Edexcel GCSE Chemistry (1CH0) — Transition Metals, Quantitative Analysis and Equilibria
Every time you leave an unprotected iron bicycle out in the rain, you witness a destructive chemical reaction taking place. Corrosion is the general term for the gradual destruction of a metal by chemical reactions with its environment. However, rusting is the specific term used ONLY for the corrosion of iron and its alloys (like steel).
Rusting is an oxidation process and a redox reaction. Iron atoms lose electrons to form ions (oxidation), while oxygen is reduced. This reaction forms a flaky, orange-brown solid called hydrated iron(III) oxide. The word "hydrated" means the structure contains water of crystallisation (water molecules chemically bound within the solid crystal). Because the iron reacts with both oxygen and water to form this new compound, the mass of the iron object increases as it rusts. Unlike aluminium, which forms a protective non-porous layer when it oxidises, rust flakes away to expose fresh iron underneath, leading to continuous destruction.
To prove that iron requires BOTH oxygen and water to rust, you must set up an experiment isolating these variables over a period of about one week.
First, clean three non-galvanised iron nails with sandpaper to remove any pre-existing oxide layers. Then, set up three separate test tubes to test different conditions.
Tube 1 (Control - Air and Water):
Tube 2 (Excluding Oxygen):
Tube 3 (Excluding Water):
After leaving the three test tubes for roughly a week (keeping temperature and nail size as control variables), you will observe the following results:
The rate of rusting in Tube 1 could be further increased by adding sodium chloride (salt) or making the conditions acidic.
The overall reaction for rusting can be summarised by the following word equation:
The balanced symbol equation uses an to represent the variable number of water molecules in the hydrated compound:
Students frequently use the word 'rusting' when discussing metals like copper or aluminium. You must only use 'rusting' for iron or steel; for all other metals, use the term 'corrosion'.
In 6-mark questions describing the three test tubes, examiners require you to explicitly state WHY you boil the water (to remove dissolved oxygen) and WHY you use oil (as a barrier to stop oxygen re-entering).
Examiners will strictly reject 'iron oxide' or 'iron(II) oxide' as the chemical name for rust — you must write the full name 'hydrated iron(III) oxide', taking care to include the Roman numeral (III).
If asked about mass change during rusting, always state that the mass increases because oxygen from the air and water combine with the iron atoms to form a solid compound.
Corrosion
The gradual destruction of a metal due to chemical reactions with substances (usually oxygen and water) in the environment.
Rusting
The specific term used exclusively for the corrosion of iron and its alloys, such as steel.
Oxidation
A type of reaction where a substance gains oxygen or loses electrons.
Redox
A reaction in which both reduction and oxidation occur simultaneously.
Hydrated iron(III) oxide
The specific chemical name for rust, consisting of iron(III) oxide chemically bonded to a variable number of water molecules.
Water of crystallisation
Water molecules that form an essential part of the crystalline structure of a compound.
Barrier method
A method of rust prevention that physically blocks oxygen and water from reaching the metal surface, such as oil, paint, or grease.
Anhydrous calcium chloride
A chemical compound that contains no water and is commonly used as a drying agent.
Desiccant
A substance that absorbs water vapour from its immediate environment to maintain dry conditions.
Put your knowledge into practice — try past paper questions for Chemistry
Corrosion
The gradual destruction of a metal due to chemical reactions with substances (usually oxygen and water) in the environment.
Rusting
The specific term used exclusively for the corrosion of iron and its alloys, such as steel.
Oxidation
A type of reaction where a substance gains oxygen or loses electrons.
Redox
A reaction in which both reduction and oxidation occur simultaneously.
Hydrated iron(III) oxide
The specific chemical name for rust, consisting of iron(III) oxide chemically bonded to a variable number of water molecules.
Water of crystallisation
Water molecules that form an essential part of the crystalline structure of a compound.
Barrier method
A method of rust prevention that physically blocks oxygen and water from reaching the metal surface, such as oil, paint, or grease.
Anhydrous calcium chloride
A chemical compound that contains no water and is commonly used as a drying agent.
Desiccant
A substance that absorbs water vapour from its immediate environment to maintain dry conditions.
