The World of Metals and Non-metals
Class 7 Science (Curiosity) — Chapter Notes & Complete Learning Material
1. Introduction
Yashwant and Anandi live in a village in Rajasthan. For their school project, they decide to learn about ironsmiths, who make useful items from metals. With their grandfather's help, they visit Sudarshan uncle, a local ironsmith, to see how he shapes iron into everyday tools such as pans, buckets and farming equipment.
Curious and amazed, they begin exploring the fascinating world of metals, how metals differ from non-metals, and the many ways these materials are used in daily life and industry.
2. Properties of Materials
Materials are identified and classified by their properties, which are observable or measurable characteristics that help us group substances as metals or non-metals. Important properties used for classification include appearance, hardness, malleability, ductility, sonority, and the ability to conduct heat and electricity. The following sections explain these properties and compare how metals and non-metals behave.
Malleability
Malleability is the property of a material that allows it to be beaten or rolled into thin sheets without breaking.
Malleability of Metals
Most metals, such as copper, aluminium and iron, are malleable. They can be flattened into sheets or shaped by hammering and rolling.
Malleability is useful in everyday life. Silver foil used to decorate sweets is made by hammering silver into extremely thin sheets, while aluminium foil used for wrapping food is made possible by the malleability of aluminium.
Certain metals, such as gold and silver, are exceptionally malleable and can be made into very thin sheets called gold leaf and silver leaf.
Some Exceptions and Special Cases
Not all metals are hard. Metals such as sodium and potassium are very soft and can be cut with a knife. They are much softer than copper or iron.
Mercury is a metal that is liquid at room temperature. It therefore behaves differently from most metals and has been used in devices such as thermometers and barometers.
Brittleness in Non-metals
Non-metals such as coal and sulphur do not flatten when hammered. Instead, they tend to break or shatter into pieces. This characteristic is called brittleness.
Brittle materials cannot be bent or shaped easily; they break under pressure or impact.
Properties of Wood
Wood does not flatten like metals when it is hit with a hammer.
Wood neither flattens into sheets nor shatters like typical brittle substances. It shows a combination of flexibility and toughness.
Thus, wood is generally considered neither malleable nor brittle in the strict sense in which these terms are used for metals and non-metals.
Ductility
Ductility is the property of a material that allows it to be drawn out into thin wires without breaking.
Ductility of Metals
Many metals, for example copper and aluminium, are ductile and can be drawn into wires.
These wires are used extensively in electrical fittings and to make ornaments such as bangles and necklaces.
Gold is highly ductile. Under suitable conditions, one gram of gold can be drawn into a wire several kilometres long.
Everyday Uses of Ductility
Electrical fittings: Copper and aluminium wires are widely used in electrical wiring for homes and appliances because they conduct electricity well.
Jewellery and ornaments: Wires of gold, silver and other metals are shaped into bangles, necklaces and earrings.
Musical instruments: Metal strings are used in instruments such as the veena, sitar, violin and guitar. The strings vibrate to produce musical sound.
Ductility of Non-metals
Non-metals are generally not ductile. They break when stretched and cannot be drawn into wires.
Examples include coal, sulphur and phosphorus, which break into pieces when we try to stretch or hammer them.
Such non-metals are brittle rather than ductile.
Sonority
Sonority is the ability of a material to produce a clear ringing sound when struck.
Metals are sonorous. Objects such as spoons or coins give a ringing sound when struck. This property is why bells, ghungroos and many musical percussion instruments are made of metal.
Non-metals such as coal and wood produce dull sounds when dropped or struck and are not sonorous.
Everyday Examples of Sonority
The ringing sound of ghungroos, which are small bells worn by dancers, is due to the sonorous nature of the metal pieces used.
The loud ringing of a school bell is produced by the sonority of the metal from which it is made.
Sound differences produced by striking wood and metal can help us identify materials and are useful in everyday tasks.
Conduction of Heat
Conduction is the transfer of heat energy through a material from the hotter part to the cooler part.
Metals, such as aluminium, copper and iron, are good conductors of heat. They are commonly used for cooking vessels because they transfer heat quickly and evenly.
Non-metals such as wood are poor conductors of heat. They are insulators, so heat travels slowly from an immersed part to a handle outside the hot water. This is why vessel handles are often made of wood or plastic.
