A student sets up the apparatus to carry out the electrolysis of concentrated sodium chloride solution using carbon electrodes as shown in Fig. 6.1. (a) Name all the positive ions present in aqueous sodium chloride. (b) Name gases C and D. (c) Explain why the solution formed is alkaline.

The long line of the power supply is the positive terminal (anode), and the short line is the negative terminal (cathode). Gas C is collected at the cathode, and gas D is collected at the anode.

At the cathode, positive ions ($\text{Na}^+$ and $\text{H}^+$ from water) are attracted. Hydrogen ions are preferentially discharged over sodium ions because hydrogen is less reactive.

Therefore, gas C is hydrogen.

At the anode, negative ions ($\text{Cl}^-$ and $\text{OH}^-$ from water) are attracted. In concentrated sodium chloride solution, chloride ions are preferentially discharged over hydroxide ions.

Therefore, gas D is chlorine.

C: $\boxed{\text{Hydrogen}}$
D: $\boxed{\text{Chlorine}}$

During the electrolysis, hydrogen ions ($\text{H}^+$) are discharged at the cathode and chloride ions ($\text{Cl}^-$) are discharged at the anode. This leaves sodium ions ($\text{Na}^+$) and hydroxide ions ($\text{OH}^-$) in the solution.

As $\text{H}^+$ ions are removed from the solution at the cathode, the equilibrium of water dissociation ($\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-$) shifts to the right to replenish the $\text{H}^+$ ions. This leads to an increase in the concentration of $\text{OH}^-$ ions.

The accumulation of $\text{OH}^-$ ions (along with $\text{Na}^+$ ions, forming sodium hydroxide) makes the solution alkaline.