At the end point, there is a sharp change of colour in the indicator. This happened because the

Options

A. `pH` a end point changes sharply.
B. Structure of the indicator changes.
C. Colour of indicator is adsorbed by water.
D. Dissociation constant of acid and base differ by `10.`

Explanation

To solve the question regarding the sharp change of color in an indicator at the endpoint of a titration, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding the Endpoint and Equivalence Point**:
- In a titration, the endpoint is the point at which the reaction is complete, and the equivalence point is where the amount of titrant added is stoichiometrically equivalent to the amount of substance in the sample.

2. **Role of Indicators**:
- An indicator is a chemical that changes color at a specific pH range. It is used to visually signal the endpoint of a titration.

3. **pH Change at the Endpoint**:
- At the endpoint of a titration, there is a significant change in the pH of the solution. This is due to the neutralization reaction that occurs, which can lead to a rapid shift in the acidity or basicity of the solution.

4. **Sharp Color Change**:
- The sharp change in color of the indicator occurs because the pH of the solution changes rapidly at the equivalence point. The indicator will change color at a specific pH, and when the pH crosses this threshold, a noticeable color change is observed.

5. **Conclusion**:
- Therefore, the sharp change of color in the indicator at the endpoint is due to the sharp change in pH that occurs at this point in the titration.

### Final Answer:
The sharp change of color in the indicator at the endpoint happens because of the sharp change in pH at the equivalence point.