If a metal spoon and a wooden spoon are left in hot water for the same time, the metal spoon feels much hotter because metal conducts heat faster from the hot water to our hand, whereas wood does not.
Good and Poor Conductors
Materials that allow heat to pass through them easily are called conductors. Metals are good conductors.
Materials that do not allow heat to pass through easily are called insulators or poor conductors. Wood, plastic and rubber are typical insulators used for handles and safety purposes.
Conduction of Electricity
Materials that allow electric current to pass through them easily are called good conductors of electricity. Materials that do not allow electric current to pass through them easily are called insulators.
Metals such as aluminium, iron and copper are excellent conductors and are used for electrical wiring and many electrical devices.
Non-metals such as sulphur, coal, wood, stone, rubber and nylon are poor conductors of electricity. They are used as insulating materials, for example, in tool handles, cable coverings and protective clothing.
Electrical Conductivity and Safety
The handle of a screwdriver used by electricians is typically made of plastic or rubber because these materials are poor conductors of electricity and help protect the user from electric shock.
Electricians also wear rubber gloves and rubber-soled shoes to reduce the risk of electrical accidents because rubber does not allow electricity to pass through easily.
Metallic Lustre
Many metals, such as copper, aluminium and iron in polished form, show a characteristic shiny appearance called metallic lustre. They reflect light well and appear bright and polished.
In contrast, non-metals such as coal, sulphur and wood are generally dull or non-lustrous and do not reflect light in the same way as metals.
Metals are often harder than many non-metals and resist scratching or denting under normal conditions.
3. Effect of Air and Water on Metals: Iron
Rusting of iron is the formation of reddish-brown, flaky deposits on iron surfaces when iron reacts with oxygen and moisture in the air. This chemical change is commonly called rusting.
The brown deposits that form on iron are known as rust.
Rusting is a chemical reaction in which iron reacts with oxygen in the presence of water or moisture.
Rusting causes iron to deteriorate and weaken over time, reducing the life and strength of iron objects.
Conditions Required for Rusting
Rusting occurs when iron is exposed to both air containing oxygen and water or moisture.
Iron does not rust when exposed to dry air alone because moisture is absent.
Iron also does not rust when completely submerged in water that has no dissolved oxygen. In ordinary water, however, dissolved oxygen may be present, so rusting can occur.
Impact
Rusting makes iron objects and structures weak and unsafe.
It causes damage and decay to iron used in buildings, vehicles, bridges and tools.
Large sums are spent every year on repairing or replacing rusted iron structures and equipment.
Prevention of Rusting
Painting iron surfaces keeps air and moisture away from the iron.
Applying oil or grease forms a protective barrier on the metal surface.
Galvanisation is the process of coating iron with a layer of zinc, which protects the iron from rusting.
Corrosion
Corrosion is the gradual deterioration of metals due to chemical reactions with substances in the environment, such as air, water, acids or salts. Rusting is a common example of corrosion in iron.
4. Effect of Air and Water on Other Metals
Magnesium
When a magnesium ribbon is burned, it produces a very bright white flame and leaves a white powder called magnesium oxide.
Magnesium oxide reacts with water to form a basic mixture or solution that turns red litmus blue.
As a general rule, oxides of metals are often basic.
Sodium
Sodium reacts vigorously with air and water, producing heat and substances such as sodium oxide or sodium hydroxide. For safety, sodium metal is stored under kerosene to prevent contact with air and moisture.
Sodium oxide is basic, consistent with the general behaviour of metal oxides.
5. Substances that Behave Differently from Metals in Air and Water
Certain substances, for example sulphur and phosphorus, behave very differently from metals when exposed to air and water.
Sulphur
When sulphur is burned, it produces sulphur dioxide gas, represented by SO₂. When sulphur dioxide dissolves in water, it forms sulphurous acid, which is acidic and turns blue litmus red.
Sulphur itself does not react with water when simply mixed with it. Its combustion product, sulphur dioxide, is responsible for the acidic behaviour of the solution.
Phosphorus
Phosphorus catches fire readily in air and is therefore usually stored under water to prevent it from coming into contact with oxygen.
Properties of Non-metals
Non-metals are usually soft and dull or non-lustrous.
