In short
A catalyst is a substance that makes a chemical reaction faster without being used up. It works by giving the reaction an easier route with a lower activation energy.
A catalyst is present in the same amounts at the end as at the start. It does not change the products or the position of equilibrium, only how fast it is reached. Examples are manganese(IV) oxide for the decomposition of hydrogen peroxide, iron in making ammonia, and enzymes in living things. Raising the temperature also speeds up a reaction, but by giving the particles more energy, not by changing the route.
Hydrogen peroxide breaks down slowly into water and oxygen. Adding a little manganese(IV) oxide makes it fizz, and the black powder can be filtered out unchanged afterwards.
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