Question

For any substance, why does the temperature remain constant during the change of state?​

Answer 1

Because with the increase of temperature, more number of particles get enough kinetic energy to go into the vapour state. Hence temperature remains constant during the change of state.

Answer 2

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It is due to the latent heat as the heat supplied to increase the temperature of the substance is used up to transform the state of matter of the substance hence the temperature stays constant.