Final answer:
The formation of water from hydrogen and oxygen demonstrates the conservation of mass because the number of atoms of each element in the reactants must be the same as in the products.
Step-by-step explanation:
The formation of water from hydrogen and oxygen demonstrates the conservation of mass because the law of conservation of matter states that matter cannot be created or destroyed. In a chemical equation, the number of atoms of each element in the reactants must be the same as the number of atoms of each element in the products. In the case of water formation, there are two hydrogen atoms and one oxygen atom in the reactants, and there are two hydrogen atoms and one oxygen atom in the product. Therefore, the atomic mass of the reactants equals the atomic mass of the product, and the product has the same number of atoms as the reactants.