What defines a catalyst in a chemical reaction?

A catalyst is defined as a substance that increases the rate of a chemical reaction without being consumed in the process. It achieves this by providing an alternative reaction pathway with a lower activation energy, which allows more reactant particles to collide with sufficient energy to form products. This process enhances the speed of the reaction.

Choosing the option that describes a catalyst as one that speeds up the reaction by providing an alternative pathway captures this essential role. When a catalyst is present, the reaction can proceed more efficiently, but the catalyst itself remains unchanged at the end of the reaction, which means it can be used repeatedly.

The other options do not align with the definition of a catalyst. For instance, a substance that is consumed in the reaction cannot act as a catalyst since it would not be available for future reactions. Similarly, a substance that slows down a reaction contradicts the fundamental role of a catalyst, which is to enhance the reaction rate. Lastly, stating that a substance has no effect on the reaction rate neglects the influence that a catalyst has in accelerating reactions. Thus, option C accurately represents the defining characteristics of a catalyst.