Which best defines a catalyst in chemistry?

• A. A substance that increases the rate of a chemical reaction without being consumed.
• B. A substance that changes the color of a solution.
• C. A substance that slows down a chemical reaction.
• D. A substance that is consumed to drive the reaction.

Answer: (A)

A catalyst speeds up a chemical reaction without being consumed. It does this by providing an alternative pathway with lower activation energy, so more reactant molecules can react at the same temperature. Because the catalyst is not used up in the process, a small amount can continuously speed up many cycles of the reaction.

Catalysts influence how fast equilibrium is reached but not the final equilibrium position itself; they make the system reach equilibrium faster without changing the amounts at equilibrium. Examples include enzymes, which are biological catalysts, and metals like platinum used in industrial processes.

In contrast, a substance that changes color is typically an indicator and not a catalyst; something that slows a reaction is an inhibitor; and something that is consumed to drive the reaction is simply a reactant, not a catalyst.