Question
Why does iron rust when exposed to air and moisture, but gold does not rust or corrode even after years of exposure? Explain in terms of reactivity.
Solution — Step by Step
Rusting is a specific type of corrosion that affects iron and its alloys. It produces a reddish-brown, flaky substance called rust, which is hydrated iron(III) oxide: .
The chemical reaction involves iron reacting with oxygen AND water simultaneously:
Both oxygen AND water are necessary — iron doesn’t rust in completely dry air or in deoxygenated water.
Metals are arranged in the reactivity series (electrochemical series) based on their tendency to lose electrons and form ions.
Position of key metals:
- Iron (Fe): moderately reactive — middle of the series
- Gold (Au): least reactive — at the very bottom
Iron has a relatively strong tendency to lose electrons: or .
This means iron readily donates electrons to oxygen (which accepts electrons). The driving force for oxidation of iron by atmospheric oxygen is thermodynamically favourable — the products (iron oxides) have lower energy than the reactants.
Gold is an extremely noble metal — it sits at the bottom of the reactivity series with a very high standard reduction potential ( V for ).
This means:
- Gold has almost NO tendency to lose electrons and form ions
- Oxygen’s oxidising power is insufficient to oxidise gold under normal conditions
- The reaction is NOT thermodynamically spontaneous under ordinary conditions
In fact, gold oxide () decomposes back to gold and oxygen above 160°C — the oxide is thermodynamically unstable.
The difference in reactivity has profound practical implications:
- Iron must be protected by galvanising (zinc coating), painting, or alloying (making stainless steel with chromium) to prevent rust
- Gold is used in jewellery, electronic connectors, dental applications, and spacecraft because it maintains its surface integrity indefinitely
- The reactivity difference is also why gold coins survived thousands of years underground (from Harappan civilisation to Roman hoards) but iron tools from the same period have often corroded away
The electrochemical series quantifies this: metals with more negative standard electrode potentials (like Fe, V) are more easily oxidised than metals with more positive potentials (like Au, V).
Why This Works
Corrosion is fundamentally an electrochemical process — it’s the spontaneous oxidation of a metal by its environment. The thermodynamic driving force is the difference in standard electrode potentials between the metal and the oxidising agent (usually O₂/H₂O).
For iron: \Delta G° < 0 for oxidation by O₂ → reaction is spontaneous → rusting occurs. For gold: \Delta G° > 0 for oxidation by O₂ → reaction is non-spontaneous → gold doesn’t corrode.
This is not just about chemistry — it’s about energy. Iron loses energy by reacting with oxygen. Gold doesn’t gain anything by reacting, so it doesn’t.
Alternative Method
A simpler way to remember: the reactivity series ranks metals from most reactive (K, Na, Ca…) to least reactive (…Cu, Ag, Au). The further down a metal is, the less it reacts with air, water, or acids. Gold is literally the least reactive metal — it doesn’t react with any single acid (only aqua regia, a mixture of concentrated HNO₃ and HCl, dissolves it).
Common Mistake
Students often say “gold is non-reactive because it’s a noble metal.” This is too vague. The specific reason is that gold has a very high standard reduction potential — it has minimal tendency to get oxidised (lose electrons). The term “noble metal” is itself defined by high reduction potential, so just saying “noble” is circular. A complete answer references the reactivity series and the tendency to lose electrons.
For CBSE Class 10, always name both conditions required for rusting: oxygen AND water. A common exam question is “prove that both air and water are necessary for rusting” — the standard experiment uses three test tubes: iron nails in (1) regular water + air, (2) boiled water with oil layer (no air), (3) anhydrous CaCl₂ (no water). Only test tube 1 shows rusting.