MYP Chemistry: Corrosion & Rusting FAQ

Corrosion is the gradual chemical breakdown of a metal when it reacts with substances in its environment, usually oxygen and water. Rusting is corrosion specifically affecting iron and steel, producing the reddish-brown flaky substance hydrated iron(III) oxide. Every metal corrodes differently — only iron actually 'rusts'.

Rusting happens when iron reacts with both oxygen and water: 4Fe(s) + 3O₂(g) + 6H₂O(l) → 4Fe(OH)₃(s). That iron(III) hydroxide then loses water (dehydrates) to form hydrated iron(III) oxide, Fe₂O₃·xH₂O — the flaky reddish-brown rust you see forming on exposed steel.

Rusting is one specific type of corrosion — it only applies to iron and steel reacting with oxygen and water to form iron oxide. Corrosion is the broader term for any metal degrading chemically through contact with its surroundings, including copper turning green (verdigris) or silver tarnishing black.

Rusting is a chemical change, not physical. Iron atoms lose electrons to oxygen in a redox reaction, forming a genuinely new substance — iron oxide — with different properties from the metal it came from. You can't scrape rust off and recover the original iron; the change isn't reversible by physical means alone.

Rusting speeds up with more dissolved oxygen, more moisture, higher temperatures, dissolved salt (which acts as an electrolyte), and acidic conditions — which is exactly why cars rust faster near the coast or on gritted winter roads. It slows down in dry air, at low temperatures, or when the surface is coated.

A reliable rusting investigation uses four identical iron nails in separate test tubes: one in boiled, oxygen-free water sealed under oil; one in ordinary tap water; one in salt water; and one in dry air with a drying agent such as calcium chloride. Compare rust formation after five to seven days.

Reactivity decides it. Metals higher on the reactivity series lose electrons more readily, so they oxidise faster. Gold and platinum barely corrode because they're unreactive; magnesium and iron corrode quickly. Aluminium looks resistant, but it actually forms a tough, invisible oxide layer that protects the metal underneath from further attack.

Rusting is prevented either by stopping oxygen and water from reaching the iron surface, or by making the metal itself more resistant to reacting. Common methods include painting, oiling, galvanising with zinc, electroplating with tin or chromium, and alloying iron into stainless steel using chromium and nickel.

Expect "state" (a fact, no explanation needed), "describe" (a detailed account with no reasoning), "explain" (give reasons using cause and effect), and "investigate" (observe, question, and collect data). In corrosion topics, "explain why" questions almost always want oxygen, water, and the redox process linked together, not listed separately.

The mistake I see most often marking mock investigations: students write "iron reacts with air" without specifying that both oxygen and water need to be present — miss either one and rusting simply doesn't start. The second common error is confusing rust (the compound) with rusting (the process) in written answers.

A corrosion investigation usually feeds into MYP Sciences Criterion B (Inquiring and investigating) and Criterion C (Processing and evaluating), alongside Criterion A (Knowing and understanding) for explaining the underlying redox chemistry. According to the IB, each MYP Sciences criterion has been marked out of a maximum of 8 since the MYP Next Chapter framework began in 2014.

A typical MYP-style question: "Explain why an iron nail left in salt water rusts faster than one kept in a sealed, dry container." A full-mark answer names both conditions needed (oxygen and water), explains salt's role as an electrolyte speeding up electron transfer, and states the product formed — hydrated iron(III) oxide.

Corrosion and rusting teach redox reactions, reaction rates, and experimental design in one accessible, everyday context — skills your child will build on directly in DP Chemistry, where oxidation states and electrochemistry return in far more depth. It's genuinely one of the better-designed bridge topics between MYP and DP.

Look for clear, concise revision notes that separate the theory (why rusting happens) from the exam skill (how to answer "explain why" questions), plus practice questions with mark schemes so your child can self-check. RevisionPrep's MYP Chemistry Revision Notes and Topical Worksheets cover both, with sample investigations already mapped to the assessment criteria.