3. Students measured the mass of the reactants and products for a combustion reaction they observed.
When they measured the mass of the products, they discovered the products had a greater mass than
the reactants. How did this unexpected change in mass occur?
A. Stadents made a measurement error, because ending with more products is impossible.
B. The reactants had hidden materials within them that had no mass until the reaction was complete.
C. The products of reactions often have greater mass, because the law of conservation is not
applicable in all chemical reactions.
D. The reaction occurred in an open system, so during the reaction a gas interacted with the reactants.
The mass of the gas could not be measured before the reaction.

The law of conversation of matter tells us that in a chemical reaction, matter is never created or destroyed, it's simply converted from one form to another. So the mass of reactants should always equal the mass of the products in a chemical reaction. If there is excess mass in the product, the students have made an error of some kind.