(a) Why is direct current (DC) not used to measure the resistance of an ionic solution?

• Step 1 (Governing Principle): [0.5 Mark] Direct current (DC) causes electrolysis of the ionic solution.
• Step 2 (Mechanism): [0.5 Mark] The passage of DC leads to a change in the concentration of the electrolyte near the electrodes, resulting in polarization effects.
• Step 3 (Final Conclusion): [0.0 Mark] Consequently, the resistance measured using DC is inaccurate. Therefore, alternating current (AC) is used to prevent electrolysis and maintain constant concentration.

💡 EL10 Concept Insight: Think of DC like a one-way street that forces chemicals to pile up at the electrodes, changing the solution's makeup; AC acts like a swinging door that moves back and forth so quickly that the chemicals don't have time to pile up, keeping the measurement steady.

(b) Why are the products of electrolysis different for the electrolysis of aqueous solution of $AgNO_3$ with silver electrodes and platinum electrodes?

• Step 1 (Governing Principle): [0.5 Mark] The nature of the electrode (inert vs. active) determines the reaction at the anode.
• Step 2 (Reaction Analysis): [0.5 Mark]
  • With Platinum (inert) electrodes: The anode does not participate. $OH^-$ ions are discharged in preference to $NO_3^-$ ions, releasing $O_2$ gas: $$4OH^- \rightarrow 2H_2O + O_2 + 4e^-$$
  • With Silver (active) electrodes: The silver anode itself undergoes oxidation: $$Ag(s) \rightarrow Ag^+(aq) + e^-$$
• Step 3 (Final Conclusion): [0.0 Mark] Thus, with Pt electrodes, $O_2$ is evolved at the anode, whereas with Ag electrodes, the silver anode dissolves into the solution.

💡 EL10 Concept Insight: An inert electrode (Platinum) is just a spectator that watches the reaction happen, but an active electrode (Silver) is a participant that jumps into the reaction itself, changing the final product.

(c) Why are magnesium blocks fixed to the iron pipelines carrying water?

• Step 1 (Governing Principle): [0.5 Mark] This is an application of sacrificial protection based on the electrochemical series.
• Step 2 (Mechanism): [0.5 Mark] Magnesium is more reactive than iron ($E^\circ_{Mg^{2+}/Mg} = -2.37\,V$ vs $E^\circ_{Fe^{2+}/Fe} = -0.44\,V$).
• Step 3 (Final Conclusion): [0.0 Mark] Magnesium acts as a sacrificial anode and undergoes oxidation in preference to iron, thereby preventing the corrosion (rusting) of the iron pipeline.

💡 EL10 Concept Insight: Magnesium acts like a "bodyguard" for the iron pipe; it chooses to get "hurt" (oxidized) first so that the iron pipe stays safe and rust-free.