What is a catalyst?

• A. A substance that speeds up a chemical reaction by being consumed.
• B. A substance that speeds up a chemical reaction without being consumed.
• C. A substance that slows down a chemical reaction.
• D. A protein that binds to water.

Answer: (B)

A catalyst speeds a chemical reaction without being consumed. It works by providing an alternative reaction pathway with lower activation energy, so more reacting molecules have enough energy to reach the transition state and react per unit time. Because the catalyst remains chemically unchanged at the end, it can catalyze many cycles and doesn’t get used up, though it may participate transiently during the reaction mechanism. Catalysts generally speed up both forward and reverse processes, affecting how fast equilibrium is reached rather than the final balance of products and reactants.

The option that says it is consumed is incorrect, and the idea that a catalyst slows a reaction describes an inhibitor, not a catalyst. A protein binding to water alone isn’t a description of catalysis; enzymes (biological catalysts) work by binding substrates and stabilizing transition states to accelerate the reaction.