Leaving Certification (LC) Chemistry Practice Exam 2025 - Free Chemistry Practice Questions and Study Guide

A catalyst is defined as a substance that alters the rate of a chemical reaction without being consumed in the process. This means that while it facilitates the reaction—often by providing an alternative pathway with a lower activation energy—it remains unchanged at the end of the reaction and can be used repeatedly. Catalysts can increase the rate of both forward and reverse reactions, thereby helping to reach equilibrium more quickly without affecting the overall dynamics of the equilibrium position.

In contrast, the first choice describes a substance that participates in the reaction and is consumed, which does not align with the role of a catalyst. The third choice incorrectly associates catalysts with temperature changes; while catalysts can influence reaction rates, they do not inherently change the temperature of the system. Lastly, the fourth option defines a reactant that initiates a reaction, which is fundamentally different from the role of a catalyst since catalysts are not reactants and do not get consumed. Therefore, the definition of a catalyst as a substance that alters the rate of a reaction without being consumed is the most accurate and fitting description.