Which of the following statements is/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.

Explanation

To determine which statements are not correct regarding catalysts, let's analyze each statement step by step.

### Step 1: Analyze Statement A
**Statement A:** "A catalyst always increases the speed of reaction."

- This statement is **not correct**. While most catalysts do increase the rate of reaction, there are also substances known as inhibitors that can decrease the rate of reaction. Therefore, this statement is misleading.

### Step 2: Analyze Statement B
**Statement B:** "A catalyst does not participate in the reaction."

- This statement is **correct**. Catalysts facilitate reactions without being consumed or permanently altered in the process. They may form temporary intermediates but do not change the overall reaction outcome.

### Step 3: Analyze Statement C
**Statement C:** "A catalyst may affect the nature of the product formed."

- This statement is **correct**. Catalysts can influence the pathway of a reaction, which can lead to the formation of different products. Therefore, this statement is true.

### Step 4: Analyze Statement D
**Statement D:** "A catalyst is always an external substance added to the reaction mixture."

- This statement is **not correct**. While many catalysts are external substances, some reactions can produce their own catalysts during the reaction. For example, in the hydrolysis of esters in acidic medium, the acid formed can act as a catalyst, making this statement false.

### Conclusion
Based on the analysis:
- **Not Correct Statements:** A and D
- **Correct Statements:** B and C

### Final Answer
The statements that are not correct are **A and D**.

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