In class, we experimented with amounts of powders and liquids and the amount of gas produced. we found that as we changed the amounts, we would see more gas produced (along the slope of the mass of powder over mass of gas in the balanced equation) up until a certain point. This change in production was brought about by limiting and excess reactants. limiting reactants are the reactants you control the amount of in an experiment, excess reactants are exactly that, excess. There is an unlimited amount of this reactant in the equations. The reactant in excess matters because you can use the amount taken by it to calculate many aspects of a reaction, including the amount of excess reactant.