The law of conservation of mass states matter can be changed from one form to another, but the total mass stays the same in an isolated system. If a group of items is mixed then the total mass is the same as the individual masses added together. Those items may change states, like liquid from a solid, but the mass of both states will be the same.
Conservation of mass is seen most commonly in chemistry, mechanics and fluid dynamics. It has become a key component of chemistry, and the discovery of the law by Antoine Lavoisier was very important in the move from alchemy to the sciences we know and use in modern times.