Which one of the following species acts only as a base ?

Options

A. `H_2S`
B. `HS^-`
C. `S^(2-)`
D. `H_2O`

Answer

To determine which species acts only as a base, we need to analyze the behavior of each given species in terms of acid-base chemistry.

### Step-by-Step Solution:

1. **Understanding Bases**:
A base is defined as a species that can accept protons (H⁺ ions).

2. **Analyzing the Options**:
We will evaluate each option to see if it can act as a base only or if it can also act as an acid.

3. **Option 1: H₂S (Hydrogen Sulfide)**:
- H₂S can donate a proton (H⁺) to form HS⁻.
- Therefore, H₂S can act as an acid.

4. **Option 2: HS⁻ (Hydrosulfide Ion)**:
- HS⁻ can donate a proton to form S²⁻, thus acting as an acid.
- It can also accept a proton to form H₂S, thus acting as a base.
- Therefore, HS⁻ can act as both an acid and a base.

5. **Option 3: H₂⁻ (Hydride Ion)**:
- H₂⁻ does not have any protons to donate.
- It can only accept a proton (H⁺) to form H₂.
- Therefore, H₂⁻ acts only as a base.

6. **Option 4: H₂O (Water)**:
- Water can donate a proton (H⁺) to form OH⁻, acting as an acid.
- It can also accept a proton to form H₃O⁺, acting as a base.
- Therefore, H₂O can act as both an acid and a base.

7. **Conclusion**:
After analyzing all the options, the only species that acts only as a base is H₂⁻.

### Final Answer:
The species that acts only as a base is **H₂⁻ (Hydride Ion)**.

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