In a chemical change, the molecules in the reactants interact to form new substances. In a physical change, like a state change or dissolving, no new substance is formed. Explain that another way to say that no atoms are created or destroyed in a chemical reaction is to say, “Mass is conserved.
They change matter by producing unique substances. Such as if I bond oxygen and hydrogen gas I'll be left with water although oxygen and hydrogen are gases when they react in a chemical reaction they can produce unique substances unlike the ones before. It's quite cool seeing that two gases can create a liquid that all life in this world relies on.