Question

The chemical used as a fixer in photography is:

• A. Sodium thiosulphate
• B. Sodium sulphate
• C. Borax
• D. Ammonium sulphate

Hint:

Think of the compound called "hypo" used in the darkroom.

Answer 1

The Correct Option is A

Step 1: Recall what a fixer does.
In black and white photography, after the film is developed, unexposed silver halide (AgBr) grains are still present on the film.
Step 2: Identify the chemical role.
A fixer dissolves this unused silver halide so the image does not darken further when exposed to light.
Step 3: Match the compound.
Sodium thiosulphate (Na₂S₂O₃), commonly called "hypo", forms a soluble complex with silver ions and washes away the unexposed silver halide. This is why it is used as the fixer.
Step 4: Check the other options.
Sodium sulphate has no complexing action on silver salts. Borax is a cleaning and buffering agent, not a fixer. Ammonium sulphate is used in fertilisers, not in film processing.
Final Answer: A) Sodium thiosulphate.