Which acid can combine with its own salt again 

Options

A. `HF`
B. `HBr`
C. `HCl`
D. `HI`

Answer

To determine which acid can combine with its own salt again, we need to analyze the given acids and their behavior in forming acid salts.

### Step-by-Step Solution:

1. **Identify the Acids**: We need to consider the four acids mentioned in the question. For this solution, we will focus on hydrofluoric acid (HF) as the primary example.

2. **Understand Acid-Salt Formation**: An acid can combine with its own salt if it can donate a proton (H+) to the anion of its salt, forming an acid salt. This typically occurs when the acid is weak and can exist in equilibrium with its salt.

3. **Analyze Hydrofluoric Acid (HF)**: Hydrofluoric acid is a weak acid that can dissociate in water to form fluoride ions (F-) and hydrogen ions (H+).

4. **Formation of Acid Salt**: When HF is in the presence of its salt (e.g., sodium fluoride, NaF), it can react with the fluoride ion (F-) to form a complex ion, which is known as an acid salt. The reaction can be represented as:
\[ HF + F^- \rightleftharpoons HF_2^- \]
Here, HF can combine with its own salt (F-) to form the di-fluoride ion (HF2^-).

5. **Conclusion**: Therefore, hydrofluoric acid (HF) is the acid that can combine with its own salt again, forming an acid salt.

### Final Answer:
The acid that can combine with its own salt again is hydrofluoric acid (HF).