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Why does iron rust, and how can we prevent it?

Class 10 ScienceNCERT chapter: Chemical Reactions and Equationsइसे हिंदी में पढ़ें

Exam-ready answer

Iron rusts when it reacts with oxygen and moisture in air to form hydrated iron(III) oxide, a reddish-brown flaky coating called rust. This is an oxidation reaction. Rusting can be prevented by painting, oiling, greasing, galvanisation (coating with zinc), or alloying to form stainless steel.

Explanation

Rusting is a chemical change where iron reacts with oxygen and water (moisture) to form hydrated iron(III) oxide, Fe2O3·xH2O. The reaction is: 4Fe + 3O2 + xH2O → 2Fe2O3·xH2O. This is an oxidation reaction because iron gains oxygen. Rust is reddish-brown and flaky, and it weakens the iron object over time.

For rusting to occur, both air (oxygen) and moisture are necessary. If either is absent, rusting does not happen. For example, iron nails do not rust in boiled distilled water (which has no dissolved oxygen) or in dry air. Rusting is faster in coastal areas because of high moisture and salt in the air.

Prevention methods stop the contact of iron with air and moisture. Common methods include painting, oiling, greasing, and galvanisation (coating iron with a layer of zinc). Alloying iron with carbon and other metals to make stainless steel also prevents rusting. These methods are used in everyday objects like bridges, car bodies, and kitchen utensils.

Points to remember

  1. Rust is hydrated iron(III) oxide, Fe2O3·xH2O, formed by oxidation of iron.
  2. Both oxygen (air) and moisture (water) are necessary for rusting.
  3. Rusting is a redox reaction: iron is oxidised, oxygen is reduced.
  4. Prevention methods: painting, oiling, greasing, galvanisation, alloying (stainless steel).
  5. Galvanisation is coating iron with zinc; zinc protects iron even if the coating is broken.

Exam tip

Write the chemical equation for rusting and mention that both air and moisture are necessary. For prevention methods, list at least three with brief explanations.

Questions students also ask

What is the chemical formula of rust?
Rust is hydrated iron(III) oxide, with the formula Fe2O3·xH2O. It is a reddish-brown flaky substance formed when iron reacts with oxygen and moisture.
Why does rusting not occur in distilled water?
Distilled water does not contain dissolved oxygen or salts. Rusting requires both oxygen and moisture. In distilled water, oxygen is absent, so rusting does not occur.
How does galvanisation prevent rusting?
Galvanisation is coating iron with a layer of zinc. The zinc layer prevents contact of iron with air and moisture. Even if the coating is broken, zinc corrodes first, protecting iron.

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