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The World of Metals and Non-metals - Effect of Air and Water on Metals: Iron

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Rusting is a slow chemical process in which iron reacts with oxygen and moisture present in the air to form a reddish-brown flaky substance called rust.

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The chemical name for rust is hydrated iron (III) oxide, represented as Fe2O3⋅xH2OFe_2O_3 \cdot xH_2O.

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For rusting to occur, two conditions are mandatory: the presence of Oxygen (O2O_2) and the presence of Water or Water vapour (H2OH_2O).

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Rusting is a destructive process as it makes iron objects weak and eventually eats them away, leading to significant economic loss.

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Prevention of rusting can be achieved by: 1. Coating with paint or grease to cut off contact with air and moisture. 2. Galvanization, which is the process of depositing a thin layer of Zinc (ZnZn) on iron. 3. Alloying, such as mixing iron with Chromium (CrCr), Nickel (NiNi), and Manganese (MnMn) to form stainless steel, which does not rust.

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The rate of rusting is higher in coastal areas because of the high moisture content and the presence of salts in the air, which accelerate the chemical reaction.

📐Formulae

4Fe+3O2+2xH2O→2Fe2O3⋅xH2O4Fe + 3O_2 + 2xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O

Iron+Oxygen (from air)+Water→Hydrated Iron Oxide (Rust)Iron + Oxygen\ (from\ air) + Water \rightarrow Hydrated\ Iron\ Oxide\ (Rust)

💡Examples

Problem 1:

Explain why an iron nail kept in boiled distilled water (covered with a layer of oil) does not rust, while an iron nail kept in ordinary tap water rusts quickly.

Solution:

Rusting requires both Oxygen (O2O_2) and Water (H2OH_2O). In boiled distilled water, the dissolved oxygen is removed during boiling, and the layer of oil prevents any air from dissolving back into the water. In tap water, both moisture and dissolved oxygen are present, fulfilling all conditions for the reaction 4Fe+3O2+2xH2O→2Fe2O3⋅xH2O4Fe + 3O_2 + 2xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O.

Explanation:

Boiling removes air, and oil acts as a barrier. Without oxygen, the chemical oxidation of iron cannot take place.

Problem 2:

Why is stainless steel preferred over pure iron for making kitchen utensils like knives and spoons?

Solution:

Pure iron reacts with oxygen and moisture to form rust (Fe2O3⋅xH2OFe_2O_3 \cdot xH_2O). Stainless steel is an alloy of Iron (FeFe) with Carbon (CC), Chromium (CrCr), Nickel (NiNi), and Manganese (MnMn). This alloy does not rust even when exposed to water and air.

Explanation:

Alloying changes the chemical properties of the metal, making it resistant to corrosion and more durable for household use.