In this post we will discuss about latent heat and its two forms. Let’s start with a question.
Why does the temperature remain constant during melting and boiling even when heat is being supplied to a substance continuously?
This is so because the heat supplied to the substance is used up in overcoming the intermolecular forces, and therefore, it does not show up as a rise in the temperature.
Thus, the heat supplied during melting and boiling remains hidden from the thermometer and is called as latent heat.
Depending upon the nature of the process, the types of latent heat are:
- Latent heat of fusion
- Latent heat of vaporization
Latent Heat of Fusion
The latent heat of fusion of a substance is the quantity of heat required to convert one unit mass, 1 g or 1 Kg, of the substance from solid state to the liquid state at its melting point without any change of temperature.
The value of latent heat of fusion of ice, at 0°C, is 3.35*105 J/Kg.
Thus, to melt 1 Kg of ice (at 0°C) into water (at 0°C), 3.35*105 J of heat energy is required.
Latent Heat of Vaporisation
The latent heat of vaporization of a substance is the quantity of heat required to convert one unit mass, 1 g or 1 Kg, of the substance from liquid state to gaseous state at its boiling point without any change of temperature.
The value of latent heat of vaporization of water, at 100°C, is 22.6*105 J/Kg.
Thus, to vaporise 1 Kg of water (at 100°C) into water vapour (at 100°C), 22.6*105 J of heat energy is required.
Why does steam at 100°C cause more severe burns than boiling water at 100°C?
Steam at 100°C has more heat in the form of latent heat than the boiling water.
So, when steam comes in contact with the skin, it releases more heat than the boiling water at 100°C. As a result, steam at 100°C causes more severe burns than the boiling water at 100°C.
Hence, it is concluded that the temperature remain constant during melting and boiling is because the heat supplied to the substance is used up in overcoming the intermolecular forces, so it does not show up a rise in the temperature.
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