Why Only Some Nails Rusted
Four clean iron nails are placed in labelled test tubes and left for five days at room temperature. Tube A contains dry air and a sachet of calcium chloride; the nail is not touching water. Tube B contains 10 mL of tap water and is open to the air. Tube C contains 10 mL of water that was boiled and then covered with a 1 cm layer of cooking oil. Tube D contains 10 mL of water with 1 g of dissolved salt and is open to the air. On day 5, Tube D shows heavy reddish-brown deposit, Tube B shows a thin patch, Tube A shows a small bright patch, and Tube C shows no visible change. The student writes: 'Salt causes rusting. Water alone does not cause rusting, because Tube C had water and did not rust.' The teacher points out that Tube C was boiled and sealed under oil and asks the student to re-examine the conclusion before writing the report.
1. Identify the independent variable(s) and the controlled variables in this experiment, and explain why Tube C cannot be used as evidence that water alone does not cause rusting.
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The main independent variable is the environment around the nail: dry air, ordinary tap water exposed to air, boiled water under oil, and salt water exposed to air. The controlled variables include the type of nail, size of test tube, volume of water (10 mL), temperature, and time (five days).
Tube C cannot show that water alone fails to cause rusting because boiling removes dissolved oxygen and the oil layer prevents oxygen from re-entering. Tube C therefore lacks both oxygen and access to air, not just 'water alone'. It is a no-oxygen control. The correct comparison for 'water alone' is Tube B, where the nail is in water with air present and does rust slowly. Tube C supports the conclusion that oxygen is needed, not that water is unnecessary.
2. Explain, using the results, the conditions required for iron to rust and why Tube D rusted fastest.
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Rusting requires iron, oxygen, and water together. Tube A has dry air but little water, so rusting is very slow. Tube B has both water and dissolved air, so a thin patch forms. Tube C has water but essentially no oxygen because it was boiled and covered with oil, so there is no visible rust. Tube D has water, dissolved air, and salt, and it rusts heavily.
Salt does not replace oxygen or water; it acts as an electrolyte. It makes the water more conducting, so the electrochemical corrosion cells on the iron surface can operate faster. Salt may also disturb protective surface layers. That is why Tube D rusts faster than Tube B. The student's claim that salt 'causes' rusting is too strong: salt accelerates rusting, but the essential reactants are still iron, oxygen, and water.
3. The student wants to test whether galvanising prevents rusting. Describe a fair test using the same set-up and state the expected result.
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Use two nails of the same size and type in identical test tubes, each with 10 mL of salt water and open to the air. One nail is untreated iron; the other is galvanised (coated with zinc). Keep both at the same temperature for the same time, and observe or weigh any deposit. A fair test must change only the surface coating, not the water volume, salt concentration, tube size, or duration.
The expected result is that the untreated nail rusts heavily while the galvanised nail shows little or no rust, because the zinc coating excludes water and oxygen from the iron. If the coating is scratched, the zinc may still protect the exposed iron by acting as a sacrificial metal, but a deep scratch could allow some corrosion. The test should state whether the coating is intact, since that affects the conclusion.