How to Use This Page
Read each section in order — start with the Key Concepts recap for a quick refresher, then work through the MCQs and short answers before tackling the detailed answers. Use the Common Mistakes section to identify pitfalls and the Exam Checklist at the end to verify you are exam-ready.
Key Concepts (Quick Recap)
| Term | Definition |
|---|---|
| Physical change | A change in which no new substance is formed; only physical properties like shape, size, or state change (e.g., melting ice) |
| Chemical change | A change in which one or more new substances with different chemical properties are formed (e.g., burning wood) |
| Reversible change | A change that can be undone to restore the original substance (e.g., freezing water back to ice) |
| Irreversible change | A change that cannot be undone; the original substance cannot be recovered (e.g., burning paper) |
| Sublimation | The direct conversion of a solid into gas without passing through the liquid state (e.g., camphor) |
| Crystallisation | The process of forming crystals from a hot saturated solution by cooling (e.g., copper sulphate crystals) |
| Galvanisation | Coating iron or steel with a thin layer of zinc to prevent rusting |
| Rusting | The slow oxidation of iron in the presence of oxygen and moisture, forming iron oxide (Fe₂O₃·nH₂O) |
| Melting | The change of state from solid to liquid upon heating (e.g., ice to water) |
| Combustion | A chemical process in which a substance reacts with oxygen to produce heat and light (e.g., burning coal) |
| Oxidation | A chemical reaction in which oxygen is added to a substance or hydrogen is removed (e.g., tarnishing of copper) |
I. Choose the Best Answer (MCQs)
Q1. Which of the following is a physical change?
- (a) Burning of paper (b) Rusting of iron (c) Melting of ice (d) Cooking of food
Q2. Which of the following is a chemical change?
- (a) Boiling of water (b) Dissolving salt in water (c) Burning of candle (d) Cutting of wood
Q3. Rusting of iron is an example of
- (a) Physical change (b) Chemical change (c) Reversible change (d) Both physical and chemical
Q4. A change in which no new substance is formed is called
- (a) Chemical change (b) Physical change (c) Irreversible change (d) Permanent change
Q5. Which of the following is an irreversible change?
- (a) Dissolving sugar in water (b) Melting of wax (c) Burning of wood (d) Stretching a rubber band
Q6. The process of changing a solid directly into gas is called
- (a) Evaporation (b) Condensation (c) Sublimation (d) Melting
Q7. Which type of change is photosynthesis?
- (a) Physical change (b) Chemical change (c) Reversible change (d) No change
Q8. Galvanisation is the process of coating iron with
- (a) Copper (b) Chromium (c) Zinc (d) Tin
Q9. Which condition is necessary for rusting of iron?
- (a) Only oxygen (b) Only moisture (c) Both oxygen and moisture (d) Neither oxygen nor moisture
Q10. Crystallisation is used to
- (a) Melt a substance (b) Purify a substance (c) Burn a substance (d) Rust a substance
Q11. Which of the following changes is BOTH physical and chemical?
- (a) Melting of ice (b) Burning of wood (c) Burning of a candle (d) Rusting of iron
II. Fill in the Blanks
1. A change in which a new substance is formed is called a chemical change. 2. A change in which no new substance is formed is called a physical change. 3. The process of converting liquid into gas is called evaporation. 4. The process of converting gas into liquid is called condensation. 5. The process of converting solid directly into gas is called sublimation. 6. Burning of paper is an irreversible change. 7. Melting of ice is a reversible change. 8. Rusting of iron requires oxygen and moisture. 9. The process of depositing a layer of zinc on iron is called galvanisation. 10. Crystallisation is a process of forming crystals from a solution.
III. True or False
IV. Match the Following
| Column I | Column II | Answer |
|---|---|---|
| Melting of ice | Physical, reversible | Melting of ice — Physical, reversible |
| Burning of wood | Chemical, irreversible | Burning of wood — Chemical, irreversible |
| Rusting of iron | Chemical, irreversible | Rusting of iron — Chemical, irreversible |
| Dissolving salt | Physical, reversible | Dissolving salt — Physical, reversible |
| Cooking of food | Chemical, irreversible | Cooking of food — Chemical, irreversible |
| Sublimation of camphor | Physical, reversible | Sublimation of camphor — Physical, reversible |
V. Short Answer Questions
Q1. Define physical change with two examples. Answer: A physical change is a change in which no new substance is formed. Only the physical properties such as shape, size, state, or colour may change, and the change is often reversible. The chemical composition of the substance remains the same before and after the change.
Examples: 1. Melting of ice (solid water becomes liquid water; no new substance is formed) 2. Dissolving sugar in water (sugar dissolves but can be recovered by evaporation)
Q2. Define chemical change with two examples. Answer: A chemical change is a change in which one or more new substances are formed with different chemical properties from the original substances. The change is permanent, irreversible, and usually involves energy changes such as heat, light, or sound being released or absorbed.
