A catalyst is something which speeds up a chemical reaction without itself being one of the reactants or products (in other words, without being consumed).
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(hr) A catalyst is a substance that accelerates a chemical reaction without being changed by the reaction. Different substances catalyze different reactions. For example, platinum catalyzes the burning of hydrogen in air, acid catalyzes the decomposition of starch into glucose, and iron catalyzes the decomposition of hydrogen peroxide into water and oxygen.