When does a chemical reaction stop? the lab is finished When the excess reactant is used up When the limiting reactant is used up Chemical reactions never stop

## Step 1<br />A chemical reaction stops when the limiting reactant is used up. The limiting reactant is the reactant that is completely consumed in a chemical reaction. It determines the maximum amount of product that can be formed from the reaction.<br /><br />## Step 2<br />Once the limiting reactant is completely consumed, the reaction stops because there is no more of it to react with the other reactants. This is why the correct answer is "When the limiting reactant is used up".<br /><br />## Step 3<br />The other options are incorrect. The lab finishing does not necessarily mean the reaction has stopped. The excess reactant is the reactant that is left over after the reaction has taken place, so it does not determine when the reaction stops.<br /><br />## Step 4<br />Chemical reactions can stop, so the statement "Chemical reactions never stop" is also incorrect.