A catalyst is a substance which :

Options

A. Increases the equilibrium concentration of the product
B. Changes the equilibrium constant of the reaction
C. Shortens the time to reach equilibrium
D. Supplies energy to the reaction

Explanation

### Step-by-Step Solution:

1. **Definition of a Catalyst**: A catalyst is a substance that increases the rate of a chemical reaction without undergoing any permanent chemical change itself. It participates in the reaction but is regenerated at the end.

2. **Mechanism of Action**: Catalysts work by providing an alternative pathway for the reaction with a lower activation energy compared to the uncatalyzed reaction. This means that more reactant molecules can overcome the energy barrier and form products more quickly.

3. **Effect on Reaction Rate**: While a catalyst speeds up the rate of reaction, it does not change the equilibrium concentrations of the reactants and products. It merely allows the system to reach equilibrium faster.

4. **Impact on Equilibrium Constant**: The presence of a catalyst does not affect the equilibrium constant of the reaction. The equilibrium constant is determined by the nature of the reactants and products and their concentrations at equilibrium.

5. **Time to Reach Equilibrium**: A catalyst shortens the time required to reach equilibrium by lowering the activation energy. This means that the reaction can proceed more quickly, allowing the system to achieve equilibrium in a shorter time frame.

6. **Conclusion**: Therefore, the correct statement about a catalyst is that it shortens the time to reach equilibrium without affecting the equilibrium concentrations or the equilibrium constant.

### Answer:
A catalyst is a substance that shortens the time to reach equilibrium.

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