The following diagram displays a chemical reaction. Observe it carefully and answer the following questions:
$(a)$ Identify the type of chemical reaction that will take place and define it. How will the colour of the salt change?
$(b)$ Write the chemical equation of the reaction that takes place.
$(c)$ Mention one commercial use of this salt.

(N/A) It is a photochemical decomposition reaction. It can be defined as the decomposition of a substance in the presence of sunlight in which a single reactant breaks down to give simpler products.
$(b)$ The white silver chloride changes to grey,as it decomposes into silver and chlorine in the presence of sunlight.
The chemical equation is:
$2 AgCl (s) \xrightarrow{\text{Sunlight}} 2 Ag (s) + Cl_2 (g)$
$(c)$ This salt is used in black and white photography.