Which of the following statements is incorrect ?

Options

A. Catalysts only accelerate the rate of a chemical reaction
B. Catalysts cannot start a chemical reaction
C. Catalysts can retard the rate of a chemical reaction
D. Catalysts can accelerate and retard the rate of a chemical reaction

Answer

To determine which statement is incorrect regarding catalysts, we need to analyze each of the provided statements systematically.

### Step-by-Step Solution:

1. **Statement A: Catalysts only accelerate the rate of chemical reaction.**
- This statement is misleading because while catalysts do accelerate reactions, they can also retard (slow down) reactions in certain contexts. The inclusion of the word "only" makes this statement incorrect.

2. **Statement B: Catalysts cannot start a chemical reaction.**
- This statement is correct. Catalysts do not initiate reactions; they only speed up the rate of reactions that can occur spontaneously.

3. **Statement C: Catalysts can retard the rate of chemical reaction.**
- This statement is also correct. Catalysts can slow down reactions in specific scenarios, such as when they are poisoned or when they interact with certain reactants in a way that decreases the reaction rate.

4. **Statement D: Catalysts can accelerate and retard the rate of chemical reaction.**
- This statement is correct as well. Catalysts can indeed both speed up and slow down reactions depending on the conditions and the specific reaction involved.

5. **Conclusion:**
- The incorrect statement among the options provided is **Statement A** because it incorrectly states that catalysts "only" accelerate reactions, ignoring their ability to also retard reactions.

### Final Answer:
The incorrect statement is **A: Catalysts only accelerate the rate of chemical reaction.**

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