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Question: The graph alongside is representing a cooling curve for a substance which is being cooled from a hig...

The graph alongside is representing a cooling curve for a substance which is being cooled from a higher temperature to a lower temperature. What happens in the regions of BC and DE?

Explanation

Solution

The latent heat of a substance, may it be the latent heat of fusion or heat of vaporization commonly represented by an uppercase L. The latent heat of fusion is the amount of energy that is needed to completely convert from solid to liquid (melting) a particular substance. The latent heat of vaporization is the amount of energy that is needed to completely convert from liquid to gas (vaporize) a substance.

Complete step-by-step answer:
In BC, we can see that there is a variation in phase of substance from vapour to liquid. Heat is being released during this phase change with temperature keeping at constant. In DE there is a change in phase of the substance from the liquid state to solid state. Heat is being released during phase change with temperature kept constant. As the region BC indicates the change of state from vapour to liquid, this is the region during which the latent heat of vaporization is required. During DE, the change is happening from the phase of liquid to solid where latent heat of fusion is needed. Here the latent heat of fusion will be less than the latent heat of vaporization. Hence, DE is shorter than BC. These are the things occurring in the regions of DE and BC. Therefore the answer for the question has been obtained.

Note: In order to calculate the energy required to convert a specific substance from solid to liquid, the product of the mass of that substance (in kilograms) and the latent heat of fusion are taken. In the same way, for calculating the energy needed to vaporize a substance, the product of the mass of the liquid and the heat of vaporization of that particular substance is taken. When the material is undergoing a phase change, all the energy is used to break the intermolecular bonds within that material. Hence the kinetic energy will remain constant, as the temperature is a constant during a phase change.