The World of Metals and Non-metals – Class 7 Science Chapter 4 Notes & NCERT PDF

Introduction

Yashwant and Anandi from a village in Rajasthan visit local ironsmiths with their grandfather for a school project. Sudarshan uncle makes tawas, baltis, chimtas and farming tools like phawras (spades), kulhadis (axes), khurpis (trowels) and jelees (rakes) from iron, uses wood for handles and coal in the furnace. He heats iron till it is red hot and beats it into an axe. Anandi wonders — can other metals be beaten flat too?

Key Concepts

4.1 Properties of materials

Appearance and hardness (Activity 4.1) — copper, aluminium and iron are lustrous (shiny — metallic lustre) and hard. Coal, sulfur and wood are non-lustrous and not as hard. But not all metals are hard: sodium and potassium can be cut with a knife, and mercury is liquid at room temperature.

Malleability — the property of being beaten into thin sheets. Copper, iron and aluminium flatten when hammered. Gold and silver are the most malleable; silver foil on sweets and aluminium foil are examples. Coal and sulfur break into pieces — they are brittle. Wood is neither malleable nor brittle.

Ductility — the property of being drawn into wires. Copper and aluminium wires are used in electrical fittings; metal wires are used in jewellery and instruments like veena, sitar, violin, guitar. One gram of gold can be drawn into a wire 2 km long! Steel ropes (iron + carbon) hold suspension bridges and cranes.

Sonority (Activity 4.2) — metals give a ringing sound when struck (spoon, coin, school bell, ghungroos); coal and wood give a dull sound. Metals are sonorous.

Conduction of heat (Activity 4.3) — a metal spoon in hot water gets hotter than a wooden spoon. Metals are good conductors of heat, so cooking vessels are metal while handles are wood or plastic.

Conduction of electricity (Activity 4.4) — using the tester, aluminium, iron and copper make the bulb glow; sulfur, coal, wood, stone, rubber eraser and nylon rope do not. Electricians use plastic-handled screwdrivers and rubber gloves because these are poor conductors.

Metals are generally hard, lustrous, malleable, ductile, sonorous, and good conductors of heat and electricity.

Holistic lens: The Harappans used copper and gold but hardly any iron. Iron came into common use later and, being strong, gave far better tools like ploughs.

4.2 Effect of air and water on iron (Activity 4.5)

Three clean nails in three bottles for 8–10 days:

Rusting needs both air and water (moist air). The brown deposit is rust.

Other metals also discolour — a green coating on copper, a black coating on silver. This gradual damage by air, water or other substances is corrosion. Rusting is prevented by painting, oiling, greasing and galvanisation (a coat of zinc).

The Iron Pillar of Delhi — made in the time of Chandragupta II, more than 1600 years ago, about 8 metres high and over 6000 kg — has barely any rust, showing India's ancient skill in metal technology.

4.3 Metals with air and water (Activity 4.6 — demonstration)

A cleaned magnesium ribbon burns with a dazzling white flame, forming a white powder — magnesium oxide. Dissolved in water, it turns red litmus blue → basic.

Metal oxides are generally basic.

Sodium is stored in kerosene because it reacts vigorously with oxygen and water, giving out a lot of heat.

4.4 Non-metals (Activity 4.7–4.8)

Sulfur burns to form sulfur dioxide gas; dissolved in water it forms sulfurous acid, which turns blue litmus red → acidic. Sulfur does not react with water. Phosphorus is stored in water because it catches fire in air.

Non-metals (sulfur, phosphorus, oxygen, hydrogen, nitrogen, carbon…) are usually soft and dull, not malleable, ductile or sonorous, poor conductors, and their oxides are acidic.

Plastic, glass, wood, rubber and paper are not metals or non-metals — they are not elements. An element cannot be broken down into simpler substances; 118 elements are known.

4.5 Non-metals in everyday life

Science and society: Alloys are mixtures of metals (or a metal and a non-metal). Special metals: zirconium (atomic energy), titanium (aerospace). India recycles iron and aluminium for sustainability.

Key Terms

Common Misconceptions

Keep the Curiosity Alive (exercise hints)