Choose the correct statement from the following

Options

A. The addition of catalyst changes equilibrium constant
B. A catalyst speeds up forward reaction and slows the backward reaction
C. The composition of equilibrtum mixture is not changed by a catalyst
D. A catalyst is active only in solution

Explanation

To solve the question of choosing the correct statement regarding catalysts, we will analyze each of the provided statements one by one.

### Step-by-Step Solution:

1. **Understanding the Role of Catalysts**:
- Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They achieve this by lowering the activation energy required for the reaction to occur.

2. **Analyzing the Statements**:
- **Statement 1**: "The addition of catalyst changes the equilibrium constant."
- This statement is **incorrect**. A catalyst does not change the equilibrium constant of a reaction; it only helps the system reach equilibrium faster.

- **Statement 2**: "Catalyst speeds up forward reaction and slows down the backward reaction."
- This statement is also **incorrect**. A catalyst speeds up both the forward and backward reactions equally, thus maintaining the ratio of the concentrations of reactants and products at equilibrium.

- **Statement 3**: "The composition of the equilibrium mixture is not changed by a catalyst."
- This statement is **correct**. A catalyst does not change the position of equilibrium; it only helps the system reach equilibrium more quickly. The final concentrations of reactants and products remain the same as they would be without the catalyst.

- **Statement 4**: "A catalyst is active only in solution."
- This statement is **incorrect**. Catalysts can be effective in various phases, including solid, liquid, and gas. They are not limited to just solutions.

3. **Conclusion**:
- After analyzing all the statements, the correct statement is **Statement 3**: "The composition of the equilibrium mixture is not changed by a catalyst."

### Final Answer:
The correct statement is: **The composition of the equilibrium mixture is not changed by a catalyst.**

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