How can you tell which reactant is in excess at the end of a reaction?

• A. The excess reactant will be visible.
• B. The mixture changes color.
• C. The gas evolves.
• D. The temperature drops.

Answer: (A)

When a reaction has a limiting reactant and an excess, the reaction can only go on until the limiting reactant is used up. After that, any leftover amount of the other reactant remains in the mixture. So the clearest sign that one reactant is in excess is that some of it is still present once the reaction seems to have stopped.

In many setups this leftover is visible, especially if that reactant is a solid. For example, if a metal reacts with an acid, the acid is used up first and solid metal pieces that didn’t react will still be there, showing which reactant was in excess.

The other observations aren’t reliable indicators of which reagent is excess: a color change, gas evolution, or a temperature change can occur during the reaction regardless of which reactant is leftover.