Properties of metals and non-metals

A visit to the ironsmith

Sudarshan, an ironsmith, heats an iron block in his coal furnace until it is red hot, then beats it with a hammer into the shape of an axe. Can other materials be shaped like this? Testing a few common materials answers the question and shows what makes metals special.

Lustre and hardness

In Activity 4.1, an iron nail, bits of copper and aluminium, a lump of sulfur about the size of a pea, some coal and a block of wood are looked at and felt, and then hammered on a hard surface. Copper, aluminium and iron are lustrous, with the shine called metallic lustre, and they are hard. These are metals. Coal, sulfur and wood are dull, or non-lustrous, and not as hard. Some metals break the pattern: sodium and potassium can be cut with a knife, and mercury is a liquid at room temperature.

Malleability

Under the hammer, copper, aluminium and the iron nail flatten. The property of being beaten into thin sheets is malleability, and most metals have it. The silver foil on some sweets and the aluminium foil used to wrap food are made this way; gold and silver beat into sheets more easily than any other metal. Coal and sulfur shatter instead, so they are brittle. Wood neither flattens into a sheet nor breaks, so it is neither malleable nor brittle.

Ductility

Metal wires are everywhere: copper and aluminium wires in electrical fittings, wire bangles, necklaces and earrings, the strings of a veena, sitar, violin or guitar, and the mesh of a tea strainer. The property of being drawn into wires is ductility, and it belongs mainly to metals; nobody has seen a wire of coal or sulfur. Gold is so ductile that a single gram can be pulled out into a wire about 2 kilometres long. Ropes of steel wire, steel being a mixture of iron and carbon, can hold heavy loads, so they are used in suspension bridges and cranes.

Sonority

Drop a metal spoon and a coin, then some coal and a wooden block (Activity 4.2). The spoon and coin give a ringing sound; coal and wood give a dull one. The ability of metals to ring is sonority, so metals are sonorous. School bells, temple bells and ghungroos ring because of it, and a person who cannot see can tell metal from wood by the sound a walking stick makes on them.

Conduction of heat

In Activity 4.3, two spoons of about the same size, one metal and one wooden, stand in a tumbler of hot water for a few minutes. The top of the metal spoon becomes hot, while the wooden spoon stays cool. Heat passing from one point of a material to another is conduction, and materials that pass heat on are conductors. Metals are good conductors of heat and wood a poor one, which is why cooking vessels are made of metal and their handles of wood or other poor conductors.

Conduction of electricity

A tester circuit, a cell, a bulb and wires with a gap, can test materials (Activity 4.4). Aluminium foil, an iron nail and a copper wire make the bulb glow; sulfur, coal, dry wood, stone, a rubber eraser and nylon rope do not. Everything that lights the bulb is a metal. Materials that let electricity flow easily are good conductors of electricity; those that do not are poor conductors. That is why electricians use screwdrivers with plastic handles and wear rubber gloves and shoes: these poor conductors protect them from electric shock.

Metals and non-metals compared

Why it matters

These properties decide what each material is used for, from wires and pans to bells and foil. How metals react with air and water is covered in rusting and corrosion and oxides of metals and non-metals. Answers to every activity and question are in the NCERT solutions for Chapter 4; see all of the Chapter 4 notes.