Lesson Plan

• Describe how a thin aluminium oxide (Al₂O₃) layer forms when aluminium is exposed to air.
• Explain how this oxide layer acts as a physical barrier, passivates the metal, and reduces its apparent reactivity.
• Compare the protective effects of aluminium’s oxide layer with those of iron and magnesium.
• Predict the outcome of reactions when the oxide layer is removed (e.g., by scratching or using a strong base).

• Projector and screen
• Aluminium foil pieces (≈2 cm²)
• Fine sandpaper
• Dilute HCl solution (1 M)
• Concentrated NaOH solution (2 M)
• Safety goggles and gloves
• Worksheet & diagram of the oxide layer

Begin with a quick poll: “Which metals react vigorously with acid?” Students will recall the reactivity series and notice aluminium’s odd behaviour. Explain that today they will uncover why aluminium often seems “inactive”. Success will be measured by their ability to explain the role of the oxide layer.

1. Do‑now (5'): Answer three short questions on the reactivity series (worksheet).
2. Mini‑lecture (10'): Explain oxide‑layer formation, show diagram, discuss passivation.
3. Demo 1 – Acid test (unprepared aluminium) (8'): Observe little or no bubbling.
4. Demo 2 – Scratch & acid test (8'): Scratch surface, add HCl, observe rapid bubbling.
5. Demo 3 – Base test (7'): Add aluminium to NaOH, note gas evolution.
6. Guided practice (10'): Pairs complete worksheet comparing oxide layers of Al, Fe, Mg.
7. Check for understanding (5'): Exit‑ticket question – “Why does scratched aluminium react with acid but bulk aluminium does not?”

Recap that aluminium’s thin, adherent Al₂O₃ layer masks its true reactivity by acting as a barrier and insulator. Collect the exit‑ticket responses to gauge understanding, and assign homework: write a short paragraph describing how removing the oxide layer changes aluminium’s behaviour in acid.