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Edexcel GCSE Chemistry · 1CH0
Edexcel 1CH0 · Suggested Practicals Check the specification (PDF) (opens in a new tab)
An iron nail left outdoors may develop a reddish-brown coating called rust. Rusting is a form of corrosion: the iron is gradually changed by a reaction with its surroundings. In this investigation, the question is which conditions allow iron to rust. Iron needs both oxygen and water for rusting to occur. Comparing nails kept in different conditions lets you test that claim rather than simply watching one nail go rusty.
Place a similar, clean iron nail in each of four labelled test tubes. Use eye protection. The tubes provide four useful conditions:
| Tube | Conditions around the nail | Purpose |
|---|---|---|
| Plain water | A little deionised water, with air above it | Provides water and oxygen for comparison |
| Boiled water and oil | Boiled deionised water covered by a layer of oil | Boiling removes dissolved air; oil limits more oxygen entering the water |
| Salt water | Salt dissolved in deionised water, with air above it | Tests the effect of salt against plain water |
| Dry air | A dry nail with anhydrous calcium chloride in a bunged tube | The calcium chloride absorbs water, so the nail has air but very little moisture |
Four conditions for testing whether iron needs water and oxygen to rust, and whether salt affects how quickly rust forms.
The dry tube tests what happens when water is excluded; the boiled-water-and-oil tube tests what happens when oxygen is largely excluded. The plain-water and salt-water tubes both contain water and have access to air, so their difference tests the effect of salt. Keep the nail type and size, the amount of liquid and the time left standing as similar as practical. Boiled water and the drying chemical should be prepared or handled with appropriate teacher supervision.
Leave the tubes for several days, then record the appearance of each nail. You can note where rust first appears and compare how much reddish-brown coating forms. Expect rust on the nail exposed to both water and air, often more on the nail in salt water. Expect little or no rust in the dry tube or under the boiled water and oil. Oxygen can be difficult to exclude completely, so a small amount of rust in the boiled-water tube does not by itself overturn the conclusion.
The comparisons show why keeping either oxygen or water away from iron helps prevent rusting. Paint, grease and other intact coatings work as barriers to air and moisture. Salt is not required for rusting, but the salt-water comparison can show that it makes rusting faster.
Iron rusts when oxygen and water are both present. Compare similar nails in plain water with air, boiled water beneath oil, dry air with a drying agent, and salt water with air. Leave them for several days and record the amount of rust.
Plain water versus dry air tests the need for water; plain water versus boiled water under oil tests the need for oxygen. Salt water versus plain water tests salt’s effect: salt can speed up rusting but is not essential. Excluding oxygen or water helps prevent rusting.
Compare the conditions in each tube, not just the amount of rust. The dry and air-free tubes test whether water and oxygen are needed.
When testing salt, compare a nail in salt water with one in plain water; keep the other conditions the same.
Describe the oil layer as reducing oxygen entering the boiled water, rather than guaranteeing that no oxygen remains.
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Last updated
Rusting
The corrosion of iron when it reacts in the presence of oxygen and water, producing a reddish-brown coating.
Corrosion
The gradual damage to a metal caused by chemical reactions with its surroundings.
Oxidation
A reaction in which a substance gains oxygen; oxidation can also be described as loss of electrons.
Control variable
A factor kept the same between experimental conditions so that a difference in results can be linked to the factor being tested.
Put your knowledge into practice — try past paper questions for Chemistry
Rusting
The corrosion of iron when it reacts in the presence of oxygen and water, producing a reddish-brown coating.
Corrosion
The gradual damage to a metal caused by chemical reactions with its surroundings.
Oxidation
A reaction in which a substance gains oxygen; oxidation can also be described as loss of electrons.
Control variable
A factor kept the same between experimental conditions so that a difference in results can be linked to the factor being tested.