Freezing

Freezing

Let us now try to reverse the process. Let us start with the liquid and cool it slowly. What happens?

How does the temperature of a liquid vary when it is cooled till it freezes?

Now let us start with the liquid naphthalene that we got from the previous experiment. Let us allow it to cool while observing the temperatures at regular intervals of time till the liquid completely freezes or solidifies. Let us plot a graph of temperature versus time. This curve is called the cooling curve. This shows that how the temperature of a pure solid changesasitiscooledtoitsfreezingpointand beyond.

Cooling Curve 

Let us now analyse the curve.

Why the temperature remains a constant between F – G?

The entire heat energy is given out at this stage as the particles of the liquid get attracted to each other. This released energy is absorbed by the surroundings.

Hence there is no increase in temperature for naphthalene. Both liquid and solid states co- exist at this stage. This hidden energy is called latent heat of freezing which is the same as latent heat of fusion, This latent heat is released when there is a change of state from liquid to solid.