CBSE Class 9 Answered

Molecular motions are associated with kinetic energy. Faster the molecules move, higher is the kinetic energy. As a liquid solidifies, its molecules comes close together.

When water is converted to ice at the same temperature, there are changes involved in internal potential energy. The molecular motion decreases as heat is released to the surroundings.

Energy involved in the phase chnage = latent heat x Mass

= - 80cal/g x 1000 g

= - 80,000 cal

(-) sign indicates that thermal energy is decreased.

Therfore, when 1 kg water is converted to ice at same temperature, 80,000 cal energy is released (exothermic).

Latent heat of fusion is the amount of heat(energy) which is taken from the surrounding or added to the system in order to change it from solid to liquid at constant temperature.