a) Impure copper is purified by electrolysis using the apparatus shown - Edexcel - GCSE Chemistry - Question 6 - 2015 - Paper 1

At the end of the electrolysis process:

1. Anode (Impure Copper Electrode): The anode loses mass as copper atoms oxidize. This occurs as copper atoms donate electrons and move into the solution as Cu$^{2+}$ ions. Thus, the anode becomes smaller and changes from a reddish-brown color to a less substantial mass.

2. Cathode (Pure Copper Electrode): The cathode gains mass as Cu$^{2+}$ ions in the solution are reduced and deposit as elemental copper on the electrode. The overall appearance changes to a shiny and reddish color due to this deposition.

The correct choice for the half equation is D (reduction), as copper ions are being reduced to form metallic copper.

Copper sulfate solution conducts electricity due to the presence of ions. When an electric current is applied, the Cu$^{2+}$ and SO$_{4}^{2-}$ ions move towards the respective electrodes. The Cu$^{2+}$ ions move towards the cathode (negative electrode), where they can gain electrons and form copper metal. Meanwhile, the sulfate ions migrate to the anode (positive electrode) where they can engage in oxidation reactions. The movement of these charged ions allows the solution to conduct electricity.

In the electrolysis of sodium sulfate solution:

1. Anode (Positive Electrode): Hydroxide ions (OH$^{-}$) and sulfate ions (SO$_{4}^{2-}$) migrate to the anode. Hydroxide ions are oxidized, leading to the overall reaction:

   $4OH^{-} → 2H_{2}O + O_{2} + 4e^{-}$

   The identification of oxygen can be confirmed by the presence of bubbling gas at the anode and a glowing splint can reignite if brought near.

2. Cathode (Negative Electrode): Sodium ions (Na$^{+}$) carry positive charge and migrate towards the cathode. Hydrogen ions (H$^{+}$) are reduced at the cathode, resulting in:

   $2H^{+} + 2e^{-} → H_{2}$

   This hydrogen gas can be identified by the 'pop' sound when a lit splint is placed in the gas.