Buffer solution have constant acidity and alkalinity because :
1. these give unionised acid or base on reaction with added acid and bases
2. acids and alkalies in these solution are shielded from attack by other ions
3. they have large excess of `H^(+) or OH^(-) ` ions
4. they have fixed value of pH

Options

A. these give unionised acid or base on reaction with added
B. acids and alkalies in these solution are shielded from attack by other ions
C. they have large excess of `H^(+) or OH^(-) ` ions
D. they have fixed value of pH

Answer

To solve the question regarding why buffer solutions have constant acidity and alkalinity, we need to analyze the provided options based on our understanding of buffer solutions. Let's go through the options step by step.

### Step 1: Understanding Buffer Solutions
Buffer solutions are special solutions that resist changes in pH when small amounts of acids or bases are added. They typically consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

### Step 2: Analyzing Each Option
1. **Option 1: These give unionized acid or base on reaction with added acid and bases.**
- This statement is not true. When a strong acid or base is added to a buffer solution, it reacts with the components of the buffer (weak acid or weak base) to form ions, rather than forming unionized species. Therefore, this option is **false**.

2. **Option 2: Acids and alkalis in these solutions are shielded from attack by other ions.**
- This statement is true. The buffer solution can resist changes in pH because the added acids or bases are neutralized by the buffer components. For example, if a strong acid is added, the weak base in the buffer will react with it, and vice versa. This shielding effect helps maintain the pH, making this option **true**.

3. **Option 3: They have large excess of H⁺ or OH⁻ ions.**
- This statement is false. Buffer solutions do not contain a large excess of H⁺ or OH⁻ ions; rather, they contain a limited amount of these ions due to the presence of a weak acid or weak base. The buffer works by utilizing these limited ions to resist changes in pH.

4. **Option 4: They have fixed value of pH.**
- This statement is also false. While buffer solutions maintain a nearly constant pH, they do not have a fixed value of pH. The pH can change if a large amount of acid or base is added, exceeding the buffer capacity.

### Conclusion
After analyzing all the options, the only correct statement is:
- **Option 2: Acids and alkalis in these solutions are shielded from attack by other ions.**

Thus, the answer to the question is **Option 2**.

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