The Law of Conservation state that matter can neither be created nor destroyed it can only be changed from one form to another.
Starting with an ice cube of one mole, #6.02x10^23# molecules, of water at 0C, you would begin melting the iceas the temperature increases. The molecules of water would change ftom the crystal solid phase and enter the liquid phase, but you would still have one mole of molecules. Eventually all #6.02x10^23# molecules would be water in a liquid form. The temperature would continue to rise and those excited water molecules would begin to jump off of your puddle of water and vaporize at 100 C. But even though these molecules are now in a gas phase, you would still have one mole of water molecules. Eventually, the heat would rise to a point where all of the water would evaporate into gas and youbwould still have one mole, #6.02x10^23# molecules of water in a gas phase.
Matter was not created, matter was not destroyed, matter only changed from one form to another.
I hope this was helpful.