Physical and Chemical Changes — Class 7 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 7 Science chapter "Physical and Chemical Changes" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 7 Science chapter "Physical and Chemical Changes" — 8 important questions with detailed answers for CBSE b…
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NCERT Solutions for Class 7 Science — Physical and Chemical Changes. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Physical and Chemical Changes — All 8 Questions Solved
Q1. Define physical change.
A physical change is a change in which only the physical properties of a substance change.
The substance remains the same chemically.
No new substance is formed.
Example: Melting of ice, boiling of water, breaking of glass.
Q2. Define chemical change.
A chemical change is a change in which a new substance with different chemical properties is formed.
The original substance is transformed into one or more new substances.
Example: Burning of wood, rusting of iron, digestion of food.
Q3. Distinguish between physical and chemical changes with two examples each.
Physical change: No new substance formed; reversible or irreversible change in physical properties only.
Examples: Melting of butter, tearing of paper
Chemical change: New substance formed; generally not easily reversible; change in chemical properties.
Examples: Burning of candle, cooking of food
Q4. What are the signs of a chemical change?
Signs of chemical change:
- Change in colour
- Evolution of gas
- Change in temperature (heat absorbed or released)
- Formation of a precipitate
- Change in smell
Q5. Is freezing of water a physical or chemical change? Why?
Freezing of water is a physical change.
Water changes from liquid state to solid state (ice).
No new substance is formed and water molecules remain the same chemically.
It is reversible - ice can be melted back to water.
Q6. Is burning of magnesium a physical or chemical change? Explain.
Burning of magnesium is a chemical change.
Magnesium reacts with oxygen to form magnesium oxide (white powder).
2Mg + O₂ → 2MgO
A new substance (magnesium oxide) is formed with different properties.
The change is not easily reversible.
Q7. Explain why rusting of iron is a chemical change.
Rusting of iron is a chemical change because:
- Iron combines with oxygen and water to form iron oxide (rust)
- A new substance with different properties is formed
- The change is not easily reversible
- Rust has different colour, texture and properties compared to iron
Q8. What is the difference between burning and rusting? Are they both chemical changes?
Burning: Rapid oxidation process; releases light and heat; occurs at high temperature; example: burning of wood
Rusting: Slow oxidation process; no light; minimal heat; occurs at normal temperature; example: rusting of iron
Both are chemical changes as new substances (oxides) are formed in both cases.
Physical and Chemical Changes — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following is an example of a physical change?
- A. Burning of wood
- B. Rusting of iron
- C. Melting of ice
- D. Cooking of food
Answer: C. Melting of ice — Melting of ice is a physical change because water changes its state from solid to liquid, but its chemical composition (H2O) remains the same. No new substance is formed.
MCQ 2. Which of these changes involves the formation of a new substance?
- A. Tearing a sheet of paper
- B. Bending a metal wire
- C. Dissolving sugar in water
- D. Burning a magnesium ribbon
Answer: D. Burning a magnesium ribbon — Burning a magnesium ribbon produces magnesium oxide, which is a new substance with different properties. This is a chemical change.
MCQ 3. What is a key characteristic of a chemical change?
- A. It is always reversible
- B. No new substance is formed
- C. The chemical composition of the substance changes
- D. Only the physical state changes
Answer: C. The chemical composition of the substance changes — In a chemical change, the original substance undergoes a change in its chemical composition, resulting in the formation of one or more new substances.
MCQ 4. Which of the following processes does NOT result in a new substance being formed?
- A. Digestion of food
- B. Ripening of fruit
- C. Boiling water
- D. Photosynthesis
Answer: C. Boiling water — Boiling water is a physical change where water changes from liquid to gas (steam), but it remains chemically H2O. No new substance is formed.
MCQ 5. Rusting of iron is an example of:
- A. A physical change
- B. A chemical change
- C. A periodic change
- D. A reversible change
Answer: B. A chemical change — Rusting is a chemical change where iron reacts with oxygen and water to form hydrated iron(III) oxide (rust), a new substance with different properties.
MCQ 6. What two conditions are essential for the rusting of iron to occur?
- A. Heat and light
- B. Water and carbon dioxide
- C. Oxygen and water
- D. Nitrogen and heat
Answer: C. Oxygen and water — Both oxygen (from air) and water (or water vapour) are necessary for iron to rust. If either is absent, rusting does not occur.
MCQ 7. Which of the following processes is generally considered reversible?
- A. Burning a candle
- B. Baking a cake
- C. Freezing water
- D. Digestion of food
Answer: C. Freezing water — Freezing water into ice is a reversible physical change, as the ice can be melted back into liquid water without changing its chemical identity.
