Changes Around Us: Physical and Chemical – Case-based Questions with Answers
Class 7
Science — Chapter 5: Changes Around Us (Physical & Chemical)
20 NCERT-aligned case-based questions with clear answers for CBSE Class 7 revision.
CBSE Board Examination
Focus: identify change types, interpret observations, and suggest protective or corrective measures.
Case-Based Questions & Model Answers
Case 1 — A student heats a beaker containing sugar until it turns brown and emits a distinct smell.
Q: Is this a physical or chemical change? Explain the observations.
A: Chemical change. Browning and a new smell indicate formation of new substances (caramelisation/thermal decomposition). The change is irreversible under ordinary conditions.
Case 2 — Two clear solutions when mixed form a cloudy suspension that settles into a white solid.
Q: Identify the type of change and name the process.
A: Chemical change — formation of a precipitate. The insoluble solid indicates a new substance formed by reaction of ions in solution.
Case 3 — A puddle of water evaporates on a hot day leaving no residue.
Q: Classify the change and justify why it is reversible or not.
A: Physical change (change of state). It is reversible by condensation as the water vapour can be cooled to form liquid water again.
Case 4 — Iron nails kept in a humid place develop reddish flaky coating over weeks.
Q: Explain the process, conditions required and why it weakens nails.
A: Rusting (chemical change) — iron reacts with oxygen and water to form hydrated iron oxides. Moisture and oxygen accelerate it; salts speed it further. Rust is flaky and non‑protective, exposing fresh iron and weakening the metal.
Case 5 — A student mixes baking soda and vinegar and observes vigorous bubbling.
Q: What gas is produced and how would you confirm it experimentally?
A: Carbon dioxide (CO₂) is produced. Confirm by bubbling collected gas through limewater; it turns milky due to CaCO₃ formation.
Case 6 — A piece of sodium metal is stored under oil in the lab.
Q: Explain why sodium is kept under oil and what would happen if exposed to air.
A: Sodium is highly reactive with oxygen and moisture; under oil it is isolated from air and water. If exposed, it oxidises and reacts vigorously with water producing NaOH and hydrogen (possible fire/explosion).
Case 7 — A student sees steam rising from a kettle and collects it; the condensate is clear water.
Q: Is the process chemical or physical? Explain the difference between the steam and condensate.
A: Physical change — boiling and condensation are state changes. Steam is water vapour (gas); condensate is liquid water; chemical composition remains H₂O throughout.
Case 8 — A metal spoon left in a beaker of dilute acid produces bubbles and the spoon gradually dissolves.
Q: Identify the change and name the gas; give a simple equation if the metal is zinc and acid is HCl.
A: Chemical change — metal reacts with acid releasing hydrogen gas. For zinc: Zn + 2HCl → ZnCl₂ + H₂↑ (bubbling is H₂).
Case 9 — Two containers: A contains sugar water, B contains salt water. Both are left to evaporate in sun; white crystals form in one while the other shows a different residue.
Q: Explain how evaporation helps identify the dissolved substance and classify the type of changes observed.
A: Evaporation (physical change) removes water leaving solute crystals. If salt crystals appear, salt was solute; sugar crystals indicate sugar. The process is physical as the solute is unchanged chemically.
Case 10 — A student pours lemon juice on a metallic coin and notices a colour change on the coin’s surface.
Q: Suggest what might be happening and whether it is chemical or physical.
A: Acidic lemon juice may react with metal surface causing metal dissolution or oxide removal leading to colour change — a chemical change (corrosion/tarnishing) rather than a simple physical stain.
Case 11 — A chemistry teacher burns magnesium in air producing bright light and a white ash.
Q: Identify the product and explain why light is emitted during the reaction.
A: Product is magnesium oxide (MgO). Light is emitted because the reaction is highly exothermic; excited electrons release energy as visible light when returning to ground state.
Case 12 — A bottle with milk turns sour after being unrefrigerated for days.
Q: Explain the change and name the biological agent responsible.
A: Chemical change due to microbial action — bacteria (lactic acid bacteria) convert lactose to lactic acid causing souring; new substances and taste change occur.
Case 13 — Mixing two clear solutions gives a solution that slowly becomes cloudy over hours.
Q: Propose a likely explanation and how you would test whether it is precipitation or bacterial growth.
A: Could be slow precipitation (chemical) or microbial growth (biological). Test: sterilise with heat — if cloudiness returns only in unsterilised sample it's biological; if filtrate shows solid on standing, it's precipitation. Microscopic exam or chemical tests help differentiate.
Case 14 — A wooden spoon left in sunlight becomes dry and brittle over months.
Q: Is this physical or chemical? Explain factors causing brittleness.
A: Mostly physical degradation (loss of moisture) and some chemical breakdown (UV-induced polymer degradation). Drying makes fibres lose flexibility causing brittleness; long-term exposure can cause oxidation of components.
Case 15 — A student notices bubbles when hydrogen peroxide is poured onto a cut; bubbles increase in presence of a catalyst.
Q: Identify the gas and explain the role of catalyst (e.g., catalase in blood or MnO₂ in lab).
A: Bubbles are oxygen (O₂) from decomposition of H₂O₂: 2H₂O₂ → 2H₂O + O₂. Catalysts like catalase (in blood) or MnO₂ speed decomposition by lowering activation energy; they are not consumed.
Case 16 — A student heats a mixture of iron powder and sulfur and obtains a black solid that is difficult to separate.
Q: Explain the observation and write the chemical equation if iron sulfide is formed.
A: Chemical change: formation of iron sulfide (FeS) — a compound with different properties. Equation: Fe + S → FeS. The product is a single substance, not a mixture, so separation is not by physical means.
Case 17 — A bicycle chain is oiled regularly and shows little rust compared to an un‑oiled chain.
Q: Explain how oil prevents rusting and whether this is a chemical or physical protection method.
A: Oil forms a physical barrier preventing contact of iron with water and oxygen, slowing corrosion. It’s a physical protective method; chemical methods (galvanisation) provide sacrificial protection.
Case 18 — A student tests whether a change is exothermic by touching the reaction vessel and feeling warmth.
Q: Is this a reliable test? Suggest safer and quantitative ways to measure heat change.
A: Feeling warmth is qualitative and not precise. Safer/quantitative: use a thermometer or simple calorimeter to record temperature change; measure with insulated container and thermometer for better accuracy.
Case 19 — A metal spoon left in a jar of sugar solution is sticky but not visibly corroded after a month.
Q: Explain why stickiness occurs and whether a chemical change might be happening slowly.
A: Stickiness likely from sugar deposition (physical) or microbial growth forming biofilm (chemical/biological). Check by rinsing — if residue dissolves, physical; if odour or spoilage signs appear, biological/chemical processes may be involved.
Case 20 — During a school demonstration, paper burns producing ash and smoke; some students argue ash is the same paper but in smaller pieces.
Q: Explain why ash is not the same as original paper and how this illustrates chemical change.
A: Burning decomposes cellulose; products include CO₂, H₂O and solid carbonaceous ash — composition is different from paper. Chemical bonds are broken and new substances form; ash cannot be converted back to original paper by simple physical means, showing irreversibility of chemical change.