This is due to the “law of conservation of mass.” This law was discovered by Antoine Lavoisier after he observed that, in an isolated system, mass cannot be created or destroyed after chemical or physical changes. The sum of the mass of the product(s) must equal the mass of the reactant. In other words, regardless of changes in appearance, no part of the substance “disappears” and nothing is “created.” When a substance changes its state the molecules simply rearrange themselves.
It can be as simple as melting an ice cube (physical change). The liquid water has the same number of atoms as the ice cube you started with. Another common example is the assembly of glucose by plants (chemical change). When you look at the equation 6CO2+6H2O=C6H12O6+6O2 you created glucose and oxygen from carbon dioxide and water. The substances changed, but if you count the number of each atom in the reactant and product side, you notice that they are equal.
One neat note is that the universe as a whole is considered an isolated system. What this means is that the atoms we have right now are the same ones we will always have; they will just keep rearranging!
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