Experiment 2: Rusting — Oxidation Over Five Days

Stand a nail in salt water and wait. Nothing seems to happen — yet day by day an orange-brown crust creeps across the metal. Rusting is chemistry on a slow clock: a redox reaction in which iron is oxidised. This five-day investigation is your first genuine long-term experiment, run the way a materials scientist would run it — four conditions side by side, one control, daily records, and a conclusion that has to wait for the data.

The chemistry behind it

Rusting needs three things at once: iron, oxygen and water. Remove any one and the reaction stalls. The overall change is:

4Fe + 3O2 + 2xH2O -> 2Fe2O3.xH2O   (hydrated iron(III) oxide — rust)

Underneath, rusting is electrochemical. At one spot on the nail, iron atoms lose electrons (oxidation: Fe → Fe²⁺ + 2e⁻). At another spot, oxygen and water gain those electrons (reduction). The electrons travel through the metal; the ions travel through the water film on the surface. That is why salt speeds rusting up: dissolved NaCl turns the water film into a much better conductor (an electrolyte), so both half-reactions run faster. It is also why a coating works: paint, oil or grease is a barrier that keeps oxygen and water from ever touching the iron.

Variables. Independent variable: the condition the nail sits in (dry air / plain water / salt water / oiled in water). Dependent variable: amount of rust, scored 0–3 (0 = none, 1 = a few spots, 2 = patchy, 3 = heavy) or estimated as percentage coverage in the virtual lab. Controlled variables: identical nails, same room and position, same water depth, observations at the same time each day. Cup A — dry air — is the control: it shows what happens with no liquid water at all, the baseline every other cup is compared against.

Safety first

• Nails have sharp points — handle them carefully and never leave them where someone might step or sit.
• Keep salt water away from eyes and wash your hands after handling rusty nails.
• Rusty metal plus broken skin is a tetanus risk: if you cut yourself on a nail, tell an adult. (The original caution stands — make sure vaccinations are current.)

Doing the investigation

Hands-on. Label four cups: A dry air, B plain water, C salt water (2 teaspoons of salt dissolved), D nail coated in vegetable oil then covered halfway with plain water. Use identical non-galvanised, non-stainless nails, half-covered in B, C and D. Photograph every nail on Day 0, then observe and photograph once a day for 5 days, scoring rust 0–3 each time.

In the virtual lab below (Rusting — Oxidation Over Five Days) you can compress the week into seconds: set Humidity, Salt and Coating (None / Oil / Paint), press Play 5 days, and read rust coverage, rust spots and corrosion rate as you drag the day slider. Follow its guided procedure — compare day-5 coverage for each coating, then push salt to 5% and watch the curve steepen. Log the coating comparison in the observation log; you will need exactly this comparison again in Experiment 17.

Worked investigation: five days of rust

One student's scores (0–3 scale):

Processing the result:

• Mean day-5 score across the three wet cups = (2 + 3 + 1) ÷ 3 = 2.0; the control scores 0.
• Salt water reaches the maximum score (3) by day 4; plain water is still at 2 on day 5; oil halves the plain-water result (1 vs 2).
• In the virtual lab at 70% humidity and 2% salt with no coating, coverage grows roughly 0% → 6% → 18% → 34% → 52% → 68% across the five days — fast at first, then slowing as rust itself covers the fresh iron.

Conclusion. Rusting needs liquid water: the dry control stayed clean for five days. Salt water rusted fastest — salt makes the surface film conductive, speeding the electrochemical reaction. Oil slowed rusting by acting as a barrier, but did not stop it completely, because a thin oil film has gaps and wears away. This is exactly why cars get thick paint and underseal rather than a wipe of oil.

Uncertainty. The 0–3 score is a judgement call: two observers can disagree by a full point (±0.5 is realistic), and a photograph's lighting changes how rusty a nail looks. Percentage coverage in the simulation is more precise, but a real nail's coverage can only be estimated to about ±10%.

Evaluation: how good is this evidence?

