CLASS 7 SCIENCE · CHAPTER 4 · STUDY NOTES
The World of Metals and Non-metals
Connect material properties with uses, follow a fair rusting test and compare metal and non-metal oxides.
01 / EXPLORE & UNDERSTAND
Choose a material for its job
A tool can use different materials for different reasons.
Shaping a useful tool
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1. Look at the ironsmith’s tool
The chapter opens with an ironsmith shaping iron into useful tools. An axe needs a strong working head. A handle has a different job: it must be held comfortably and safely. Saying “metal is best” is less useful than naming the property needed for each part.
2. Connect a property to an effect
A cooking pan should transfer heat to food, while its handle should slow heat reaching your hand. A material choice is explained by both the property and why that property helps the particular use.
Your turn
Question
Why can one cooking utensil have a metal body and a wooden handle?
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02 / EXPLORE & UNDERSTAND
Shiny and hard are useful clues—not rules for every metal
Look at several properties together.
1. Start with common examples
Copper, aluminium and iron have metallic lustre when their surfaces are clean. Their everyday solid pieces are generally hard. A dull coating can hide the shine, so a dirty or corroded surface may not show the underlying metal clearly.
2. Keep the exceptions
Sodium and potassium are soft metals. Mercury is a liquid metal at room temperature. These examples show why “all metals are hard solids” is incorrect. They are examples to learn about, not materials for unsupervised handling.
Your turn
Question
A metal surface looks dull after being left outside. Does that prove it is no longer a metal?
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03 / EXPLORE & UNDERSTAND
Malleability: shape a sheet
Flattening is different from breaking into pieces.
Changing the shape
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1. Connect the test with foil
Many metals change shape under hammering rather than immediately crumbling. Aluminium foil and thin silver foil are familiar results of malleability. The ironsmith heats iron to make it easier to shape; working with hot metal requires specialist skill and equipment.
2. Distinguish brittle behaviour
A brittle sample cracks or breaks when struck. Sulfur is a useful example. The chapter compares particular samples, so do not turn one wood-block result into “wood can never split”. The form, grain and conditions can matter.
Your turn
Question
Which observation shows malleability: a sample becomes a sheet or shatters into fragments?
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04 / EXPLORE & UNDERSTAND
Ductility: draw a wire
A wire is long and thin; a sheet is broad and flat.
1. Compare two shape changes
Drawing copper into a long wire uses ductility. Pressing aluminium into foil uses malleability. Both involve changing shape, but the resulting forms are different. A metal can show both properties.
2. Connect wires with useful objects
Electrical cables, a metal mesh strainer and some musical strings use metal wires. Steel-wire ropes can support heavy loads in bridges and cranes because suitable steel combines useful strength with the ability to be formed into wire.
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Question
Why is “ductile” the better word for making copper cable than “malleable”?
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05 / EXPLORE & UNDERSTAND
Sonority: a ringing sound
The material helps explain why a bell rings.
1. Compare sounds carefully
Many metal objects, such as a bell or metal plate, produce a clear ringing sound when struck. A wood block or piece of coal usually gives a duller sound in the chapter’s comparison. We call metals generally sonorous.
2. Connect the whole object to the sound
Material is not the only factor. Shape, size and how an object is held affect its vibration. A metal object held tightly may not ring for long. Use the property to explain bells and ghungroos, not to identify every object from sound alone.
Your turn
Question
Why might gripping a metal bell tightly reduce its ringing?
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06 / EXPLORE & UNDERSTAND
Heat travels along a metal spoon
The spoon’s handle can warm even when it is above the hot water.
Same hot water, different materials
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1. Trace where the heat goes
In the teacher’s comparison, similar metal and wooden spoons are placed in the same hot water for the same time. Heat travels from the immersed end towards the handle. The metal transfers it more quickly than the wood.
2. Make the comparison fair
Similar size, the same water and the same time help us compare the materials. Metal cooking vessels transfer heat well; wood or suitable insulating handles slow it. Do not touch heated objects to test their temperature yourself.
Your turn
Question
Why should both spoons be placed in the same water for the same time?
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07 / EXPLORE & UNDERSTAND
Metals carry current
Heat conduction and electrical conduction are different properties.
1. Connect with the lamp tester
In a suitable small-cell tester, a metal sample can bridge a gap and let current pass, making the lamp glow. Copper, aluminium and iron are common conductors. Dry rubber and plastic generally do not complete that conducting path in this test.
2. Explain the material choice accurately
Copper in a wire is useful because it conducts electricity and can be drawn into wire. Copper in a pan is useful because it conducts heat. Saying only “it is a conductor” leaves out which transfer matters.
Your turn
Question
Which property directly helps a pan heat its contents: electrical conduction or heat conduction?
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08 / EXPLORE & UNDERSTAND
Rusting needs moisture and oxygen
Compare three conditions, not just three nails.
A: air kept dry
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B: water kept away from fresh air
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C: both air and water
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1. Remove one needed condition at a time
Bottle A contains a nail in air dried by silica gel. Bottle B contains a nail in boiled and cooled water, protected by oil to keep air out. Bottle C exposes the nail to both air and water. The conditions are deliberately different.
