What term describes a substance that slows down a chemical reaction or prevents it from occurring?

• A. Catalyst
• B. Acid
• C. Inhibitor
• D. Base

Answer: (C)

A substance that slows down or stops a chemical reaction is an inhibitor. It works by interfering with the reaction steps—such as blocking active sites on a catalyst or enzyme, or neutralizing reactive intermediates—so collisions between reactant molecules are less likely to lead to products. This reduces the reaction rate or can halt the reaction entirely.

In contrast, a catalyst speeds up a reaction by providing an alternative pathway with lower activation energy. Acids and bases can influence reaction rates through changes in mechanism or proton transfer, but they are not defined by the action of slowing or stopping the reaction. So the best-fit term is inhibitor.