Examples: 1. Burning of wood (forms ash, CO₂, and water vapour) 2. Rusting of iron (forms iron oxide)
Q3. How can rusting of iron be prevented? Answer: Rusting can be prevented by creating a barrier between iron and the oxygen and moisture that cause it. Common methods include: 1. Painting: Coating iron with paint prevents contact with air and moisture. 2. Galvanisation: Coating iron with a layer of zinc prevents rusting. 3. Oiling and greasing: Applying oil or grease to iron surfaces prevents contact with moisture. 4. Alloying: Making stainless steel by mixing iron with chromium and nickel prevents rusting. 5. Keeping iron dry: Moisture and oxygen are needed for rusting, so keeping iron dry prevents it.
Q4. Why is burning of a candle considered both a physical and a chemical change? Answer: A burning candle involves two simultaneous processes. In the physical change, the wax near the flame melts and drips down — it changes from solid to liquid state, but no new substance is formed. In the chemical change, the melted wax vaporises and burns, reacting with oxygen in the air to form carbon dioxide and water vapour. Since new substances are produced in this step, it is a chemical change. Because both processes occur together, burning a candle is considered both a physical and a chemical change.
Q5. What is galvanisation? Answer: Galvanisation is the process of coating iron or steel objects with a thin layer of zinc to prevent rusting. Zinc acts as a physical barrier between the iron and the surrounding air and moisture. Even if the zinc layer is scratched, zinc sacrifices itself by corroding preferentially (sacrificial protection), thereby continuing to shield the iron beneath. Common examples of galvanised items include iron pipes, buckets, and roofing sheets.
Q6. What is crystallisation? Give an example. Answer: Crystallisation is the process of forming pure crystals from a solution by first preparing a hot, saturated solution and then allowing it to cool slowly. As the solution cools, the excess solute can no longer remain dissolved and separates out as well-defined crystals. Impurities remain in the solution, making crystallisation an effective purification method.
Example: When a hot saturated solution of copper sulphate is allowed to cool, blue crystals of copper sulphate (CuSO₄·5H₂O) are formed. This method is widely used in laboratories and industry to obtain pure substances.
Q7. What is sublimation? Give two examples. Answer: Sublimation is the process in which a solid changes directly into a gas without passing through the liquid state. This occurs when the substance's vapour pressure at a given temperature exceeds atmospheric pressure before the melting point is reached.
Examples: 1. Camphor — when heated, camphor sublimes directly into vapour. 2. Naphthalene balls — they gradually sublime at room temperature, reducing in size over time. 3. Dry ice (solid CO₂) — sublimes directly into carbon dioxide gas.
VI. Detailed Answer Questions
Q1. Compare physical and chemical changes. Answer:
| Property | Physical Change | Chemical Change |
|---|---|---|
| New substance | No new substance formed | New substance(s) formed |
| Reversibility | Usually reversible | Usually irreversible |
| Energy change | May or may not involve energy change | Usually involves energy change (heat, light, sound) |
| Chemical composition | Remains the same | Changes completely |
| Examples | Melting ice, dissolving sugar | Burning wood, rusting iron |
| Mass | Mass remains the same | Mass may change (gases released) |
| Original substance | Can be recovered | Cannot be recovered |
Key distinctions to remember:
- The most reliable test for a chemical change is the formation of a new substance with different chemical properties. Reversibility alone is not a reliable indicator because some physical changes (like breaking glass) are irreversible.
- Chemical changes often produce observable signs such as colour change, gas evolution, precipitate formation, or temperature change.
- In a physical change, the molecules remain the same — only their arrangement or state changes. In a chemical change, the molecules themselves break apart and recombine to form new substances.
Q2. Explain the different types of changes with examples. Answer: Changes can be classified into two main types:
1. Physical Changes: Changes in which no new substance is formed. Only physical properties change, and the original substance can often be recovered.
Examples:
- Change of state: Melting of ice (solid to liquid), boiling of water (liquid to gas)
- Change of shape: Cutting paper, stretching a rubber band
- Dissolving: Sugar dissolving in water, salt dissolving in water
- Sublimation: Camphor changing directly from solid to gas
2. Chemical Changes: Changes in which one or more new substances are formed with different chemical properties. The change is permanent and the original substances cannot be recovered.
Examples:
- Combustion: Burning of wood, paper, or coal — produces ash, smoke, and gases
- Oxidation: Rusting of iron (iron + oxygen + moisture forms iron oxide), tarnishing of silver
- Reaction with acid: Vinegar reacting with baking soda produces carbon dioxide gas
- Biological processes: Digestion of food, ripening of fruit, fermentation of batter
- Cooking: Cooking food changes raw ingredients into new substances with different taste, texture, and nutritional value
Some changes are both physical and chemical:
- Burning of a candle (melting wax = physical; burning wax = chemical)
- Making bread (mixing ingredients = physical; baking = chemical)
How to identify the type of change:
- Ask yourself: "Is a new substance formed?" If yes, it is a chemical change.
- Look for signs: colour change, gas bubbles, heat/light produced, precipitate formed — these indicate a chemical change.
- If only state, shape, or size changes and no new substance appears, it is a physical change.
