Hinglish

Malleability, Ductility and Sonority

Class 7 Science · Chapter 4: The World of Metals and Non-metals · NCERT Curiosity section 4.1.1–4.1.3

Learn

Zoya and Kabir visit a village ironsmith. He heats an iron block until it is red hot, then beats it with a hammer into the shape of an axe. Pans, buckets, tongs, spades and trowels all start as iron. "Can other materials be beaten flat too?" Zoya wonders.

Copper, aluminium and iron are lustrous (shiny) and hard: these are metals. Coal, sulfur and wood are dull. Hammered, the metals flatten: the property of being beaten into thin sheets is malleability. Silver leaf on sweets and aluminium foil are examples; gold and silver are the most malleable. Coal and sulfur break: they are brittle. Wood does neither.

Not all metals are hard: sodium and potassium can be cut with a knife, and mercury is a liquid at room temperature. Metals can also be drawn into wires: this is ductility. Copper and aluminium wires, sitar and guitar strings, and steel ropes on cranes and bridges all use it. One gram of gold can make a wire about 2 km long!

Drop a steel spoon or a coin and it rings; drop coal or wood and you hear a dull thud. The ringing property of metals is sonority: think of school bells, temple bells and ghungroos. A person who cannot see may tap a stick to tell metal from wood by sound. Iron tools such as ploughs helped Indian civilisation grow, long after copper and gold were first used.

Explore: hammer test.

  1. Hammer each object and sort it.

Remember

  • Metals are lustrous, malleable (sheets), ductile (wires) and sonorous (ring).
  • Coal and sulfur are brittle; mercury is a liquid metal; sodium is soft.

Quick check

Which property lets aluminium be made into foil?

  1. malleability
  2. ductility
  3. sonority
  4. brittleness
Answer

malleability. Being beaten into thin sheets is malleability.

Think about it

Why is it believed that copper was used by people earlier than iron?

One answer

Copper is sometimes found almost pure and can be shaped easily at lower temperatures, while getting iron from its ore needs much hotter furnaces and more skill.

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.