Define a thermal dissociation reaction with a suitable example. Give an example of a photochemical decomposition reaction. Name a metallic oxide which on thermal decomposition is reduced to a metal.

Thermal dissociation reaction is a decomposition reaction in which a substance dissociates into two or more simpler substances on application of heat.

For example, on application of heat, ammonium chloride decomposes into Ammonia and HCl:

NH₄Cl $\xrightleftharpoons[\enspace\text{cool}\enspace]{\enspace\text{heat}\enspace}$ NH₃ + HCl

An example of photochemical decomposition reaction is the decomposition of silver salts in the presence of light:

2AgCl $\xrightarrow{\enspace☼\enspace}$ 2Ag + Cl₂

HgO is a metallic oxide which reduces to a metal on thermal decomposition as per the equation below:

2HgO $\xrightarrow{\enspace\Delta\enspace}$ 2Hg + O₂