. Instead, the ice keeps melting while the temperature remains at 0°C until all of it has turned into water. So where does the supplied heat go?
The answer lies in the latent heat of fusion. This heat is used to change the state of a substance rather than increase its temperature. The same idea applies when a liquid changes into a gas.
In this article, we will understand what is latent heat of fusion, its definition, formula and value. We will also look at the latent heat of fusion of ice, examples and the difference between latent heat of fusion and latent heat of vaporisation.

Latent heat of fusion is the heat required to change unit mass of a substance from solid to liquid at its melting point without changing its temperature. During this process, the supplied heat is used for the change of state rather than raising the temperature. Its SI unit is J/kg.
For example, ice at 0°C absorbs heat and changes into water at 0°C. The temperature remains constant until the melting is complete. The latent heat of fusion is a property of the substance and can also depend on pressure.

The latent heat of fusion formula is:
Q = mLᶠ
where:
• Q = heat supplied
• m = mass of the substance
• Lᶠ = specific latent heat of fusion
Therefore:
Lᶠ = Q/m
This formula can be used to calculate the heat required to melt a known mass of a substance.
For example, if the specific latent heat of fusion of a substance is 200 J/kg, the heat required to melt 2 kg of it at its melting point would be:
Q = 2 × 200 = 400 J
The formula applies when the substance is undergoing a change of state at its melting point.

The latent heat of fusion of ice is approximately:
Lᶠ = 3.34 × 10⁵ J/kg
This can also be written as:
334 J/g
This means that about 334 J of heat is required to convert 1 g of ice at its melting point into 1 g of water at the same temperature.
NCERT material gives the latent heat of fusion of water as approximately 334 J/g.
Why Does Ice Not Get Hotter While Melting?When ice at its melting point is supplied with heat, the energy is used in changing the arrangement of its particles and overcoming intermolecular forces. Therefore, the temperature remains constant during the phase change.
Once all the ice has melted, further heating of the water increases its temperature.

The standard school-level latent heat of fusion value for ice is:

Understanding latent heat of fusion examples can make the concept easier to remember.
Example 1: Melting IceSuppose 500 g of ice is at its melting point. How much heat is required to melt it completely?
Given:
m = 500 g = 0.5 kg
Lᶠ = 3.34 × 10⁵ J/kg
Using:
Q = mLᶠ
Q = 0.5 × 3.34 × 10⁵
Q = 1.67 × 10⁵ J
Therefore, approximately 1.67 × 10⁵ J of heat is required.
Example 2: Why Does Ice Help Cool a Drink?Ice absorbs heat from the surrounding drink while it melts. A significant amount of energy is required for this change of state because of the latent heat of fusion. The ice can therefore absorb heat while remaining at its melting temperature until it has melted.

Both processes involve a change of state and heat transfer without a temperature change during the phase transition. However, they involve different changes of state.

Once you understand why temperature stays constant during a change of state, latent heat becomes much easier to remember. Keep the idea simple: the heat is being used to change the state, not raise the temperature. From there, the formula and numerical questions become much easier to handle.