A layer of copper will be deposited on the carbon rod.
Explanation
Electroplating is an application of the chemical effect of electric current, known as electrolysis. In this process, dissolved metal ions from an electrolyte solution are reduced and deposited onto a conductive surface acting as the cathode (negative electrode).
Detailed Analysis:
- Conductivity of Carbon: Carbon (specifically in the form of graphite) is a good conductor of electricity. Therefore, replacing the copper cathode with a carbon rod does not break the circuit; the current continues to flow through the electrolyte.
- Ion Migration: The electrolyte, typically copper sulphate ($CuSO_4$), dissociates into positively charged copper ions ($Cu^{2+}$) and negatively charged sulphate ions ($SO_4^{2-}$).
- Reaction at the Cathode: The $Cu^{2+}$ ions are attracted to the negative electrode (the carbon rod). Upon contact, they gain electrons (reduction) and neutralize to form solid copper atoms ($Cu^{2+} + 2e^- \rightarrow Cu$).
- Conclusion: As the reaction proceeds, a layer of metallic copper deposits on the surface of the carbon rod.
Evaluation of Incorrect Options:
- The electrolysis process will stop immediately. is incorrect: The process does not stop because carbon is a conductor, allowing the electrolytic circuit to remain complete.
- The carbon rod will dissolve into the solution. is incorrect: The cathode is the site of deposition (reduction). Dissolution (oxidation) occurs at the anode. Therefore, the carbon rod will not dissolve.
- The solution will turn colourless. is incorrect: If the anode is copper, it dissolves to replenish the $Cu^{2+}$ ions, keeping the solution blue. If the anode were inert, the blue colour would fade gradually as ions are depleted, but the solution would not turn colourless immediately. The primary observation remains the deposition on the cathode.
Key Takeaway:
In electroplating, the material to be coated is always made the cathode. As long as the cathode is conductive, metal ions from the electrolyte will reduce and deposit upon it, regardless of the cathode's material composition.