Sodium thiosulphate is used in photography because of its:

Options

A. oxidising behaviour
B. reducing behaviour
C. complexing behaviour
D. photochemical behaviour

Explanation

To answer the question regarding why sodium thiosulphate is used in photography, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Sodium Thiosulphate**: Sodium thiosulphate is a chemical compound with the formula Na2S2O3. It is commonly known as "hypo" in photography.

2. **Role of Silver Bromide (AgBr)**: In traditional photography, silver bromide (AgBr) is used as a light-sensitive material. When exposed to light, AgBr decomposes into metallic silver (Ag) and bromine gas (Br2).

3. **Formation of Black and White Images**: The decomposition of AgBr results in the formation of metallic silver, which appears black on the photographic film, creating the image.

4. **Need for a Fixer**: After the exposure, unreacted AgBr remains on the film. To obtain a clear photograph, this unreacted AgBr must be removed. This is where sodium thiosulphate comes into play.

5. **Complex Formation**: Sodium thiosulphate reacts with the unreacted AgBr to form a soluble complex. The reaction can be represented as:
\[ \text{AgBr} + \text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Na}_3[\text{AgS}_2\text{O}_3] + \text{NaBr} \]
This complex is soluble in water, allowing the unreacted AgBr to be washed away.

6. **Conclusion**: Therefore, sodium thiosulphate is used in photography primarily due to its **complexing behavior** with unreacted AgBr, which allows for the removal of excess light-sensitive material, resulting in a clear photograph.

### Final Answer:
Sodium thiosulphate is used in photography because of its **complexing behavior** with unreacted AgBr.

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