Catalyst definition

• A. A catalyst is a substance that increases the rate of a reaction but remains chemically unchanged at the end.
• B. A catalyst is consumed in the reaction and becomes part of the product.
• C. A catalyst is a substance that lowers the pressure of the system.
• D. A catalyst is a substance that changes the products of the reaction.

Answer: (A)

A catalyst works by providing an alternative, lower-energy route for a reaction to proceed. Because this pathway requires less energy to reach the transition state, more molecules can react at the same temperature, so the rate increases. Crucially, the catalyst is not consumed in the reaction; it emerges unchanged at the end and can be used again. This means it speeds up the reaction without changing the final amounts of reactants and products (it just gets there faster). The other statements would imply the catalyst is used up, changes the products, or lowers the system’s pressure, none of which define what a catalyst does.