The bimetallic effect

You will need: five cleaned iron nails, agar jelly, sodium chloride, phenolphthalein indicator, potassium ferricyanide, Petri dishes, strips of magnesium, zinc, copper and lead, water

1. Add a little sodium chloride, phenolphthalein and potassium ferricyanide to about 250 cm³ of water. Boil the solution, add about a tablespoon of agar jelly and stir well.
2. Take five Petri dishes. Put an iron nail alone in the first. Wrap strips of magnesium, zinc, copper and lead tightly around the other four nails so they touch, and place them in the other four dishes.
3. Pour the warm jelly into all five dishes so the nails are completely covered. Leave to cool and observe after about an hour. Record your observations.

• Phenolphthalein turns pink where OH^- ions are present.
• Fe²⁺ ions give a blue colour with potassium ferricyanide.

A pink colour is seen around the iron nails: OH^- ions have formed near the iron. No blue shows, so no Fe²⁺ has formed. The nails here are joined to magnesium and zinc, which are more active than iron, so the cathode reaction happened at the iron nail. The more active magnesium and zinc act as the anode and are oxidised; the Mg²⁺ and Zn²⁺ ions they form give no colour with potassium ferricyanide.

A blue patch forms around the iron nails, showing that Fe²⁺ ions have formed: the iron has corroded, acting as the anode. Copper and lead are below iron in the activity series, and iron joined to such a metal rusts. A pink patch around the copper and lead strips shows that OH^- ions formed there — the cathode reaction happened at the copper and lead.