They are generally not malleable or ductile and therefore cannot be made into sheets or wires.
Non-metals are not sonorous; they produce dull sounds when struck.
They are poor conductors of heat and electricity and generally act as insulators.
Oxides of non-metals are generally acidic. For example, sulphur dioxide forms an acidic solution in water.
Examples of non-metal elements include oxygen, hydrogen, nitrogen, carbon, chlorine and iodine.
Materials such as plastic, glass, wood, rubber and paper are not elements. Therefore, they are not classified as metals or non-metals in the elemental sense.
6. Are Non-metals Essential in Everyday Life?
Although metals are highly visible because of their shine, strength and conductive properties, non-metals perform essential roles that keep life and industry functioning.
Importance of Some Non-metals
Oxygen: We breathe oxygen every day. It is essential for life and is also used in hospitals, combustion and welding processes.
Carbon: Carbon is the building block of life. It forms the basis of proteins, fats and carbohydrates, which provide energy and enable growth.
Nitrogen: Nitrogen is widely used in the manufacture of fertilisers and chemicals. It is also a vital nutrient for plant growth.
Chlorine: Chlorine is used in water purification to make drinking water safe.
Iodine: Iodine is used as an antiseptic in iodine solutions applied to wounds to help prevent infection.
Science and Society
Many metals and alloys, which are mixtures of two or more metals or of metals with non-metals, are used daily in utensils, tools and machines. Metals are essential in modern technology and almost every industry. Examples include the use of zirconium in atomic energy and titanium in aerospace engineering. In India, recycling metals such as iron and aluminium is common and helps reduce waste and promote sustainability.
7. Points to Remember
Metals are generally shiny and hard. They are malleable, meaning that they can be beaten into sheets; ductile, meaning that they can be drawn into wires; sonorous, meaning that they produce ringing sounds; and good conductors of heat and electricity.
Non-metals are usually dull, soft or brittle and are poor conductors of heat and electricity.
Malleability allows metals such as copper and gold to be made into thin sheets used as foils and in tools.
Ductility allows metals to be drawn into wires used in electrical systems and jewellery.
Metals produce clear ringing sounds and are therefore used in bells. Non-metals generally produce dull sounds.
Metals conduct heat and are used for cooking pots. Non-metals such as wood are used for handles because they are poor conductors.
Metals such as copper and aluminium conduct electricity, while materials such as rubber and plastic act as insulators for safety.
Iron rusts when exposed to both air and water, forming reddish-brown rust.
Copper and silver also form surface coatings when exposed to air and moisture. Copper may form a green patina, while silver may develop a black coating.
The Iron Pillar of Delhi, which is over 1600 years old, demonstrates how ancient Indian metallurgists produced iron that resists rust.
Magnesium burns in air to form a basic oxide, while sodium must be stored in kerosene to prevent its rapid reaction with air and moisture.
Sulphur burns to form sulphur dioxide, which forms acidic solutions in water. Sulphur itself does not react with water under ordinary conditions.
Phosphorus is stored under water to prevent it from catching fire in air. Oxides of non-metals are generally acidic.
Non-metals such as oxygen, carbon and nitrogen are vital for life, agriculture and industry.
There are 118 known elements. They include metals and non-metals; some occur naturally and some are made in laboratories.
Metals and alloys are widely used in tools, machines and industries. Recycling metals helps protect the environment.
8. Difficult Words and Their Meanings
Malleability: The ability of a material to be beaten into thin sheets without breaking.
Ductility: The ability of a material to be drawn into thin wires.
Sonority: The property of a material to produce a ringing sound when struck.
Conduction: The transfer of heat or electricity through a material, for example, heat transfer through a metal spoon.
Conductor: A material, such as copper, that allows heat or electricity to flow easily.
Insulator: A material, such as rubber, that does not allow heat or electricity to flow easily.
Rusting: The process in which iron forms brown deposits called rust when exposed to both air and water.
Corrosion: The gradual damage to a metal's surface by air, water or other substances, for example, rust on iron or a green coating on copper.
Oxide: A substance formed when a metal or non-metal reacts with oxygen, for example, magnesium oxide or sulphur dioxide.
Element: A basic substance, such as iron or oxygen, that cannot be broken down into simpler substances by ordinary chemical methods. There are 118 known elements.
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