Whenever the heat is supplied to the water at normal atmospheric pressure, its molecules gain the thermal energy and their kinetic energy increases, the inter-molecular bonding decreases due to which its molecules leave the surface of water and convert into the vapor form.
The action can be reversed by condensation.
The mass of the matter remains conserved irrespective of its phase.
There is a loss of weight in the water phase because some of the molecules are now in the gaseous state but if we calculate the total mass of all the water particles before and after the process it remains constant.