2. Use the result to explain the cause
In the chapter’s expected results, C develops rust while A and B do not show the same rusting. This supports the need for both moisture and oxygen. Ordinary water contains dissolved air, which is why B uses boiled water and an oil layer.
Your turn
Question
Why would ordinary water without an oil layer be a poor test of “water without oxygen”?
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09 / EXPLORE & UNDERSTAND
Stop contact to slow corrosion
A coating helps only when it protects the surface effectively.
1. Distinguish rust from other coatings
Rusting is the corrosion of iron. Copper may develop a green coating and silver may tarnish dark; these are not called iron rust. The chapter’s Iron Pillar example shows remarkable resistance to rusting, not that iron can never corrode.
2. Choose a protective method
Paint, oil or grease can reduce iron’s contact with water and air. Galvanisation coats iron with zinc. A suitable protective layer must stick, last and suit the use; a material is not a good coating merely because it does not react with water.
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Question
Why should a scratch in protective paint be repaired?
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10 / EXPLORE & UNDERSTAND
Magnesium burns to form an oxide
A new material forms when magnesium reacts with oxygen.
Magnesium changes into a white oxide
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Red litmus changes in the wet oxide test
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1. Follow the teacher’s demonstration
Burning magnesium produces a dazzling white light and white magnesium oxide. This is a teacher demonstration: do not stare at the bright flame or attempt it yourself. The white powder is a reaction product, not simply the original ribbon cut into smaller pieces.
2. Connect the product with an indicator test
When the oxide is mixed with water, the mixture is basic and turns red litmus blue. Many metal oxides are basic. This is a general pattern, not a claim that every oxide of every metal behaves identically.
A little deeper: Metals differ in how strongly they react
Sodium reacts vigorously with water and air, so trained laboratories store it under a suitable protective liquid such as kerosene. Iron rusts much more slowly in the conditions discussed here. “Metal” does not mean every member has the same reactivity.
Your turn
Question
What evidence supports calling the water mixture basic?
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11 / EXPLORE & UNDERSTAND
Sulfur gives a different oxide
Compare the products and their indicator results.
The sulfur-oxide solution is acidic
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1. Follow sulfur to sulfur dioxide
In the teacher’s demonstration, sulfur burns in oxygen to form sulfur dioxide gas. When dissolved in water, it gives an acidic solution, which turns blue litmus red. The gas is harmful to breathe; this is not a home experiment.
2. Separate sulfur from its oxide
Putting unburnt sulfur in water does not produce the same result. Sulfur dioxide is a new compound with different properties. Many non-metal oxides are acidic, but this is not universal: some oxides are neutral.
Your turn
Question
Why is “sulfur in water” not the same test as “sulfur dioxide in water”?
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12 / EXPLORE & UNDERSTAND
Not every non-metallic object is a non-metal element
Wood and plastic are materials; oxygen and sulfur are elements.
1. Separate the material from the element group
Iron, copper and aluminium are metallic elements. Sulfur, oxygen, carbon and nitrogen are non-metal elements. Wood, plastic and glass are not single elements, so we do not classify them as non-metal elements just because they are not metal objects.
2. Use general properties with care
Solid non-metals generally are not malleable or ductile, and many conduct heat and electricity poorly. There are exceptions: graphite, a form of carbon, conducts electricity. Also, the storage-in-water example refers specifically to white phosphorus, not every form of phosphorus.
Your turn
Question
Is a wooden ruler a non-metal element? Explain.
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13 / EXPLORE & UNDERSTAND
Non-metals matter too
Useful does not mean metallic.
1. Connect non-metals with life
Oxygen supports respiration in humans. Carbon is part of carbohydrates, fats and proteins. Plants need nitrogen-containing nutrients; fertilisers can supply suitable compounds. These are important uses even though the materials do not have a metallic shine.
2. Distinguish controlled use from the raw element
Chlorine is used under controlled conditions in water treatment, and iodine-containing preparations have antiseptic uses. A useful chemical still needs the correct form and amount. Do not interpret an application as permission to handle or use the raw element.
Your turn
Question
Why is “non-metals are unimportant” incorrect?
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14 / EXPLORE & UNDERSTAND
Combine properties, then reduce waste
The best material depends on the job and the resources used.
1. Understand why steel differs from pure iron
Steel is an alloy based on iron with carbon and sometimes other elements. Suitable composition and processing can give useful strength and other properties. Alloys are chosen for tools, utensils, buildings and machines because the final material suits the job.
2. Keep materials in use
Repairing, reusing and recycling suitable metals reduce waste and the need for fresh material. Sorting matters because different metals and alloys need appropriate processing. Recycling still uses energy, so preventing unnecessary waste remains useful.
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Question
Why should metal scraps be sorted before recycling?
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Take these ideas with you
- Malleable means sheets; ductile means wires; sonorous means ringing.
- Explain a use by naming the property and how it helps.
- Rusting needs iron, moisture and oxygen.
- Many metal oxides are basic; many non-metal oxides are acidic. These patterns have exceptions.
- Wood and plastic are not non-metal elements.
- Metals and non-metals are both important in daily life.
Use your understanding in a new situation
Question
Explain why a metal can be suitable for a pan body but unsuitable for an unprotected handle.
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A nail in dry air does not rust like one exposed to moist air. Which missing condition explains the difference?
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