### Correct Option: D. the litmus turns red and oxygen is evolved #### Explanation of the Correct Answer: 1. **Understanding Electrolysis**: Electrolysis is a process that uses electrical energy to drive a non-spontaneous chemical reaction. In this case, we are looking at the electrolysis of copper(II) sulfate (CuSO₄) solution using platinum electrodes. 2. **Electrolyte Composition**: Copper(II) sulfate dissociates in water to form copper ions (Cu²⁺) and sulfate ions (SO₄²⁻). The solution also contains water, which can undergo electrolysis. 3. **Electrode Reactions**: - **At the Anode (Positive Electrode)**: Oxidation occurs. In the case of copper(II) sulfate, the sulfate ions (SO₄²⁻) can be oxidized to produce oxygen gas (O₂) and hydrogen ions (H⁺). The half-reaction can be represented as: \[ 2 \text{H}_2\text{O} \rightarrow \text{O}_2 + 4 \text{H}^+ + 4 \text{e}^- \] - **At the Cathode (Negative Electrode)**: Reduction occurs. Copper ions (Cu²⁺) are reduced to form copper metal (Cu): \[ \text{Cu}^{2+} + 2 \text{e}^- \rightarrow \text{Cu} \] 4. **Effect on Litmus Solution**: When litmus solution is added to the anode compartment: - The production of hydrogen ions (H⁺) from the oxidation of water increases the acidity of the solution. - Litmus is a pH indicator that turns red in acidic conditions. Therefore, the litmus solution will turn red due to the formation of H⁺ ions. 5. **Gas Evolution**: Oxygen gas is evolved at the anode as a result of the oxidation of water. This can be observed as bubbles forming at the anode. #### Why Other Options Are Incorrect: - **Option A: the litmus turns blue but no gas is evolved** - This option is incorrect because the litmus does not turn blue; it turns red due to the increase in H⁺ ions. Additionally, oxygen gas is indeed evolved at the anode. - **Option B: the litmus turns blue and oxygen is evolved** - This option is also incorrect for the same reason as Option A. The litmus does not turn blue; it turns red due to the acidic environment created by the H⁺ ions. - **Option C: the litmus turns blue and hydrogen is evolved** - This option is incorrect because hydrogen is not evolved at the anode; it is evolved at the cathode. At the anode, oxygen is produced, and the litmus turns red, not blue. ### Summary of Key Points: - During electrolysis of CuSO₄, oxidation occurs at the anode, producing oxygen gas and H⁺ ions. - The increase in H⁺ ions causes litmus to turn red, indicating an acidic solution. - Oxygen gas is evolved at the anode, while copper is deposited at the cathode. - Understanding the reactions at each electrode is crucial for predicting the behavior of indicators like litmus in electrolysis.