Question

A light sensitive compound ‘X’ of silver is used in black and white photography. On exposure to sunlight its color changes to grey.
a) Identify X
b) Write a chemical equation to express the above changes.
c) Identify the chemical reaction.

Answer 1

Hint: The silver halides are the compounds which are sensitive to the lights. They will undergo chemical reactions to give the black and white photography.

Complete answer:
The light sensitivity property of the silver halides is the key reason why it is used in photographic processes. Tiny crystals of these silver halides are used for making the photographic films. When these crystals are exposed to sunlight, that time a chemical reaction takes place. This causes the film to darken and hence the image will be produced.
Photography is a technique which is based on photochemical decomposition reaction. Photochemical decomposition reaction is a type of chemical reaction in which a compound is broken down into simpler substances in the presence of a light. The photographic films contain silver salts. When the picture is taken, these salts in the film absorb light and it will get converted into metallic silver, which produces a dark image on the negative.
Hence the light sensitive compound of silver can be silver chloride, silver bromide and silver iodide.
The chemical reaction taking place are shown below:
$2AgC{l}_{(s)}\to 2A{g}_{(s)}+C{l_2}_{(g)}$
The chemical reaction which is taking place in this process is photochemical decomposition.

Additional Information:
- All silver halides can be used in the photographic films.
- Silver halides such as Silver bromide and Silver iodide can also give the following reaction.
$2AgB{r}_{(s)}\to 2A{g}_{(s)}+B{r_2}_{(g)}$
$2Ag{I}_{(s)}\to 2A{g}_{(s)}+{I_2}_{(g)}$

Note: The photochemical and thermal decomposition are different from each other.
- Thermal decomposition is a decomposition reaction in which the compound is broken down into simpler forms in the presence of heat.
- Photochemical reaction is a decomposition reaction in which compound is broken down into simpler substances in the presence of light.