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The law of conservation of mass states that matter can be neither created nor destroyed. Because matter is composed of atoms that are unchanged in a chemical reaction, it follows that mass must be conserved as well. In order to demonstrate the validity of the law we will need to design an experiment that appears to “destroy” one substance to “create” another but be able to show that although the character of the substances in the reaction has changed, the mass remains constant.

The experiment we are going to conduct involves the addition of zinc metal (Zn⁰) to an acidic solution of copper (II) sulfate (CuSO₄ containing Cu⁺² and SO₄^-2 ions). If we know the initial masses of both the solution and the metal pieces before the reaction, we can track how the masses have changed after the reaction. There are actually two reactions that occur with these reactants, and they are summarized below:

Zn⁰(s) + Cu⁺²(aq) Zn⁺² + Cu⁰(s)
Zn⁰(s) + 2HCl(aq) Zn⁺² + 2Cl^-(aq) + H₂(g)
Net Reaction: 2 Zn⁰(s) + Cu⁺²(aq) + 2HCl(aq) 2 Zn⁺² + Cu⁰(s) + 2Cl^-(aq) + H₂(g)

These are both called oxidation-reduction reactions, and the two reactions can be summed to give the indicated net reaction. It is important to note in the above equation that in addition to solid copper being produced, some zinc metal is being ionized to Zn⁺², and hydrogen gas (H₂) is produced. This gas production adds another part of the mass that we would like to be able to quantify if we are going to keep an accurate tracking of what happens to the mass of the reagents as the reaction occurs. We will do this by providing a container to “trap” the gas as it evolves. We will measure the volume of the H₂ gas formed and from that we can calculate its mass. Since we measure the mass of the solution, it will include any change in mass of the Cu⁺² reactant and the Zn⁺² product. There is no change in the chloride ions.