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 given options based on their ability to accept protons (H⁺) and donate protons. A species that acts only as a base will only accept protons and will not have the ability to donate them.

### Step-by-Step Solution:

1. **Identify the Species**: We have four species to evaluate. Let's denote them as:
- Option 1: H₂
- Option 2: HS⁻
- Option 3: S²⁻
- Option 4: H₂O

2. **Analyze Option 1 (H₂)**:
- H₂ can donate a proton (H⁺) to form H⁻. Therefore, it can act as an acid.
- **Conclusion**: H₂ can act as both acid and base. **Not the answer**.

3. **Analyze Option 2 (HS⁻)**:
- HS⁻ can donate a proton to form S²⁻. Hence, it can act as an acid.
- It can also accept a proton to form H₂S, which means it can act as a base as well.
- **Conclusion**: HS⁻ can act as both acid and base. **Not the answer**.

4. **Analyze Option 3 (S²⁻)**:
- S²⁻ has no hydrogen atoms to donate as protons. It can only accept protons to form HS⁻.
- **Conclusion**: S²⁻ acts only as a base. **This is a potential answer**.

5. **Analyze Option 4 (H₂O)**:
- H₂O can donate a proton to form OH⁻, thus acting as an acid.
- It can also accept a proton to form H₃O⁺, acting as a base.
- **Conclusion**: H₂O can act as both acid and base. **Not the answer**.

6. **Final Conclusion**:
- The only species that acts solely as a base is **Option 3: S²⁻**.

### Answer:
**Option 3: S²⁻ acts only as a base.**