Rust and How to Prevent It
AdvancedRusting needs both air and water; oiling, painting, metal coating or keeping dry can prevent it. Water's changes of state reverse, but rusting and burning cannot be undone.
- Rusting needs air plus water — neither can be missing.
- Change one condition at a time; that is a fair test.
- Prevent rust: oil, paint, keep dry, metal coat, stainless steel.
文體小知識：記敘文記下探究的過程。這一篇跟着科學小組做三隻鐵釘的實驗，一步步找出鐵生鏽的秘密。
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1In Grandpa's old iron gate, reddish-brown rust has grown in the corners, crumbling into powder when touched; the bicycle chain downstairs, never oiled for months, has rusted stiff. Why do iron things rust? Rusting is a slow chemical reaction between iron, air and water. The rust formed is brittle and crumbly, so iron things flake away layer by layer.
2To find the conditions needed for rusting, a science group decided to run a control experiment: three identical iron nails go into three bottles, each with one condition changed, and after a week we see which nails rust. Changing only one factor at a time — a fair test — is exactly how scientists work.
3The three bottles are arranged with care. Bottle A holds a drying agent: the nail touches dry air only, with no water. In bottle B, the water is first boiled to drive out dissolved air and sealed with a layer of oil: the nail touches water but no air. Bottle C is ordinary: the nail sits half in water and half in the open air, with both air and water.
4A week later the bottles are opened: the nail in dry air shines like new; the one sealed under oil shows no rust; only the third nail, touching both air and water, is covered in reddish-brown rust. The experiment proves: air and water are both necessary for rusting — neither can be missing.
5Knowing the cause gives us the cures: coat iron with oil or paint to shut out air and water; oil the bicycle chain regularly; wipe kitchen knives dry after use; factories coat iron with zinc or chrome, or make stainless steel that hardly rusts, so bridges, ships and buildings last longer.
6Think again about kinds of change: water evaporating, condensing, freezing and melting can all go backwards and forwards — reversible changes. But rusted iron cannot become shiny iron again, and burnt paper cannot turn back — irreversible changes. Knowing the difference helps us protect iron things before trouble starts.
1Summary 重點摘要
Rusting needs both air and water; oiling, painting, metal coating or keeping dry can prevent it. Water's changes of state reverse, but rusting and burning cannot be undone.
2Vocabulary cards Vocabulary
The slow chemical reaction of iron with air and water that turns it to rust.
The reddish-brown, brittle substance formed when iron rusts.
An experiment changing only one condition at a time so results can be compared fairly.
A protective layer painted on iron to shut out air and water.
Covering iron with zinc or chrome to shield it from air and water.
Steel mixed with special metals so it hardly rusts, used for cutlery and kitchens.
A change that can go backwards, such as water freezing and melting again.
A change that cannot be undone, such as paper burning or iron rusting.
Water turning into vapour when heated.
Vapour turning back into droplets when cooled, like drops on a cold drink bottle.
3Step by step Steps
- Step 1: Observe rusting around usLook around: old iron gates, railings, bicycle chains and nails, after long use, all grow a reddish-brown crumbly layer — rust — and the process is called rusting. Rust is not hard like iron; it flakes off when rubbed, making iron things thinner until they break. Rusting damages bridges, cars and water pipes, costing the world vast sums of money every year, which is why scientists study its causes and prevention so seriously.
- Step 2: Make a hypothesis and design a fair testThe first step of inquiry is a hypothesis: does rusting need air, water, or both together? To test it, we need a fair test: three nails of exactly the same size and material, kept at the same temperature, with only one condition changed at a time. Then any difference in results can be traced to that one condition. The fair test is the cornerstone of all scientific experiments — a method even primary students can master.
- Step 3: Set up three comparison bottlesBottle A holds a drying agent, so the nail meets dry air with no water, testing air alone. In bottle B the water is boiled first to expel dissolved air and sealed with a layer of oil, so the nail meets water but no air, testing water alone. Bottle C holds a nail half in water and half in the open air, with both conditions, testing the pair together. All three bottles sit by the window for a week, observed and recorded every day.
- Step 4: Analyse the results and concludeAfter a week the results are crystal clear: the nail in bottle A's dry air shines like new; the nail in bottle B's oiled water is also rust-free; only the nail in bottle C, touching both air and water, is covered in reddish-brown rust. Comparing the three results, we conclude: iron needs both air and water at the same time to rust — neither condition can be missing. This also explains why iron rusts especially fast on rainy days, when air and water are both abundant.
- Step 5: Apply the ways to prevent rustSince rusting needs air and water, the key to prevention is keeping them away from iron. The methods: oiling, such as lubricating bicycle chains; painting, such as repainting gates and railings regularly; keeping dry, such as wiping kitchen knives and woks after use; metal coating, where factories cover iron with zinc or chrome; and making stainless steel, so cutlery and cookware hardly rust. Which method to choose depends on the iron's use and surroundings.
- Step 6: Tell reversible from irreversible changesWater evaporates into vapour when heated, and vapour condenses into droplets when cooled; water freezes into ice, and ice melts back to water — these changes can reverse, with no new substance made: reversible changes. But once iron rusts it has become a different substance and cannot turn back into shiny iron; burnt paper becomes ash and gases that cannot be restored — irreversible changes. Knowing the difference, we understand why iron must be protected early: once rusted, the damage is hard to undo.
5Worked example Example
Example: A science group puts three identical iron nails into three bottles. Bottle A holds dry air only (with a drying agent); bottle B holds freshly boiled water sealed with a layer of oil; bottle C holds a nail half in water and half in the open air. After a week, which nail rusts, and why?
- Bottle A: the nail meets air, but the drying agent absorbs all moisture — the condition is air without water.
- Bottle B: boiling has driven the air out of the water, and the oil seals the surface so no air can dissolve in — the condition is water without air.
- Bottle C: the nail is half in water and half in air — both conditions are present.
- Result: the nails in bottles A and B do not rust; only the nail in bottle C grows reddish-brown rust.
- Conclusion: air and water are both necessary for rusting — they must be present together, neither can be missing.
- Answer: Only the nail in bottle C rusts. Rusting needs air and water together: bottle A has air but no water, bottle B has water but no air, so their conditions are incomplete and no rust forms. The nail in bottle C sits half in water and half in air, with both conditions, so it slowly oxidises and rusts. The experiment proves that air plus water are the necessary conditions for rusting — neither can be missing.
6Common mistakes Mistakes
7Quick check Quick check
Q1Which two conditions must exist together for iron to rust?
Q2Why does the oil layer on bottle B's water matter?
Q3How is a bicycle chain protected from rust?
Q4What kind of change are evaporation and condensation of water?
Q5Is rusting reversible or irreversible?
8Recap Recap
- Rusting needs air plus water — neither can be missing.
- Change one condition at a time; that is a fair test.
- Prevent rust: oil, paint, keep dry, metal coat, stainless steel.
- Water's changes reverse; rusting and burning never do.
- Check and maintain iron things before trouble starts.