Question

While boiling the water a student observed that temperature remains constant at 100°c tell whole of water vaporises. explain?

Answer 1

The temperature of a substance reflects the average kinetic energy of the particles. If addition of heat increases the kinetic energy, as is usually the case, the temperature goes up. But at a phase transition, the heat energy goes into bond breaking, in a sense being converted to potential energy (potential to re-form a bond).

When the bond between neighboring molecules in a boiling liquid is broken, the freed molecule can evaporate, leaving the system with the same average kinetic energy per molecule, and the same temperature.

Evaporation is key to maintaining constant temperature, as the more energetic molecules escape.