Materials: Metals and Non-Metals – Short Answer Type Questions
Class 8
Science — Chapter 4: Materials: Metals and Non-Metals
CBSE Board Examinations
How to use:
- Attempt each question in 2–4 sentences; check answers for concept clarity.
- Use topic groupings to focus study on properties, reactions, alloys, corrosion and applications.
- These are strictly based on NCERT Class 8 Chapter 4 and suitable for CBSE exam preparation.
Topic A — Physical Properties of Metals and Non‑metals (1–12)
1. Define metallic lustre with an example.
Answer: Metallic lustre is the shiny appearance of metals when freshly cut or polished. Example: a polished copper surface appears bright and shiny.
2. Explain why metals are good conductors of electricity.
Answer: Metals have free-moving electrons in their structure which can carry electric charge easily, allowing current to pass through.
3. What is malleability and give one use that depends on this property.
Answer: Malleability is the ability of a metal to be beaten into thin sheets. Aluminium foil is produced because aluminium is malleable.
4. How is ductility different from malleability?
Answer: Ductility refers to the ability to be drawn into wires, while malleability is the ability to be beaten into sheets. Copper is ductile (used for wires).
5. Name a metal which is liquid at room temperature and mention one use.
Answer: Mercury is liquid at room temperature and is used in some thermometers and electrical switches (though its use is decreasing due to toxicity).
6. Why are most metals hard and dense?
Answer: Metals have closely packed atoms and strong metallic bonding, leading to high densities and often high hardness compared to many non‑metals.
7. Give two typical physical properties of non‑metals.
Answer: Non‑metals are often dull and brittle (if solid) and are poor conductors of heat and electricity; many are gases at room temperature (e.g., oxygen).
8. Explain why graphite (a form of carbon) is an exception among non‑metals.
Answer: Graphite conducts electricity due to delocalised electrons between layers and has a shiny appearance, unlike most non‑metals which are insulators and dull.
9. Why is copper used for making electrical wires?
Answer: Copper is highly ductile and an excellent conductor of electricity, making it suitable for drawing into wires used in electrical circuits.
10. What property makes aluminium useful in aircraft construction?
Answer: Aluminium is lightweight (low density) and has good strength and corrosion resistance, making it suitable for aircraft parts where low weight is essential.
11. How can you distinguish a metal from a non‑metal using simple tests?
Answer: Test conductivity (metals conduct electricity), check malleability (metals can be beaten into sheets), observe appearance (metals are generally shiny) — safe tests under supervision.
12. Mention one use each for sulphur and oxygen (non‑metals).
Answer: Sulphur is used in fertilizers and manufacturing sulphuric acid; oxygen is essential for respiration and used in medical and industrial processes.
Topic B — Chemical Properties & Reactions (13–24)
13. What happens when a reactive metal reacts with dilute acid? Give an example.
Answer: A reactive metal reacts with dilute acid to produce a salt and hydrogen gas. Example: Zinc + HCl → ZnCl₂ + H₂ (hydrogen gas bubbles are observed).
14. Write a simple word equation for magnesium burning in oxygen.
Answer: Magnesium + Oxygen → Magnesium oxide (Mg + O₂ → MgO); magnesium burns with a bright white flame.
15. Explain displacement reaction with a suitable example involving metals.
Answer: In a displacement reaction, a more reactive metal displaces a less reactive metal from its compound. Example: Zn + CuSO₄ → ZnSO₄ + Cu (zinc displaces copper).
16. Why do metals form positive ions?
Answer: Metals tend to lose electrons to achieve a stable electronic configuration, forming positive ions (cations) in chemical reactions.
17. How do non‑metal oxides generally react with water?
Answer: Many non‑metal oxides react with water to form acidic solutions; for example SO₂ + H₂O → H₂SO₃ (sulphurous acid).
18. Mention one test to show evolution of hydrogen gas when a metal reacts with acid.
Answer: Collect the gas and bring a burning match near it — a characteristic 'pop' sound indicates hydrogen gas.
19. Why does sodium react more vigorously with water than zinc?
Answer: Sodium is higher in the reactivity series and has one valence electron which it loses easily, making its reaction with water much more vigorous than zinc.
20. What is meant by a basic oxide? Give an example.
Answer: Basic oxides are metal oxides that react with acids to form salt and water. Example: Na₂O or CaO react with acids to form corresponding salts.
21. Give an example of a non‑metal oxide and its acidic property.
Answer: Carbon dioxide (CO₂) reacts with water to form carbonic acid (CO₂ + H₂O → H₂CO₃), showing acidic behaviour.
22. How is hydrogen gas identified in the lab?
Answer: Hydrogen burns with a 'pop' sound when a burning splinter is brought near the collected gas, indicating its presence.
23. Explain why gold does not corrode easily.
Answer: Gold is very unreactive (noble metal) and resists oxidation and corrosion in air and water, so it retains its lustre and does not rust.
24. What is meant by 'reactivity series' of metals?
Answer: The reactivity series is a list of metals arranged in order of decreasing reactivity; it predicts which metals displace others and how they react with water/acids.
