Materials: Metals and Non-Metals – Case-based Questions with Answers
Class 8
Science — Chapter 4: Materials: Metals and Non-Metals
CBSE Board Examinations
How to use:
- Read each case carefully; answer the questions in full sentences with reasoning.
- Use NCERT concepts (properties of metals/non‑metals, reactivity, corrosion, alloys, density) to justify answers.
- These cases test application and explanation — suitable for long & value-based questions in CBSE exams.
Case 1 — A Rusty Gate
A school gate made of iron has started showing reddish-brown flaky deposits. The gate is near a road where salt is spread during winter.
Q1. Identify the phenomenon and name the two main factors causing it.
Q2. Suggest two methods to protect the gate and explain briefly how each method works.
Q2. Suggest two methods to protect the gate and explain briefly how each method works.
Answer: (1) The phenomenon is rusting — corrosion of iron. Main factors: moisture (water) and oxygen, with salt accelerating the process by increasing conductivity. (2) Methods: Painting/coating — creates a barrier preventing contact with oxygen/water; galvanisation — coating with zinc which provides sacrificial protection as zinc corrodes preferentially and protects exposed iron.
Case 2 — Choosing Wires
An electrician is selecting material for home wiring. He considers copper and aluminium.
Q1. Which metal is generally preferred for household wiring and why?
Q2. Name one advantage of aluminium and one trade-off when using it for wiring.
Q2. Name one advantage of aluminium and one trade-off when using it for wiring.
Answer: (1) Copper is preferred due to its higher electrical conductivity, ductility, and reliable long-term connections. (2) Aluminium advantage: lighter and cheaper; trade-off: higher resistivity and greater thermal expansion which can cause loosening and heating at joints unless special connectors are used.
Case 3 — Displacement in Cylinder
A student drops an irregular metal piece into a measuring cylinder. Water rises from 40.0 mL to 46.5 mL. The piece mass is 52.56 g.
Q1. Calculate the density of the metal in g/cm³.
Q2. If the density is close to 8.9 g/cm³, which metal might it be?
Answer: (1) Volume = 46.5 − 40.0 = 6.5 cm³. Density = mass/volume = 52.56 / 6.5 = 8.085 g/cm³ ≈ 8.09 g/cm³. (2) 8.09 g/cm³ is slightly lower than pure copper (8.96) and iron (7.87); it could be an alloy or impure copper/bronze. Mention comparison: if teacher gives values, pick closest—iron ~7.87, copper ~8.96.
Case 4 — Metal Reacting with Acid
A small piece of metal X is added to dilute HCl and bubbles are observed that pop with a burning splint test.
Q1. What gas is produced and how is it identified?
Q2. What does the reaction indicate about metal X's position relative to hydrogen in the reactivity series?
Q2. What does the reaction indicate about metal X's position relative to hydrogen in the reactivity series?
Answer: (1) Gas is hydrogen, identified by the characteristic 'pop' with a burning splint. (2) Metal X is more reactive than hydrogen (above hydrogen in reactivity series) since it displaces hydrogen from acid.
Case 5 — Alloy Selection
A manufacturer needs material for musical instrument valves requiring ease of shaping and resistance to corrosion. They consider pure copper and brass.
Q1. Which is better — copper or brass — and why?
Q2. Explain how alloying improves properties.
Q2. Explain how alloying improves properties.
Answer: (1) Brass (copper + zinc) is better for valves as it is harder, more durable and resists corrosion better than pure copper while retaining workable properties. (2) Alloying modifies crystal structure and bonding, increasing hardness, strength and corrosion resistance — tailoring properties for intended use.
Case 6 — Salted Roads Effect
A car parked near the coast shows quicker rusting compared to one inland. The coastal roads are salted in winter.
Q1. Explain why coastal exposure and salt accelerate corrosion.
Q2. Suggest two maintenance practices car owners should follow.
Q2. Suggest two maintenance practices car owners should follow.
Answer: (1) Salt increases the conductivity of moisture on metal surfaces, accelerating electrochemical corrosion; chloride ions also disrupt protective oxide layers causing pitting. (2) Practices: frequent washing to remove salt, apply protective coatings or wax, and inspect/repair paint chips to prevent exposure.
Case 7 — Conductivity Check
In a lab, students test two samples: A (shiny) and B (dull). A completes an electric circuit and B does not.
Q1. Which sample is likely a metal and why?
Q2. Name one non-metal that can be an exception and explain.
Q2. Name one non-metal that can be an exception and explain.
Answer: (1) Sample A is likely a metal — it's shiny and conducts electricity due to free electrons. (2) Exception: graphite (a form of carbon) is a non‑metal that conducts electricity because of delocalised electrons in its layered structure.
Case 8 — Choosing Utensil Material
A kitchen designer must choose a metal for cookware that heats evenly and resists corrosion.
Q1. Which metals are commonly used and why?
Q2. Explain why composite constructions (layers) are often used.
Q2. Explain why composite constructions (layers) are often used.
Answer: (1) Copper and aluminium heat evenly due to high thermal conductivity; stainless steel resists corrosion. (2) Composite cookware (aluminium core with stainless-steel exterior) combines excellent heat distribution with corrosion resistance and durability — engineers balance properties for performance and safety.
Case 9 — Density Identification
A student suspects a metal sample is aluminium. Mass = 54 g, volume = 20 cm³.
Q1. Calculate density and check if it matches aluminium (~2.70 g/cm³).
Q2. If not matching, suggest reasons for discrepancy.
Q2. If not matching, suggest reasons for discrepancy.
Answer: (1) Density = 54 / 20 = 2.7 g/cm³ — matches aluminium. (2) If not matching, reasons could be measurement error, impurities, alloying or trapped air in measurement apparatus.
