Question

Hypo solution is used in photography because of its:
A. Reducing behavior
B. Oxidizing behavior
C. Complex-forming behavior
D. Reaction with light

Answer 1

Hint: Hypo solution is nothing but sodium thiosulphate. Photography film contains silver halide crystals that define resolution, sensitivity, and contrast of the film. Try to figure out how silver halide reacts with complexity.

Complete step by step answer: We know that photography film is coated with a layer of silver halide crystals. During the development of the image, this plate is exposed to a low amount of sunlight. This develops a partial image on the plate which is almost invisible which is called a latent image. As this plate contains silver halide crystals when they are exposed to sunlight silver ion gets reduced silver. These silver atoms form traces on the photography plate which obstruct light and appear black.
Silver halide molecules that are unconverted in the exposure to light needed to be removed to form a clear image. To remove leftover silver halide crystals we need to use sodium thiosulphate solution. When we wash the photographic plates in the sodium thiosulphate solution the silver halide molecules will be converted to a soluble complex and separate.
The reaction will be:
$$2N{a}_2{S}_2{O}_3+AgBr\to N{a}_3[Ag(S_2{O}_3{)}_2]+NaBr$$
Therefore, option C is correct.
Note: Removal of silver halide crystals process is called fixing of image. Ammonia can also be used as a complex-forming agent but it creates an unpleasantness which is harmful to the photographer so nowadays we are not using an ammonia solution as a complexing agent.