Q3. Describe an activity to show that rusting requires both air and moisture. Answer: Activity: Take three test tubes labelled A, B, and C. Place an iron nail in each.
- Test tube A: Contains boiled water (to remove dissolved oxygen) with a layer of oil on top (to prevent air from dissolving). The nail is exposed to water but not air.
- Test tube B: Contains dry air. Calcium chloride is placed inside to absorb moisture. The nail is exposed to air but not moisture.
- Test tube C: Contains ordinary tap water. The nail is exposed to both air and moisture.
Leave the test tubes for a few days.
Observations:
- Test tube A: No rusting (water without air)
- Test tube B: No rusting (air without moisture)
- Test tube C: Rusting occurs (both air and moisture present)
Conclusion: Rusting of iron requires both air (oxygen) and moisture (water). Absence of either prevents rusting.
Additional points for exam answers:
- Boiling the water in test tube A removes dissolved oxygen; the oil layer on top prevents atmospheric oxygen from dissolving back in.
- Calcium chloride in test tube B is a drying agent that absorbs any moisture from the air inside the tube.
- This experiment clearly demonstrates that both conditions — presence of oxygen and presence of moisture — must be satisfied simultaneously for rusting to occur. Removing either one is sufficient to prevent rusting.
Q4. Explain the different methods of preventing rusting of iron. Answer: Rusting occurs when iron comes in contact with oxygen and moisture. Prevention methods create a barrier between iron and these elements:
1. Painting: Coating iron surfaces with paint prevents contact with air and moisture. Used for gates, bridges, and car bodies. Paint must be reapplied periodically because scratches expose the iron underneath.
2. Galvanisation: Coating iron with a thin layer of zinc. Zinc forms a protective barrier and also acts as a sacrificial metal — even if the coating is scratched, zinc corrodes preferentially to protect the iron. Used for buckets, pipes, and roofing sheets.
3. Oiling and greasing: Applying oil or grease on iron parts (like bicycle chains, tools, and machinery) prevents direct contact with moisture. This method is used mainly for moving parts where painting is impractical.
4. Alloying: Mixing iron with other metals like chromium and nickel to form stainless steel, which does not rust. Used for utensils, surgical instruments, and chemical equipment.
5. Chrome plating: Coating iron with a layer of chromium gives a shiny, rust-resistant surface. Used for car parts and bathroom fittings.
6. Tinning: Coating iron with a layer of tin. Used for food cans and cooking vessels because tin is non-toxic and resistant to corrosion.
7. Electroplating: Depositing a thin layer of a less reactive metal (like nickel or chromium) on iron using electricity. Used for decorative items and hardware.
Summary for quick revision: All rust-prevention methods work by either (a) creating a physical barrier between iron and air/moisture, or (b) altering the composition of iron itself through alloying to make it resistant to oxidation.
Common Mistakes
✅ Correct approach: Crystallisation is a separation and purification technique. The original substance is recovered in a purer form, so it is a physical process.
✅ Correct approach: Remember that galvanisation offers dual protection (barrier + sacrificial), while painting offers only barrier protection.
- Calling all physical changes reversible: Some physical changes are irreversible (cutting paper, breaking glass). Reversibility is not a defining feature of physical changes.
- Confusing physical and chemical changes based on reversibility: The key distinction is whether a new substance is formed, not whether the change is reversible.
- Forgetting that burning a candle involves both physical and chemical changes: The melting of wax is physical; the burning of wax vapour is chemical.
- Saying rusting only needs oxygen: Rusting requires both oxygen AND moisture. Absence of either prevents rusting.
- Confusing galvanisation with electroplating: Galvanisation specifically involves coating with zinc, while electroplating can use various metals.
- Calling dissolving a chemical change: Dissolving is a physical change — no new substance is formed, and the solute can be recovered.
- ❌ Thinking crystallisation is a chemical change: Crystallisation is a physical change. No new substance is formed — the solute simply separates from the solution as crystals. The chemical composition of the solute remains unchanged.
- ❌ Assuming galvanisation and painting are the same type of protection: Painting only provides a barrier — if the paint scratches off, the iron starts rusting immediately. Galvanisation provides both a barrier and sacrificial protection — even if the zinc layer is scratched, zinc corrodes instead of iron.
Exam Checklist
- [ ] Can you define and distinguish between physical and chemical changes?
- [ ] Can you give at least five examples each of physical and chemical changes?
- [ ] Can you explain why burning a candle involves both physical and chemical changes?
- [ ] Can you explain why rusting requires both air and moisture (with the test tube experiment)?
- [ ] Can you list at least five methods of preventing rusting?
- [ ] Can you define sublimation with examples?
- [ ] Can you define crystallisation and explain its process?
- [ ] Can you compare physical and chemical changes in a table?
- [ ] Can you identify whether a given change is physical or chemical?
- [ ] Can you explain galvanisation and its advantages?
- [ ] Can you state why crystallisation is a physical change, not a chemical one?
- [ ] Can you describe the difference between barrier protection (painting) and sacrificial protection (galvanisation)?