Define a catalyst .

Explanation

(i) The substance in whose presence the rate of a chemical reaction changes, without causing any chemical change to it is called a catalyst.
(ii) e.g. Potassium chlorate `(KClO_(3))` decomposes rapidly in presence of manganese dioxide `(MnO_(2))` to liberate `O_(2)` gas, where manganese dioxide acts as a catalyst.
(iii) `2KClO_(3)overset(Delta)underset(MnO_(2))to2KCl +3O_(2)`