Which of the following best defines a catalyst?

• A. Speeds up the rate of a chemical reaction without being consumed or changed.
• B. Consumes itself to drive the reaction.
• C. Slows down the reaction.
• D. Is a reactant that gets converted.

Answer: (A)

A catalyst is a substance that speeds up a chemical reaction by providing an alternative pathway with a lower activation energy, and it remains chemically unchanged at the end of the reaction. Because it is not consumed, the same catalyst can facilitate many cycles of the reaction, appearing in the mechanism but not changing the overall reactant–product balance. This is why the statement that describes it as speeding up the rate without being consumed or changed is the best fit. The other ideas describe substances that are used up during the reaction, slow the reaction, or are simply reacting participants, none of which match how a catalyst behaves.