Latent Heat

Take a few pieces of ice in a beaker. Insert a thermometer. Note the temperature. You will observe it to be 0⁰C.

Now place the beaker on a flame and heat it. Ice starts melting. As you supply heat to ice, its temperature should rise. But you will find that the temperature of ice remains at 0⁰C until all the ice melts into water. If heating is continued you will notice that the temperature starts rising. Why does the temperature not rise until all the ice melted into water?

The heat supplied to a solid at its melting points is used up by the solid molecules for the change of state process, i.e., the solid molecules use the heat supplied to break free from intermolecular attraction so that the solid melts into liquid form.

The heat supplied to a solid at its melting point is used up for change of state process and is known as the latent heat of fusion.

Latent heat of fusion of a solid is defined as the heat energy required by 1 kg of the solid at its melting point to completely melt into 1 kg of liquid

Latent heat of fusion of ice = 3.35 x 10⁵J/kg

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Experiment 2

Experiment 2

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Now, heat the water at 0⁰C further. Observe that the temperature of water rises till the water starts boiling. At sea level the water boils at 100⁰C. As soon as water starts boiling, its temperature remains constant, even if heating is continued. 'The temperature at which a liquid starts boiling is known as boiling point'. Boiling point is a characteristic property of a given liquid.

What happens when a liquid starts boiling? The liquid molecules escape from the surface of the liquid and move into vapor or gaseous state. The process in which liquid molecules escape from the surface of the liquid to become vapor is known as evaporation. At boiling point evaporation is maximum.

Heat supplied to a liquid at its boiling point is utilized by liquid molecules to turn into vapor form. 'The heat energy required by 1 kg of the liquid at its boiling point to completely turn into vapor form is known as the Latent heat of Vaporization'.

Latent heat of vaporization of water = 2.26x10⁶J/kg.

We have seen that during the process of change of state from liquid to vapor state, the liquid absorbs heat energy. In the reverse process i.e. when the vapors are cooled to condense into liquid state, they liberate the same amount of energy. This is why steam burns are more severe than burns due to boiling water.

Similarly, when liquids are cooled sufficiently to become solids, they liberate energy.