Chapter 3 · Question 15

What is corrosion? Describe the conditions necessary for the rusting of iron using Activity 3.14. List any four methods of preventing corrosion and explain the principle of galvanisation. Also, define alloys and give two examples of alloys with their composition and one specific property each.

Q15

What is corrosion? Describe the conditions necessary for the rusting of iron using Activity 3.14. List any four methods of preventing corrosion and explain the principle of galvanisation. Also, define alloys and give two examples of alloys with their composition and one specific property each.

Answer Revealed

Direct Answer:

Corrosion is the slow degradation of a metal due to reaction with substances in its environment (moisture, acids, etc.). Activity 3.14 shows that iron rusts only when BOTH air (oxygen) and water (moisture) are present — nails in a tube with water and air rust, nails in boiled water covered with oil (no dissolved air) do not rust, and nails with anhydrous CaCl₂ (no moisture) do not rust. Prevention methods: (i) painting, (ii) oiling/greasing, (iii) galvanisation (coating with zinc), (iv) chrome plating, (v) anodising, (vi) alloying. Galvanisation coats iron with a thin layer of zinc — even if the zinc coating is scratched, zinc (being more reactive) corrodes preferentially and still protects the underlying iron. An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. Example 1: Stainless steel — iron + nickel + chromium; hard and does not rust. Example 2: Solder — lead + tin; has a low melting point, used for welding electrical wires.

Simple Explanation

Corrosion is metal being eaten away by its surroundings — like rust turning a shiny iron gate into a crumbling brown mess. Rust needs both water and air to form — take away either one, and rusting stops. To prevent it, you block the metal's surface with paint, oil, or a layer of zinc (galvanisation). Zinc is the clever one — even if you scratch it, zinc rusts first and still protects the iron underneath. Alloys are like 'metal cocktails' — mixing metals creates new materials with better properties. Stainless steel (iron + chromium + nickel) never rusts. Solder (lead + tin) melts easily and is used to join wires.

Exam-Ready Structure

Corrosion and its prevention, alongside alloys, form the real-world applications chapter that connects chemical principles to everyday materials. 1. Corrosion: The process in which a metal is attacked and damaged by substances in its environment, such as moisture and acids. It is an unwanted oxidation that gradually destroys metal articles. 2. Examples of corrosion: (a) Rusting of iron: When iron is exposed to moist air for a long time, it develops a reddish-brown, flaky coating called rust (composition: hydrated iron(III) oxide,

\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}

). (b) Copper: Slowly turns green in moist air due to formation of basic copper carbonate (

\text{CuCO}_3 \cdot \text{Cu(OH)}_2

). (c) Silver: Develops a black coating of silver sulphide (

\text{Ag}_2\text{S}

) when exposed to sulphur compounds in the air. 3. Activity 3.14 — Conditions for rusting: Three test tubes with clean iron nails: (a) Tube A (water + air present): Nails rust — proves both air and water are needed. (b) Tube B (boiled distilled water + oil layer, no dissolved air): No rust — only water, no oxygen. (c) Tube C (anhydrous CaCl₂, dry air): No rust — only air, no moisture. Conclusion: Both air (oxygen) AND water (moisture) are essential for rusting. 4. Methods of prevention: (a) Painting — creates a physical barrier. (b) Oiling/greasing — blocks moisture and air. (c) Galvanisation — coating with a thin layer of zinc. Zinc is more reactive than iron; even if scratched, zinc corrodes preferentially (sacrificial protection) and prevents iron from rusting. (d) Chrome plating, anodising, and alloying are other methods. 5. Alloys: (a) Definition: An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. It is prepared by melting the primary metal and dissolving other elements in it in definite proportions, then cooling. (b) Properties: Alloys generally have lower electrical conductivity and lower melting points than pure constituent metals. (c) Examples: Stainless steel (Fe + Ni + Cr) — hard and rust-resistant. Brass (Cu + Zn) — good for decorative items. Bronze (Cu + Sn) — tough and corrosion-resistant. Solder (Pb + Sn) — low melting point, used for welding electrical wires. Amalgam — an alloy containing mercury. (d) Gold jewellery uses 22-carat gold (22 parts Au alloyed with 2 parts Ag or Cu) because pure 24-carat gold is too soft.

Key Points

Corrosion: degradation of metal by reaction with environment (moisture, air, acids)
Rusting needs BOTH air (O₂) and water — proved by Activity 3.14 with 3 test tubes
Rust = $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$ (hydrated iron(III) oxide); copper goes green; silver goes black
Prevention: painting, oiling, galvanising (Zn coating — sacrificial protection), alloying, chrome plating
Alloy: homogeneous mixture of metals/metal + non-metal (e.g., stainless steel, brass, bronze, solder, amalgam)

What is corrosion? Describe the conditions necessary for the rusting of iron using Activity 3.14. List any four methods of preventing corrosion and explain the principle of galvanisation. Also, define alloys and give two examples of alloys with their composition and one specific property each.