Rusting needs water and oxygen
Rusting is the corrosion of iron and steel and needs BOTH water AND oxygen at once — neither alone is enough. Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O, where x is variable. The three-tube test proves it: a nail with a drying agent has oxygen but no water and does not rust; a nail in boiled water under oil has water but no oxygen and does not rust; a nail in ordinary water open to air rusts. Salt and acid rain speed it up.
Barrier and sacrificial protection
Barrier protection keeps water and oxygen off the iron: paint, oil, plastic, tin plating and chromium plating all work this way, but rusting restarts wherever the coating is scratched. (Extended) Sacrificial protection attaches a MORE reactive metal that corrodes preferentially, protecting the iron even where scratched. Galvanising coats steel with zinc; magnesium blocks guard ship hulls and pipelines. The reactive metal is used up and must be replaced.
(Extended) Why the metal must be more reactive
Sacrificial protection works only if the protecting metal is HIGHER than iron in the reactivity series, so it is oxidised more easily. Zinc and magnesium qualify; tin, being LESS reactive than iron, does not. In contact through water, the reactive metal loses electrons (Zn → Zn²⁺ + 2e⁻) and these flow to the iron, keeping it electron-rich so it cannot be oxidised. This is why a scratched tin can rusts faster — iron then becomes the metal oxidised preferentially.
Drawn from real examiner reports.
Both water AND oxygen, not one
The mark scheme needs BOTH water and oxygen named; "iron rusts when exposed to oxygen" or "when it gets wet" gives only one condition and scores nothing. The three-tube experiment (dry oxygen gives no rust; boiled, sealed water gives no rust) shows neither alone works. Answer "both water and oxygen from the air". Think "both or nothing" — one condition never rusts iron.
Rust is hydrated iron(III) oxide
Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O; calling it just "iron oxide" or "Fe₂O₃" drops the "hydrated" mark. "Hydrated" means water is chemically combined in the solid, not merely present as solvent — do not confuse a hydrated compound with an aqueous solution. When asked for the name or formula of rust, include the combined water.
S23 P31 Q3(d)(i) — "hydrated" requires water chemically combined, not merely present; confused with aqueous.
(Extended) Tin plating is barrier only
Tin plating protects only as a BARRIER — it is not sacrificial. Tin is LESS reactive than iron, so once the tin is scratched the exposed iron is oxidised preferentially and rusts FASTER at the scratch. Do not claim tin "sacrifices itself". Only a coating MORE reactive than iron (zinc, magnesium) gives sacrificial protection that survives a scratch.
(Extended) Galvanising: give the mechanism
At Extended level, "the zinc keeps water and oxygen away" is only the Core barrier answer and earns partial credit. Asked WHY galvanising still protects a scratched surface, give the sacrificial mechanism: zinc is more reactive than iron (higher in the series), so zinc is oxidised preferentially and the iron is not oxidised. Use "oxidised preferentially", not "reacts first".
Only iron rusts; others corrode
Rusting is specific to iron and steel. Aluminium, copper and zinc corrode, but they do not "rust" — using "rust" for another metal loses precision marks. Corrosion is the general term for a metal reacting with its environment; rusting is the particular case for iron forming hydrated iron(III) oxide. Reserve "rust" for iron only, and use "corrode" for the rest.
(Extended) Sacrificial metal must be more reactive
A sacrificial metal must be MORE reactive than iron, so copper (less reactive) is useless — attaching copper would make iron the preferentially oxidised metal and rust FASTER. Candidates sometimes pick any metal, or reverse the logic. The block is chosen because it is higher in the series than iron, and is consumed as it protects, so it needs periodic replacement.
Name both water and oxygen
Whenever you explain rusting or protection, name BOTH water AND oxygen — "prevents air" or "keeps water out" alone loses marks. Say a barrier "stops water and oxygen reaching the iron", and name rust as "hydrated iron(III) oxide", not just "iron oxide".
Barrier vs sacrificial scaffold
Split the question two ways. Barrier: stops water and oxygen reaching the iron, and fails when scratched. Sacrificial (Extended): the attached metal is more reactive than iron and is oxidised preferentially, so the iron is not oxidised even when scratched.
Use precise corrosion language
Precise wording earns marks. Use "oxidised preferentially", not "reacts first". Use "prevents water AND oxygen", not "prevents air". Name rust "hydrated iron(III) oxide", not "iron oxide". Link reactivity to electron loss, not just "blocking".
Corrosion — the destruction of a metal by chemical reaction with substances in its environment (oxygen, water, acids).
Rusting — the specific corrosion of iron (or steel) in the presence of both water and oxygen to form hydrated iron(III) oxide (rust).
Rust — hydrated iron(III) oxide, formula , where is variable.
Barrier protection — a method that prevents rusting by physically keeping water and/or oxygen away from the iron surface (paint, oil, grease, plastic coating, tin plating, chromium plating).
(Extended) Sacrificial protection — a method that attaches a more reactive metal (zinc or magnesium) to the iron. The more reactive metal is oxidised preferentially (corrodes instead of the iron), so the iron is protected even if the coating is damaged.
State the two conditions required for iron to rust.
The iron(III) oxide component of rust has the formula .
Write a balanced symbol equation for the reaction of iron with oxygen to form iron(III) oxide. Include state symbols.
