The shape of the lesson
By the end of the lesson, learners can
- What rusting needs: Three iron nails, three test tubes, one week: only one nail rusts.
- Preventing rust: Match the method to the object: paint for a bridge, grease for a bicycle chain, galvanising for a dustbin or a fence.
- Rust = hydrated iron(III) oxide.
- Barriers: paint, grease, plastic keep out water and oxygen.
- Galvanising: zinc coating.
- Extended: sacrificial metal is more reactive and corrodes instead.
One picture every idea returns to
Iron and steel rust only when both water and oxygen are present; rust is hydrated iron(III) oxide. Barrier methods (painting, greasing, coating with plastic) stop rusting by keeping out water and oxygen. Galvanising coats iron with zinc. Extended: zinc, or a block of magnesium, gives sacrificial protection: being more reactive, it loses electrons and corrodes instead of the iron, even if the surface is scratched.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Keep out air and water.. Rust costs a fortune in repairs every year, and it needs just two things: oxygen and water. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: What rusting needs | Three iron nails, three test tubes, one week: only one nail rusts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Preventing rust | Match the method to the object: paint for a bridge, grease for a bicycle chain, galvanising for a dustbin or a fence. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Preventing rust, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A barrier only works while it is unbroken: once paint is scratched, iron underneath rusts. | Galvanising protects even when scratched because zinc is more reactive than iron and corrodes in its place. Tin-plating does the opposite: tin is less reactive, so a scratched tin can rusts faster. |
Rusting and its prevention: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
State the two conditions needed for iron to rust, and give the chemical name of rust.
Describe an experiment using three iron nails to show that both water and oxygen are needed for rusting.
Suggest a suitable method of rust prevention for (a) a car body, (b) a bicycle chain and (c) a steel dustbin.
Explain how painting prevents rusting, and why a scratched painted surface starts to rust.
(Extended) Explain, in terms of reactivity and electrons, how blocks of zinc attached to a steel ship's hull prevent the hull rusting.
(Extended) Suggest why copper blocks could not be used for sacrificial protection of iron.
Answer key and marking notes
- Oxygen (air) and water.
- Hydrated iron(III) oxide.
- Tube 1: nail in water open to the air; it rusts.
- Tube 2: nail in boiled water with a layer of oil on top (no oxygen); it does not rust.
- Tube 3: nail in air dried with anhydrous calcium chloride (no water); it does not rust.
- (a) painting.
- (b) oiling or greasing.
- (c) galvanising (coating with zinc).
- Paint forms a barrier that keeps water and oxygen away from the iron.
- When scratched, water and oxygen reach the exposed iron, so it rusts.
- Zinc is more reactive than iron.
- Zinc loses electrons (is oxidised) more readily than iron, so it corrodes instead of the iron.
- The iron is protected as long as zinc remains; this is sacrificial protection.
- Copper is less reactive than iron.
- So iron would lose electrons and corrode instead of the copper (rusting would be faster).
7 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The diagram shows four test tubes, each containing an iron nail, set up at the same time. Which two tubes should be compared to show that water is needed for rusting?
- AA and C
- BA and B
- CB and C
- DA and D
An iron nail is weighed, left in damp air for two weeks and weighed again. Its mass has increased. What is the reason?
- Airon has combined with oxygen and water from the air to form rust
- Brust is less dense than iron, so it weighs more
- Cthe nail has absorbed nitrogen from the air
- Dsome of the iron has turned into a heavier element
Rusting is an example of which type of chemical change?
- Aoxidation
- Bneutralisation
- Cthermal decomposition
- Dreduction
Steel food cans are coated with a thin layer of tin. If a can is deeply scratched, the steel underneath rusts quickly. Why does tin not protect the steel in the way zinc does?
- Atin is less reactive than iron, so it acts only as a barrier and the exposed iron corrodes instead of the tin
- Btin is more reactive than iron, so it corrodes first
- Ctin reacts with the food to remove the protective layer
- Dtin is a non-metal, so it cannot give sacrificial protection
An underground steel pipe is connected by a wire to a buried block of magnesium. Which half-equation shows the reaction that protects the pipe?
- AMg → Mg2+ + 2e−
- BFe → Fe2+ + 2e−
- CMg2+ + 2e− → Mg
- DFe2+ + 2e− → Fe
Blocks of magnesium bolted to the steel hull of a ship must be replaced every few years. Why?
- Athe magnesium is oxidised and corrodes away in place of the iron
- Bthe magnesium is slowly converted into iron
- Cthe magnesium blocks rust because they contain iron
- Dseawater dissolves magnesium without any chemical reaction
Three iron nails are left in salt water for a week: one bare, one wrapped with a strip of zinc and one wrapped with a strip of copper. Which shows the order of the amount of rust, most first?
- Acopper-wrapped, bare, zinc-wrapped
- Bzinc-wrapped, bare, copper-wrapped
- Cbare, copper-wrapped, zinc-wrapped
- Dall three rust by the same amount
Quiz key
Tube A has both air and water; tube C has air but no water, because calcium chloride absorbs the water vapour. Only the presence of water differs, so comparing A and C shows that water is needed. A and B differ in oxygen (the boiled water under oil contains almost no dissolved air), so that pair shows oxygen is needed. B and C both lack one condition and neither nail rusts. A and D differ only in salt, so they show that salt speeds up rusting.
Rust is hydrated iron(III) oxide, so the iron has combined with oxygen and water from the air. The added atoms increase the mass. Density does not change the mass of a sample. Nitrogen is not involved in rusting. Elements are not changed into other elements in a chemical reaction.
Iron gains oxygen to form hydrated iron(III) oxide, so the iron is oxidised. Neutralisation is the reaction of an acid with a base. Thermal decomposition needs heat to break a compound down, but rusting builds a compound up at room temperature. Reduction is loss of oxygen, the opposite of what happens to the iron.
Tin is below iron in the reactivity series, so tin does not lose electrons more readily than iron. It protects only while the barrier is unbroken; once scratched, the iron loses electrons and rusts, often faster than bare steel. Zinc is more reactive than iron, so zinc corrodes instead of the iron. If tin were more reactive it would protect the steel sacrificially. Tin is a metal, and food does not remove the coating in this way.
Magnesium is more reactive than iron, so magnesium atoms lose electrons and form Mg2+ ions instead of the iron doing so. The iron stays as the metal. The equation for iron losing electrons describes the pipe corroding, which is what the magnesium prevents. Magnesium ions gaining electrons would be the reverse of corrosion. Fe2+ gaining electrons is not the protecting reaction: sacrificial protection works because the magnesium is oxidised, not because iron ions are reduced.
Magnesium is more reactive than iron, so it loses electrons more readily and is oxidised to magnesium ions, which dissolve in the seawater. The block gets used up while the iron is protected, so it has to be replaced. One element cannot turn into another in a chemical reaction. Pure magnesium contains no iron and so does not rust. The magnesium is lost by a chemical reaction (oxidation), not by simply dissolving.
Zinc is more reactive than iron, so zinc corrodes instead of the iron and that nail hardly rusts. Copper is less reactive than iron, so the iron loses electrons in preference to the copper and that nail rusts even faster than the bare nail. The order is therefore copper-wrapped, bare, zinc-wrapped. Reversing the order treats copper as sacrificial, which it cannot be. Wrapping with copper does not protect iron, and the metals clearly affect the rate, so equal rusting is wrong.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.