Name a species which can act both as conjugate acid and conjugate base.

Explanation

To answer the question of naming a species that can act both as a conjugate acid and a conjugate base, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Conjugate Acids and Bases**:
- A conjugate acid is formed when a base accepts a proton (H⁺).
- A conjugate base is formed when an acid donates a proton (H⁺).

...