A solution of an acid with pH 5.1 is given. Which of the following can be done to increase its pH?
i. add distilled water to it
ii. add a solution of a different acid with pH 5.8
iii. add few drops of a base with an unknown pH

Options

A. only i
B. only iii
C. only i and ii
D. any of i, ii and iii

Answer

To determine how to increase the pH of a solution with a pH of 5.1, we will analyze each option provided:

1. **Add distilled water to it**:
- When distilled water is added to an acidic solution, it dilutes the acid. This dilution decreases the concentration of hydrogen ions (H⁺) in the solution, which in turn increases the pH. Since the pH scale ranges from 0 (very acidic) to 14 (very basic), diluting an acid will move the pH closer to neutral (7), thus increasing it.
- **Conclusion**: This option will increase the pH.

2. **Add a solution of a different acid with pH 5.8**:
- The pH of the second acid (5.8) is higher than that of the first acid (5.1). When you mix two acids, the resulting pH will depend on their concentrations and the strength of the acids. However, since the second acid has a higher pH, it is less acidic than the first. Mixing it with the first acid will likely result in a pH that is still acidic but closer to neutral than 5.1. Therefore, this could potentially increase the pH of the original solution.
- **Conclusion**: This option may also increase the pH.

3. **Add a few drops of a base with an unknown pH**:
- Adding a base to an acidic solution will neutralize some of the hydrogen ions present in the solution, which will increase the pH. Even if the pH of the base is unknown, as long as it is above 7 (which is typical for bases), it will definitely increase the pH of the acidic solution.
- **Conclusion**: This option will increase the pH.

### Final Answer:
All three options (i, ii, and iii) can be done to increase the pH of the solution.