Which of the following statements are not correct ?

Options

A. A catalyst always increases the speed of a reaction.
B. A catalyst does not take part in the reaction
C. A catalyst may affect the nature of the products formed.
D. A catalyst is always an external substance added to the reaction mixture.

Answer

To determine which statements about catalysts are not correct, we need to analyze each statement provided. Here’s a step-by-step breakdown:

### Step 1: Analyze the Statements

1. **Statement 1**: "Catalysts always increase the rate of reaction."
- **Analysis**: This statement is incorrect because there are negative catalysts (or inhibitors) that can decrease the rate of reaction.

2. **Statement 2**: "A catalyst does not take part in the reaction."
- **Analysis**: This statement is generally correct. Catalysts facilitate the reaction but do not undergo permanent changes themselves.

3. **Statement 3**: "Catalyst may affect the nature of products."
- **Analysis**: This statement is incorrect. While catalysts can speed up reactions, they do not change the nature of the products formed; they only help in reaching the products faster.

4. **Statement 4**: "A catalyst is an external substance added to the reaction mixture."
- **Analysis**: This statement is correct. By definition, a catalyst is indeed an external substance that is added to the reaction mixture to affect the rate of reaction.

### Step 2: Identify Incorrect Statements

From the analysis, we find that:
- Statement 1 is incorrect.
- Statement 3 is incorrect.

### Conclusion

The statements that are not correct are:
- **Statement 1**: "Catalysts always increase the rate of reaction."
- **Statement 3**: "Catalyst may affect the nature of products."