On the basis of the form of energy required for the reaction, these reactions are of three types : ( a ) Thermal decomposition reaction : It is a type of decomposition reaction , which uses the energy in the form of heat . Examples : ( i ) Ferrous sulphate on heating , lose water of crystallization and forms dehydrated FeSO4 , which on decomposition gives ferric oxide , sulphur dioxide and sulphur trioxide . ( iii ) On heating, lead nitrate , decomposes to give yellow lead monoxide , nitrogen dioxide and oxygen gas . ( b ) Electrolysis ( Electrolytic decomposition ) : This reaction involves the use of electricity for the decomposition of the reactant molecules . Example : When electric current is passed through water , it decomposes to give oxygen and hydrogen. ( c ) Photochemical decomposition ( photolysis ) : This is a type of decomposition reaction which involves the use of light energy for decomposition . Example : ( i ) When silver chloride is exposed to sunlight, it decomposes to give silver metal and chlorine gas. In this reaction white color of silver chloride changes to grayish white due to the formation of silver metal. ( ii ) Similarly, silver bromide gives silver metal and bromine gas on photolytic decomposition.