How could you know which reactant is in excess?

• A. At the end of the reaction, the excess reactant will be visible
• B. The product changes color
• C. The reaction stops early
• D. The mixture becomes gas

Answer: (A)

In a reaction, the amount that gets used up first determines which reactant is in excess. The one that remains after the reaction has gone as far as it can is the excess reactant. So you can tell which is in excess by looking for leftover material once the reaction has finished—anything that hasn’t been consumed is the excess.

That’s why this option is the best: at the end you can often see leftovers of the excess reactant, whereas color changes, the reaction stopping early, or gas formation don’t reliably indicate which reactant is in excess.