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Difference between latent heat of fusion and latent heat of vaporization

Latent Heat of Fusion Definition

Heat energy required to change unit mass of a substance from solid to liquid state  at its melting point without change in its temperature is called its latent heat of fusion.

When a substance is changed from solid to liquid state by adding heat, the process is called melting or fusion. The temperature at which a solid starts melting is called its fusion point or melting point.When a process is reversed i.e., when a liquid is cooled, it changes into solid state. The temperature at which a substance changes from liquid to solid state is called its freezing point. Different substances have different melting points. However, the freezing point of a substance is the same as its melting point.

It is denoted by Hf.

Hf = ΔQf/ m

or   Δ Qf  = m Hf … …. … … (4)

Examples

• Ice changes at 0° C into water. Latent heat of fusion of ice  is 3.36 × 105 J kg -1. That is; 3.36 × 105 Joule heat is required to melt 1 kg of ice into water at 0°C.

Latent heat of vaporization

Heat energy required to change unit mass of a substance from liquid to gaseous state at its boiling point without change in its temperature is called latent heat of vaporization.It is represented by H v. Its S.I unit is J/kg. Its formula is given as:

Hv = ΔQv/m

Example

When water is heated,it boils at 100 °C under standard pressure.Its temperature remains 100 °C until it is changed completely into steam.Its latent heat of vaporisation is 2260000 J/kg.That is one kilogram of water requires 2260000 Joule heat to change it completely into steam at its boiling point.

Table of latent heat of vaporisation of some substances

 Substance Heat of vaporisation (Hv) in kJ/kg Aluminium 10500 Copper 4810 Gold 1580 Helium 21 Lead 858 Mercury 270 Nitrogen 200 Oxygen 210 Water 2260