If you were to measure the mass of magnesium and hydrochloric acid before combining them in the test tube, how would that mass compare to the mass of reactants left in the test tube after the reaction? Explain your answer and how it corresponds to the law of conservation of mass.

The mass of product left in the test tube will be less than the initial measured mass of the reactants.

Explanation: This is because one of the products of the reaction is a gas (hydrogen) and it escapes as the reaction happens in an open system.

This is the reaction,

Mg(s) + 2HCl(aq) > MgCl2(aq) + H2(g)

So, evidently, only MgCl2 is left in the reaction test tube together with unreacted reactants.

But, the hydrogen gas that escapes accounts for the lesser mass at the end of the reaction.

The mass in the test tube at the end of the reaction +

the mass of Hydrogen gas that escapes = the mass of reactants before the reaction; consolidating the law of conservation of mass.