Which term defines a substance that increases the rate of a chemical reaction?

• A. A substance that slows down a chemical reaction
• B. A substance that acts as a solvent
• C. A substance that increases the rate of a chemical reaction
• D. A substance that changes color of a solution

Answer: (C)

A substance that speeds up a chemical reaction is a catalyst. It works by providing an alternative reaction pathway with lower activation energy, so more molecules have enough energy to react in the same amount of time. Importantly, the catalyst is not consumed in the reaction, so it can catalyze multiple cycles. The other ideas aren’t the right term here: something that slows a reaction is an inhibitor, not a catalyst; a solvent is the medium in which the reaction occurs and can influence rate indirectly but does not define what speeds up the reaction; a color change describes an observable result (often from an indicator) rather than the mechanism that increases the rate. Enzymes are examples of biological catalysts.