Chapter 4 — The World of Metals and Non-metals

Chapter 4, 'The World of Metals and Non-metals', explores the physical and chemical properties that distinguish metals from non-metals, including malleability, ductility, sonority, heat and electrical conductivity, and how metals react with air and water to form rust and corrosion.

Malleability is the property by which materials can be beaten into thin sheets. Most metals possess this property. Gold and silver are the most malleable metals; aluminium foil used for wrapping food and thin silver foil on sweets are common examples.

Ductility is the property by which materials can be drawn into wires. This property is mainly possessed by metals. Gold is extremely ductile — one gram of gold can be drawn into a 2 kilometre-long wire.

Sonority is the property of metals that enables them to produce a ringing sound when dropped or struck. A metal spoon or metal coin produces a ringing sound when dropped, whereas coal or wood produces a dull sound. The ringing sound of a school bell and ghungroos is due to the sonority of metals.

Metals are good conductors of heat, meaning heat transfers through them easily (conduction), making them ideal for cooking vessels. Wood is a poor conductor of heat, so handles made of wood do not get hot, protecting the user.

Both water and air (moist air) must be present simultaneously for rusting to occur. An iron nail kept in dry air alone (bottle A) or in water from which air has been excluded (bottle B) does not rust. Only when the nail is exposed to both water and air (bottle C) does brown rust form.

Rusting is specifically the process of brown deposit (rust) forming on iron objects when exposed to moist air. Corrosion is the broader term for the gradual deterioration of any metal surface caused by air, water, or other substances — for example, the green coating on copper objects or the black coating on silver objects.

The chapter states that rusting can be prevented by painting, oiling, greasing, and galvanisation — which is the process of applying a protective layer of zinc metal over iron.

Metal oxides are generally basic in nature. For example, magnesium burns in air to form magnesium oxide, whose solution turns red litmus paper blue. Non-metal oxides are acidic in nature — sulfur burns in oxygen to form sulfur dioxide, which dissolves in water to form sulfurous acid, turning blue litmus red.

Sodium is stored in kerosene because it reacts vigorously with both oxygen and water, generating a lot of heat. Phosphorus is stored in water because it catches fire when exposed to atmospheric air.

Not all metals are hard and solid. Sodium and potassium are so soft that they can be cut with a knife. Mercury is a metal that exists in a liquid state at room temperature and is found in thermometers.

Non-metals are elements (substances that cannot be broken down into simpler substances) that lack metallic properties — they are generally soft or brittle, non-lustrous, non-sonorous, and poor conductors of heat and electricity. Examples include sulfur, phosphorus, oxygen, hydrogen, nitrogen, and carbon. Plastic, glass, wood, rubber, and paper are not classified as metals or non-metals because they are not elements.

Oxygen (non-metal) is essential for respiration. Carbon is the building block of all life forms and is a key component of proteins, fats, and carbohydrates. Nitrogen is used in manufacturing fertilisers and is essential for plant growth. Chlorine is used in water purification. Iodine solution is applied on wounds as an antiseptic.

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