MCQ 8. When a sugar solution is prepared by dissolving sugar in water, what type of change occurs?
- A. Chemical change
- B. Physical change
- C. Irreversible change
- D. Combustion
Answer: B. Physical change — Dissolving sugar in water is a physical change because the sugar molecules are still present, just spread out in the water. No new chemical substance is formed.
MCQ 9. When baking soda (sodium bicarbonate) is mixed with lemon juice (citric acid), bubbles of gas are produced. This indicates a:
- A. Physical change
- B. Chemical change
- C. Reversible change
- D. Change of state only
Answer: B. Chemical change — The production of gas bubbles (carbon dioxide) signifies the formation of a new substance through a chemical reaction between baking soda and lemon juice. This is a chemical change.
MCQ 10. Which of these changes involves an absorption of energy (endothermic process)?
- A. Burning wood
- B. Explosion of firecrackers
- C. Melting ice
- D. Rusting of iron
Answer: C. Melting ice — Melting ice requires the absorption of heat energy from the surroundings to change its state from solid to liquid, making it an endothermic process.
MCQ 11. Galvanization is a method of preventing rusting of iron by:
- A. Painting it with oil paint
- B. Applying grease
- C. Coating it with zinc
- D. Dipping it in distilled water
Answer: C. Coating it with zinc — Galvanization involves coating iron with a layer of zinc. Zinc is more reactive than iron and preferentially corrodes, protecting the iron underneath.
MCQ 12. Crystallisation is a process used to obtain pure solid from its solution. Is it a physical or chemical change?
- A. Chemical change
- B. Physical change
- C. Both chemical and physical
- D. Neither chemical nor physical
Answer: B. Physical change — Crystallisation is a physical change because the substance maintains its chemical identity; it merely changes from a dissolved state to a solid crystalline state.
MCQ 13. What happens to the chemical composition of a substance during a physical change?
- A. It changes completely
- B. It changes partially
- C. It remains the same
- D. It depends on the substance
Answer: C. It remains the same — During a physical change, only the physical properties (like state, shape, size) of a substance change, but its chemical composition stays the same.
MCQ 14. The burning of a candle typically involves:
- A. Only a physical change
- B. Only a chemical change
- C. Both physical and chemical changes
- D. Neither physical nor chemical change
Answer: C. Both physical and chemical changes — The melting of wax is a physical change, while the burning of the wax to produce carbon dioxide and water vapour is a chemical change. Both occur simultaneously.
MCQ 15. Which of the following is NOT typically an indication of a chemical change?
- A. Change in colour
- B. Evolution of gas
- C. Formation of precipitate
- D. Change in shape
Answer: D. Change in shape — While some chemical changes can cause a change in shape, a change in shape alone (e.g., crushing a can) is often a physical change and not a definitive indicator of a chemical reaction.
MCQ 16. Photosynthesis, where plants convert carbon dioxide and water into glucose and oxygen, is an example of a:
- A. Physical change
- B. Chemical change
- C. Reversible change
- D. Mechanical change
Answer: B. Chemical change — Photosynthesis is a chemical change because new substances (glucose and oxygen) are formed from carbon dioxide and water, with different chemical properties.
MCQ 17. When an iron nail is placed in copper sulphate solution, the blue colour of the solution fades, and a reddish-brown deposit is observed on the nail. This is an example of:
- A. A physical change
- B. A chemical change
- C. A phase change
- D. A dissolving process
Answer: B. A chemical change — This is a chemical change (a displacement reaction) where iron reacts with copper sulphate to form iron sulphate (colourless) and copper metal (reddish-brown deposit), both new substances.
MCQ 18. Why is the souring of milk considered a chemical change?
- A. Its temperature changes
- B. Its colour changes
- C. New substances like lactic acid are formed
- D. It separates into layers
Answer: C. New substances like lactic acid are formed — Souring of milk is a chemical change because bacteria convert lactose (milk sugar) into lactic acid, a new substance that changes the taste and properties of the milk.
MCQ 19. Which of these changes involves an exothermic reaction (releases heat)?
- A. Melting ice
- B. Dissolving salt in water
- C. Burning natural gas
- D. Boiling water
Answer: C. Burning natural gas — Burning natural gas is a combustion reaction that releases a significant amount of heat and light, making it an exothermic process.
MCQ 20. When a piece of paper is torn into smaller pieces, what kind of change occurs?
- A. Chemical change
- B. Physical change
- C. Irreversible change
- D. Exothermic change
Answer: B. Physical change — Tearing paper is a physical change because the paper's chemical composition remains the same; only its size and shape are altered.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.