In which of the compounds, the oxidation state of hydrogen is `-1`

Options

A. `H_2O`
B. `CaH_2`
C. `HBr`
D. `H_2S`

Answer

To determine in which of the given compounds the oxidation state of hydrogen is -1, we need to analyze the compounds based on the concept of oxidation states.

### Step-by-Step Solution:

1. **Understanding Oxidation States**:
- The oxidation state of hydrogen is typically +1 when it is bonded to more electronegative elements (like oxygen, nitrogen, etc.).
- However, hydrogen can have an oxidation state of -1 when it is bonded to less electronegative elements, particularly metals, forming hydrides.

2. **Identifying Hydrides**:
- Hydrides are compounds formed between hydrogen and metals. In these compounds, hydrogen usually exhibits an oxidation state of -1.
- Examples of hydrides include NaH (sodium hydride), CaH₂ (calcium hydride), and LiH (lithium hydride).

3. **Analyzing the Given Compounds**:
- We need to check the electronegativity of the elements that hydrogen is bonded to in each compound.
- If hydrogen is bonded to a metal (which is less electronegative than hydrogen), its oxidation state will be -1.

4. **Conclusion**:
- From the analysis, we find that in compounds like CaH₂ (calcium hydride), the oxidation state of hydrogen is -1 because calcium is less electronegative than hydrogen.
- Therefore, the correct answer to the question is that the oxidation state of hydrogen is -1 in calcium hydride (CaH₂).

### Final Answer:
The oxidation state of hydrogen is -1 in the compound **CaH₂** (calcium hydride).

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