The World of Metals and Non-metals — Class 7 Science

This chapter distinguishes metals from non-metals through their physical and chemical properties. Metals such as copper, aluminium, and iron are generally lustrous (metallic lustre), hard, malleable (can be beaten into thin sheets), ductile (can be drawn into wires), sonorous (produce a ringing sound), and good conductors of heat and electricity. There are exceptions: sodium and potassium are soft enough to cut with a knife, mercury is a liquid at room temperature, and gold and silver are the most malleable and ductile. Non-metals such as coal, sulfur, and phosphorus are usually dull, soft or brittle, neither malleable nor ductile, not sonorous, and poor conductors of heat and electricity. The chapter examines the effect of air and water on metals: iron develops brown deposits called rust only when both water and air (moist air) are present, a process called rusting; the slow deterioration of metal surfaces in general is called corrosion. Burning magnesium ribbon forms white magnesium oxide, whose solution turns red litmus blue, showing metal oxides are basic. Burning sulfur forms sulfur dioxide, which dissolves in water to give sulfurous acid, showing non-metal oxides are acidic. Metals and non-metals are sub-categories of elements — substances that cannot be broken into simpler ones, of which 118 are presently known. Non-metals are essential to life too: we breathe oxygen, carbon is the building block of living things, nitrogen helps make fertilisers, chlorine purifies water, and iodine acts as an antiseptic.