A catalyst changes the rate of a chemical reaction but is not used up during it. At the end of the reaction you have the same mass of catalyst as you started with. Different reactions need different catalysts. In living things, enzymes act as catalysts in biological systems.
A catalyst works by providing a different pathway for the reaction, and this pathway has a lower activation energy. With a lower activation energy, more collisions have enough energy to react, so the reaction is faster and the rate of reaction increases. On a reaction profile, the peak for the catalysed reaction is lower than the peak for the uncatalysed reaction.
You can identify a catalyst in two ways. It speeds up the reaction, and it does not appear in the chemical equation for the reaction because it is not used up. For example, hydrogen peroxide decomposes slowly: 2H2O2 → 2H2O + O2. A solid added to it can make oxygen bubble off quickly, yet the solid is not in the equation, so it is a catalyst. You can compare the effect of different metal salts this way.
