Class 7 Science Chapter 4: The World of Metals and Non-metals Notes

What is this chapter about?

Yashwant and Anandi visit Sudarshan, an ironsmith in their Rajasthan village, for a school project. He makes tawas, buckets, tongs, spades, axes, trowels and rakes from iron, with wooden handles, heating the iron in a coal furnace. When he beats a red-hot iron block into the shape of an axe, Anandi wonders whether other metals can be flattened too. The chapter explores what metals can do, how they react with air and water, and how non-metals differ.

Lustre and hardness

Hammering pieces of copper, aluminium, iron, coal, sulfur and wood shows a clear divide. Copper, aluminium and iron are shiny, or lustrous, and hard: they are metals, and their shine is called metallic lustre. Coal, sulfur and wood are dull and not as hard. Not every metal is a hard solid, though: sodium and potassium are soft enough to cut with a knife, and mercury is a liquid at room temperature.

Sheets, wires and sound

When hammered, the metals flatten into sheets. This is malleability: silver foil on sweets and aluminium foil for wrapping food come from it, and gold and silver beat into sheets more easily than any other metal. Coal and sulfur break into pieces instead, so they are brittle, and wood neither flattens nor breaks. Metals can also be drawn into wires, which is ductility: electrical wires, bangles, the strings of a veena or sitar, and steel ropes for bridges and cranes all depend on it. Gold is so ductile that one gram can make a wire about 2 km long. Metals are also sonorous: a dropped spoon or coin rings, as do school bells and ghungroos, while coal and wood just thud. Read more in properties of metals and non-metals.

Iron changed Indian history. The Harappans used copper and gold for tools and jewellery but hardly any iron, which came into everyday use much later. Once it did, strong iron ploughs and tools were far superior to earlier ones and helped civilisation advance.

Conductors of heat and electricity

Stand a metal spoon and a wooden spoon in hot water, and only the metal spoon’s top gets hot: metals are good conductors of heat, which is why pots are metal and their handles wood or plastic. In a tester circuit, aluminium foil, an iron nail and a copper wire make the bulb glow, while sulfur, coal, wood, stone, rubber and nylon do not. Metals are good conductors of electricity; rubber gloves and plastic handles protect electricians because they are poor conductors.

Rusting and corrosion

Three iron nails tell the story. A nail in dry air does not rust, nor does one under boiled water sealed with oil, but a nail in an open bottle, partly in water, gets brown deposits. Iron rusts only when it meets both air and water. The brown deposit is rust, and its formation is rusting. Other metals suffer corrosion too: copper gets a green coating and silver a black one. Painting, oiling, greasing and galvanisation, a layer of zinc, keep iron safe. The Iron Pillar of Delhi, made more than 1600 years ago, has barely any rust. Read rusting and corrosion.

Oxides of metals and non-metals

A magnesium ribbon burns with a dazzling white flame and turns into a white powder, magnesium oxide. Its solution turns red litmus blue, so it is basic: oxides of metals are generally basic. Sodium reacts so vigorously with oxygen and water that it is kept in kerosene. Burning sulfur forms sulfur dioxide, which dissolves in water to give sulfurous acid, turning blue litmus red: oxides of non-metals are acidic. Read oxides of metals and non-metals.

Non-metals in our lives

Non-metals such as sulfur and phosphorus are usually soft and dull, neither malleable nor ductile, not sonorous and poor conductors; phosphorus is kept in water because it catches fire in air. We cannot live without them: we breathe oxygen, carbon is the building block of all living things, nitrogen goes into fertilisers, chlorine purifies water and iodine is an antiseptic. Metals and non-metals are elements, of which about 118 are known; plastic, glass, wood, rubber and paper are not elements. Many metals and alloys, such as steel, run modern industry, and India recycles iron and aluminium.

In a nutshell

Metals and non-metals differ in how they look, feel, sound and conduct, and in how they react with air and water, and these properties decide their uses. Answers to every activity, Tables 4.1 to 4.3 and all twelve questions are in the NCERT solutions for Chapter 4.