Topic C — Corrosion & Its Prevention (25–34)
25. Define rusting and state the conditions required for rusting of iron.
Answer: Rusting is the corrosion of iron in the presence of oxygen and water forming hydrated iron(III) oxide. Moisture, oxygen, and often salts accelerate rusting.
26. How does painting help prevent rust?
Answer: Painting forms a protective barrier that prevents oxygen and water from contacting the iron surface, thus slowing or preventing rust formation.
27. What is galvanisation and why is it used?
Answer: Galvanisation is coating iron or steel with a layer of zinc. Zinc acts as a sacrificial metal and corrodes preferentially, protecting the iron beneath.
28. Explain sacrificial protection with an example.
Answer: A more reactive metal like zinc is attached to iron; zinc corrodes instead of iron. Example: Zinc blocks attached to ship hulls or pipelines act as sacrificial anodes.
29. Why does salt water speed up rusting?
Answer: Salt water increases the conductivity of the solution around metal, accelerating electrochemical reactions involved in rusting.
30. Mention two methods other than painting and galvanisation to prevent corrosion.
Answer: Oiling/greasing metal surfaces to block moisture and using stainless steel (alloying with chromium) which resists rusting.
31. How does alloying iron with chromium help prevent rust?
Answer: Chromium forms a passive oxide layer on the surface of steel which protects it from further oxidation; stainless steel contains chromium and resists rusting.
32. Why is corrosion an economic concern?
Answer: Corrosion damages infrastructure and machinery, requiring repair or replacement which leads to high maintenance costs and economic loss.
33. What role does oxygen play in the corrosion of iron?
Answer: Oxygen reacts with iron in presence of water to form iron oxides (rust); oxygen is an essential reactant in rusting reactions.
34. Describe one classroom demonstration to show the effect of water on rusting.
Answer: Place two iron nails in separate containers — one in water and one in oil. The nail in water will show signs of rust over days while the one in oil will remain uncorroded.
Topic D — Alloys and Applications (35–44)
35. What is steel and why is it important?
Answer: Steel is an alloy of iron and carbon (and sometimes other elements) that is stronger and more useful in construction and manufacturing than pure iron.
36. Explain how brass is useful and its composition.
Answer: Brass is an alloy of copper and zinc. It is used in musical instruments, decorative items and plumbing because it is malleable, corrosion-resistant and has good acoustic properties.
37. Why are alloys often preferred over pure metals for engineering applications?
Answer: Alloys can be engineered to have superior properties — increased strength, hardness, corrosion resistance or lower melting points — tailored to application needs.
38. Give an example of an alloy used in coins and explain why.
Answer: Cupro-nickel (copper + nickel) is used in coins because it is durable, resists corrosion and has an appealing colour and hardness suitable for circulation.
39. How does adding carbon change the properties of iron?
Answer: Adding carbon to iron produces steel which is harder and stronger than pure iron; the carbon content controls hardness and ductility.
40. Mention two uses of aluminium alloys.
Answer: Aluminium alloys are used in aircraft bodies and automotive parts due to their light weight and good strength; also used in cookware for lightness and heat conduction.
41. What properties make copper indispensable in electrical wiring?
Answer: Copper's high electrical conductivity, ductility (can be drawn into wires) and resistance to corrosion (with proper insulation) make it ideal for wiring.
42. Define bronze and give one common use.
Answer: Bronze is an alloy of copper and tin; it is used in statues, bearings and musical instruments because it is hard and wears well.
43. Why are precious metals like gold and platinum used in jewellery?
Answer: Gold and platinum are unreactive (noble metals), resist corrosion and tarnishing, and have attractive luster — making them suitable for long-lasting jewellery.
44. Explain one environmental advantage of recycling metals.
Answer: Recycling metals reduces the need for mining, saves energy in production and decreases habitat destruction and pollution associated with extraction.
Topic E — Practical Tests, Measurements & Numerical Concepts (45–50)
45. How do you measure the volume of an irregular metal piece?
Answer: Use the water displacement method: note initial water level in a measuring cylinder, immerse the object, and subtract initial volume from final volume to get the object's volume.
46. Give the formula for density and explain its units.
Answer: Density ρ = mass / volume. Common units are g/cm³ or kg/m³ — e.g., density in g/cm³ when mass is in grams and volume in cm³.
47. A student finds that a metal sample has density 8 g/cm³ and mass 240 g. Calculate its volume.
Answer: Volume = mass / density = 240 g / 8 g/cm³ = 30 cm³.
48. Why is it important to clean and sort metals before recycling?
Answer: Cleaning and sorting prevent contamination, improve recycling efficiency and ensure recovered metals meet quality standards for reuse.
49. How can you test whether a metal sample is pure or an alloy (simple observation)?
Answer: Observe uniformity of properties: alloys often have different hardness, colour or behaviour (e.g., improved strength) compared to pure metals; for precise determination, laboratory analysis is needed.
50. Suggest a simple classroom activity to demonstrate that metals conduct heat.
Answer: Place identical spoons of different materials (metal and plastic) in hot water; touch handles to observe heat conduction — metal spoon handle becomes hot faster, showing conduction.