(a) Some chemical reactions take place when reactants are subjected to high pressure. e.g: Nitrogen and hydrogen when subjected to high pressure produce ammonia gas.
(b) Some chemical reactions can take place in the presence of light. Ex. Photosynthesis.
(c) A catalyst can either increases or decreases the rate of chemical reaction and some chemical reactions need a catalyst to change the rate of the reaction, in case it is too slow or too fast.
1. Positive catalyst: When a catalyst increases the rate of reaction finely divided iron is used as a positive catalyst in the manufacturing of ammonia from hydrogen and nitrogen.
2. Negative Catalyst: When a catalyst decreases the rate of reaction.
Ex. Phosphoric acid act as a negative catalyst to decrease the rate of the decomposition of hydrogen peroxide.
(d) Some chemical reactions take place only in the presence of heat. e.g. When lead nitrate is heated, it breaks into lead monoxide, nitrogen dioxide, and oxygen.
