Question

when the water gets soldified into ice,then the heat is evolved.how?

Answer 1

In matter the molecules posses thermal energy because of which they can move . In case of solids the thermal energy is too low to allow their movement (however they can oscillate about their mean position ) that is why solids have fixed shape and volume. In liquids and the gases the molecules have sufficient thermal energy so that they can move. This is the reason liquids and gases do not have fixed shape.

When water is in liquid state the molecules have high thermal energy so they are in continuous motion but when water freezes the average thermal energy of the molecules decreases in the solid state. This extra energy is liberated in the form of heat to the surroundings.

Freezing is just the reverse case of melting of ice.In case of melting heat is supplied to the molecules in solid ice which increases their thermal energy and thus ability of motion and the ice becomes water (liquid state).