Which of the following statements is not true :

Options

A. Among halide ions, iodide is the most powerful reducing agent
B. Fluorine is the only halogen that does not show a variable oxidation state
C. HOCl is a stronger acid than HOBr
D. HF is a stronger acid than HCl

Answer

To determine which of the following statements is not true, let's analyze each statement one by one:

1. **Statement A**: Among halide ions, iodide is the most powerful reducing agent.
- **Analysis**: This statement is true. The iodide ion (I⁻) has the lowest standard reduction potential among the halide ions, making it the most powerful reducing agent.

2. **Statement B**: Fluorine is the only halogen that does not show a variable oxidation state.
- **Analysis**: This statement is also true. Fluorine only exhibits an oxidation state of -1 because it has no d-orbitals available for electron promotion, unlike other halogens which can show multiple oxidation states.

3. **Statement C**: HOCl is stronger than HOBr.
- **Analysis**: This statement is true as well. HOCl (hypochlorous acid) is stronger than HOBr (hypobromous acid) because chlorine is more electronegative than bromine, which makes HOCl a stronger acid.

4. **Statement D**: HF is a stronger acid than HCl.
- **Analysis**: This statement is not true. Hydrochloric acid (HCl) is a stronger acid than hydrofluoric acid (HF). The bond in HCl is weaker than in HF, allowing HCl to dissociate more easily in water to release H⁺ ions, making it a stronger acid.

**Conclusion**: The statement that is not true is **Statement D**: HF is a stronger acid than HCl.

### Summary of the Solution:
- Statement A is true.
- Statement B is true.
- Statement C is true.
- Statement D is not true (HCl is stronger than HF).

Thus, the answer to the question is **Statement D**.