• Subjective scoring (biggest weakness). A visual scale compresses real differences. Improvement: weigh each nail before and after on a ±0.01 g balance (rust adds mass as oxygen joins the iron) or photograph against a grid and count covered squares.
• No repeats. One nail per condition means one odd nail (a scratch, a greasy fingerprint) can fake a result. Improvement: three nails per cup and average the scores.
• Uneven conditions. Cups near a window get more warmth and light; evaporation concentrates the salt water over five days. Improvement: keep all cups side by side, top up to a marked water line daily.
• Half-covered nails rust fastest at the waterline — where oxygen and water meet — so "amount of rust" depends on where you look. Record the waterline separately.

Common mistakes

• Using galvanised or stainless nails: both are *designed* not to rust, so the experiment "fails" — check the nails are plain iron or steel.
• Handling nails with bare fingers before the test: skin grease is a mini barrier coating and creates patchy results.
• Scoring rust from memory at the end instead of photographing daily — day-2 evidence is gone by day 5.
• Moving the cups between observations, changing light and temperature mid-investigation.

Exam Experience

Question 1 — analysis [4 marks]. Using the worked-investigation table: (a) state the day-5 score for each wet condition; (b) calculate how many times greater the salt-water score is than the oiled-nail score on day 5; (c) explain, in terms of electrons and ions, why salt water rusts the nail faster than plain water.

Scoring guide:

• B = 2, C = 3, D = 1 [1]
• 3 ÷ 1 = 3 times greater [1]
• Salt water contains mobile ions, making the surface film a better conductor/electrolyte [1]
• So iron loses electrons (oxidation) and oxygen gains them (reduction) at a higher rate [1]

Question 2 — design [4 marks]. A student claims a new "miracle spray" stops rust completely. Design a fair five-day test of the claim using nails and salt water, including how you would make the rust measurement objective.

Scoring guide:

• Includes an unsprayed control nail and identical nails, same salt water, same position [1]
• Applies the spray exactly as directed to the test nail(s); uses at least 3 nails per condition [1]
• Measures objectively: mass change on a ±0.01 g balance or grid-counted photo coverage, same method daily [1]
• States a clear decision rule in advance, e.g. spray "works" only if mean day-5 coverage is less than half the control's [1]

Multiple choice 1. Which cup is the control in this investigation, and why?

• A. Salt water — it rusts fastest
• B. Oiled nail — it tests a barrier
• C. Dry air — it shows what happens without liquid water, the baseline for comparison
• D. Plain water — it is the most natural condition

Answer: C. A control shows the effect of removing the key variable; here it proves rusting needs liquid water, not just air.

Multiple choice 2. Salt speeds up rusting mainly because it:

• A. Reacts directly with iron to form rust
• B. Makes the water film conduct ions better, speeding the electrochemical reaction
• C. Warms the water as it dissolves
• D. Removes the iron's protective coating

Answer: B. NaCl does not appear in the rust equation; it provides the electrolyte that lets the oxidation and reduction half-reactions run faster.

Going deeper: a rusting nail is a battery eating itself

Look again at where a half-dipped nail rusts fastest: at the waterline. That is no accident. Deep under water, oxygen is scarce, so that part of the nail becomes the site where iron loses electrons (Fe → Fe²⁺ + 2e⁻). Near the surface, oxygen is plentiful, and that is where the electrons are consumed (O₂ + 2H₂O + 4e⁻ → 4OH⁻). The nail is literally a short-circuited battery: an anode and a cathode on the same piece of metal, with the salt water as its electrolyte. The Fe²⁺ and OH⁻ meet and precipitate as rust between the two sites.

Because rust contains added oxygen and water, a rusting nail gains mass — a measurable, objective alternative to the 0–3 score. Rust (Fe₂O₃·H₂O) has a molar mass of about 177.7 g/mol, of which the iron is 2 × 55.85 = 111.7 g:

Rust mass = iron oxidised x (177.7 / 111.7) = iron x 1.59
0.30 g of iron oxidised -> 0.48 g of rust formed

Corrosion engineers fight this reaction on an enormous scale: rust is estimated to cost around 3–4% of GDP in industrialised countries — bridges repainted, pipelines wrapped or cathodically protected, ships fitted with the sacrificial zinc blocks you will meet again in Experiment 17. Your five-day cups are the same science in miniature: control the water, the oxygen or the electron path, and you control the rust.