In black-and-white photography, what are the major chemical changes involved in exposing, developing, and fixing?

A black and white photography undergoes certain chemical changes during exposing, developing and fixing.

Theprocess of film exposure is defined as the photons from the light entering into the lenses during exposure that causes oxidation as well as reduction of atoms in that film.

Theprocess of image development involves the image from exposure to be reduced further. This image is also known as latent image.

The final process that takes place in the photographic changes is known as image fixation..

This photographic film is composed ofAgBrmicrocrystals that react depending on the amount of light entering these lenses. The greater the area getting exposed, the greater is the number of photons. This is the reason as to why it appears lighter in the image.

The chemical reaction is given by:

$AgBr\to Ag+Br$

More $A{g}^{+}$ions are reduced by a weak reducing agent known as hydroquinone $H_{2}Q$and thus make the picture clearer. Finally, at the end of the process, black clusters of silver are formed.

The chemical reaction is given by:

$2A{g}_{\left(s\right)}^{+}+H_{2}Q\left(reduced\right)\to 2A{g}_{\left(s\right)}+Q\left(oxidized\right)$

After the reaction with hydroquinone, the reduction needs to be stopped. Otherwise, the film will turn black. Image fixation helps in the removal of excess silver ions by adding sodium thiosulfate (hypo). This helps in the formation of a soluble complex with silver and can be washed away.