The compositions of equilibrium mixture is not changed by the catalyst.

Answer

**Step-by-Step Solution:**

1. **Understanding the Role of a Catalyst:**
A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It achieves this by lowering the activation energy required for the reaction to occur.

2. **Effect on Activation Energy:**
When a catalyst is added to a reaction mixture, it provides an alternative pathway for the reaction with a lower activation energy (Ea'). This means that the reactants can convert to products more easily and quickly.

3. **Equilibrium Consideration:**
In a reversible reaction, such as A + B ⇌ C + D, both the forward reaction (A + B → C + D) and the reverse reaction (C + D → A + B) occur simultaneously. The catalyst affects both reactions equally, increasing their rates.

4. **Impact on Equilibrium Composition:**
Although the catalyst speeds up the rate at which equilibrium is reached, it does not change the position of the equilibrium itself. The ratio of concentrations of reactants and products at equilibrium remains the same, regardless of the presence of a catalyst.

5. **Conclusion:**
Therefore, the statement "the compositions of equilibrium mixture is not changed by the catalyst" is true. The catalyst only accelerates the rate at which equilibrium is achieved but does not alter the equilibrium concentrations of reactants and products.

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