A solution which is resistant to change of pH upon the addition of an acid or a base is known as

Options

A. A colloid
B. A crystalloid
C. A buffer
D. An indicator

Explanation

**Step-by-Step Solution:**

1. **Understanding the Concept of pH:**
- pH is a measure of the acidity or basicity of a solution. It is defined as the negative logarithm of the hydrogen ion concentration in the solution.

2. **Definition of a Buffer Solution:**
- A buffer solution is a special type of solution that resists changes in pH when small amounts of an acid or a base are added. This means that even if you add an acid (which would normally lower the pH) or a base (which would normally raise the pH), the pH of the buffer solution remains relatively stable.

3. **Components of a Buffer Solution:**
- There are generally two types of buffer solutions:
- **Acidic Buffer:** This consists of a weak acid and its conjugate base. For example, a mixture of acetic acid (weak acid) and sodium acetate (conjugate base).
- **Basic Buffer:** This consists of a weak base and its conjugate acid. For example, a mixture of ammonia (weak base) and ammonium chloride (conjugate acid).

4. **Function of a Buffer Solution:**
- When an acid is added to an acidic buffer, the weak acid present in the buffer will react with the added hydrogen ions, minimizing the change in pH. Similarly, when a base is added, the conjugate base will react with the hydroxide ions, again minimizing the change in pH.

5. **Conclusion:**
- Therefore, the answer to the question is that a solution which is resistant to change of pH upon the addition of an acid or a base is known as a **buffer solution**.

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