What is a catalyst?

• A. A substance that speeds up the rate of a chemical reaction
• B. A substance that slows down the rate
• C. A substance that is consumed in the reaction
• D. A substance that changes colour

Answer: (A)

A catalyst speeds up the rate of a chemical reaction by providing an alternative pathway with a lower activation energy, so more particles have enough energy to react at the same temperature. It isn’t consumed in the reaction, so it can be reused to catalyze more rounds. The other ideas don’t fit: something that slows the rate is an inhibitor, not a catalyst; something that is consumed in the reaction would be a reactant; and a colour change is typically due to indicators or dyes, not the general function of a catalyst.