In a chemical reaction, what distinguishes a catalyst from a reactant?

A catalyst is a substance that increases the rate of a chemical reaction without being consumed or permanently altered during the process. This key characteristic distinguishes it from reactants, which are consumed during the chemical change and lead to the formation of products.

When a catalyst is involved in a reaction, it typically provides an alternative pathway with a lower activation energy, allowing the reaction to proceed more quickly. However, once the reaction is complete, the catalyst remains unchanged in both identity and amount, ready to participate in further reactions. This is why the statement that a catalyst does not change after the reaction is the correct answer; it highlights the fundamental role of catalysts in chemical reactions and their unique position compared to reactants.

Other options do not accurately represent the nature of catalysts. While catalysts do lower the activation energy required for a reaction to occur (which enhances the rate), they are not consumed, do not alter the reaction products, and thus can facilitate repeated executions of the same reaction cycles.