(a) The order of reactivity is: Sodium > Magnesium > Calcium. Sodium reacts with both potassium chloride and magnesium chloride, magnesium reacts with magnesium chloride, and calcium does not react with any of the solutions.

(b) Sodium is more reactive than calcium and magnesium. It has a greater tendency to lose electrons and form a positive ion. The reaction with potassium chloride and magnesium chloride involves the displacement of a less reactive metal (K or Mg) by the more reactive sodium. Calcium chloride does not react because calcium is less reactive than sodium, and therefore cannot displace the calcium ions from the solution.

Experiment: Demonstrating the Protective Nature of the Aluminium Oxide Layer

Apparatus:

• Two identical pieces of aluminium metal (e.g., aluminium foil).
• Two identical containers (e.g., glass jars or beakers).
• Distilled water.
• Salt (e.g., sodium chloride, NaCl).
• A stirring rod.
• A stopwatch or timer.
• A magnifying glass or microscope (optional, for observation).

Procedure:

1. Prepare two solutions of saltwater: one with a concentration of 1M NaCl and another with a concentration of 0.1M NaCl. Ensure the solutions are identical in volume.
2. Place one piece of aluminium metal into each container.
3. Pour the 1M saltwater solution into one container and the 0.1M saltwater solution into the other.
4. Record the appearance of the aluminium in both containers at regular intervals (e.g., every 15 minutes) for a period of 24 hours. Observe for any signs of corrosion (e.g., discoloration, pitting, or the formation of rust-like products).
5. If a magnifying glass or microscope is available, examine the surface of the aluminium for any evidence of corrosion.

Expected Results:

The aluminium in the 1M saltwater solution will show very little or no signs of corrosion. The surface will remain largely unchanged. In contrast, the aluminium in the 0.1M saltwater solution will corrode significantly. You should observe discoloration, pitting, and the formation of corrosion products on the surface of the aluminium.

Explanation:

The results demonstrate that the aluminium oxide layer protects the underlying metal from corrosion. The 1M saltwater solution, being a higher concentration of ions, is more aggressive and can penetrate the oxide layer to some extent. However, the oxide layer still prevents significant corrosion. The lower concentration of ions in the 0.1M saltwater solution allows the corrosive ions to more easily penetrate the oxide layer, leading to corrosion of the underlying aluminium metal. This supports the claim that aluminium is unreactive due to the protective nature of its oxide layer.

The student's observation is consistent with the reactivity series of metals. The reactivity series is a list of metals arranged in order of their reactivity, with the most reactive metal at the top and the least reactive at the bottom. A simplified reactivity series is:

Zinc can displace copper because zinc is higher up in the reactivity series than copper. This means that zinc has a greater tendency to lose electrons and form positive ions (Zn²⁺). When zinc is added to a solution of copper ions (Cu²⁺), the zinc ions (Zn²⁺) are reduced to zinc atoms (Zn), and the copper ions (Cu²⁺) are oxidized to copper atoms (Cu). The reaction proceeds because zinc is more reactive than copper, and therefore has a greater affinity for electrons. Silver is below zinc in the reactivity series, so zinc cannot displace silver from its solution. Silver is less likely to lose electrons than zinc, so it will not be displaced.