Question

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

Answer 1

Temperature remains the same during processes like boiling and melting as heat provided to the system gets used as latent heat. Latent heat during boiling is known as latent heat of vaporization and latent heat during melting is known as latent heat of fusion.

Complete step by step answer:
- On increasing the temperature of solid, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start to vibrate with high speed. The energy supplied by the heat overcomes the force of attraction between the particles. The particles start moving freely. A stage is reached when solid melts and is converted to liquid.
- The temperature at which solid melts to liquid at the atmospheric pressure is known as melting point. Melting is also known as fusion. When a solid melts, its temperature remains the same because during melting the heat provided to solid for its melting gets used up in changing the state by overcoming the forces of attraction between the particles.
- It is considered that this heat during melting gets hidden into the content of the beaker and is known as latent heat. Latent means hidden. The amount of heat energy that is required to change $1{\text{Kg}}$ of a solid into liquid at atmospheric pressure is known as latent heat of fusion.
- Similarly the temperature at which liquid starts converting to gas at atmospheric pressure is known as boiling point. Particles in the steam have more energy than water at the same temperature because water vapour has more energy than water as particles of steam absorb extra energy in the form of latent heat of vapourization.

Thus, for any substance, the temperature remains constant during the change in state.

Note:
The process of melting and boiling can be explained by the first law of thermodynamics which states that if some quantity of heat is supplied to system capable of doing work, then the quantity of heat absorbed by the system is equal to the sum of the increase in internal energy and the external work done by the system.