Case 10 — Sacrificial Protection
Pipelines buried underground are fitted with metal blocks that corrode first to protect the pipeline.
Q1. Name the technique and explain why it works.
Q2. Which metals are commonly used as sacrificial anodes?
Q2. Which metals are commonly used as sacrificial anodes?
Answer: (1) Technique: sacrificial protection. A more reactive metal corrodes instead of the protected metal because it is anodic relative to the structure. (2) Common anodes: zinc, magnesium or aluminium alloys.
Case 11 — Electrical Contacts
A circuit designer chooses a metal for reliable, corrosion-resistant electrical contacts.
Q1. Which metals are suitable and why?
Q2. Explain why plating (e.g., gold) is used for contacts.
Q2. Explain why plating (e.g., gold) is used for contacts.
Answer: (1) Suitable metals: gold, silver, copper — due to excellent conductivity; gold and silver resist tarnish (gold > silver). (2) Gold plating prevents oxidation and ensures low contact resistance and long-term reliability despite thin layer cost, used in critical connectors.
Case 12 — Mixing Metals
A blacksmith mixes small amounts of carbon into iron to make tools.
Q1. What is the product called and how does carbon change iron's properties?
Q2. Why is controlled carbon content important?
Q2. Why is controlled carbon content important?
Answer: (1) Product: steel (an alloy). Carbon increases hardness and strength but excessive carbon makes steel brittle. (2) Controlled carbon content balances hardness and ductility for intended application (tools vs structural beams).
Case 13 — Acid Spill
A laboratory spill of dilute hydrochloric acid occurs near metal samples.
Q1. Which metals are at highest risk of damage and why?
Q2. What immediate safety steps should be taken (lab protocol)?
Q2. What immediate safety steps should be taken (lab protocol)?
Answer: (1) Reactive metals like zinc, magnesium and iron are at risk — they react with acid producing hydrogen and salts. Noble metals (gold, platinum) are safe. (2) Safety steps: evacuate area if fumes, neutralize acid with appropriate base (under teacher/safety officer), use PPE, ventilate, collect and dispose of waste per lab rules; secure metal items and rinse if safe to do so.
Case 14 — Coin Durability
A mint chooses alloy composition for new coins to resist wear and corrosion.
Q1. What alloy properties are desirable for coins?
Q2. Suggest an alloy type and justify.
Q2. Suggest an alloy type and justify.
Answer: (1) Desirable properties: hardness, corrosion resistance, low cost, good appearance and formability. (2) Cupro‑nickel (copper + nickel) or bronze are common — durable, corrosion-resistant and maintain appearance under handling.
Case 15 — Heating Elements
An engineer selects a metal for a heater coil that must withstand high temperatures without melting.
Q1. Which property is most important and which metals/alloys suit this role?
Q2. Name one alloy commonly used for heating elements.
Q2. Name one alloy commonly used for heating elements.
Answer: (1) Important property: very high melting point and tensile strength at high temperature. (2) Alloys like nichrome (nickel + chromium) are used because they maintain strength at high temperatures and have suitable resistivity for heating.
Case 16 — Corroded Statue
A bronze statue develops greenish patches over years exposed to rain and pollution.
Q1. What is the likely cause and compound formed?
Q2. How can conservationists protect such statues?
Q2. How can conservationists protect such statues?
Answer: (1) Cause: corrosion of copper-based bronze by oxygen and pollutants forming green copper salts (patina), such as basic copper carbonate. (2) Protection: apply protective coatings (wax, lacquer), regular cleaning, or controlled environment; in some cases stable patina is preserved intentionally for aesthetic/protective reasons.
Case 17 — School Bell
A school bell made of brass loses tonal quality over years and darkens.
Q1. Why does brass darken and how does it affect sound?
Q2. How can maintenance restore function?
Q2. How can maintenance restore function?
Answer: (1) Darkening due to surface corrosion/tarnish (oxide/sulfide formation) which may slightly dampen vibrations and alter tone. (2) Cleaning and polishing remove tarnish restoring surface properties and sound; protective coating slows re-tarnishing.
Case 18 — Laboratory Density Check
Two students measure density; Student A gets 7.8 g/cm³ and Student B gets 7.3 g/cm³ for similar iron samples.
Q1. List possible reasons for mismatch.
Q2. How should they improve measurement accuracy?
Q2. How should they improve measurement accuracy?
Answer: (1) Reasons: measurement errors (mass or volume), trapped air bubbles during displacement, impurities/alloying, or reading meniscus incorrectly. (2) Improvements: repeat measurements, use clean apparatus, ensure sample fully submerged without air, calibrate balance, read meniscus at eye level and average multiple trials.
Case 19 — Recycling Scrap
A community collects metal scrap to sell to recyclers. They notice mixed metals lower the price.
Q1. Why does separation increase recycling value?
Q2. Suggest a simple sorting approach for the community.
Q2. Suggest a simple sorting approach for the community.
Answer: (1) Separation reduces contamination, improves quality of recyclate and lowers processing costs — sorted materials fetch higher prices. (2) Simple approach: separate ferrous (magnetic) and non‑ferrous using a magnet; further sort non‑ferrous by appearance or by testing (e.g., density tests) and keep clean and dry to improve value.
Case 20 — Choosing Fasteners
An engineer must choose bolts for outdoor construction near the sea.
Q1. Which material/treatment would you recommend and why?
Q2. Explain the role of coatings and cathodic protection if needed.
Q2. Explain the role of coatings and cathodic protection if needed.
Answer: (1) Recommend stainless steel fasteners (high chromium content) or hot‑dip galvanised bolts; both resist corrosion in marine environments. (2) Coatings (zinc, epoxy) provide barriers; cathodic protection (sacrificial anodes) protects larger structures where coatings may be damaged — anodes corrode instead of structural metal.