Changes Around Us — Exercise Solutions

Samacheer Kalvi Tamil Nadu State Board Class 7 Science Term 2 Chapter 3 exercise solutions. Covers MCQs, fill-in-the-blanks, true/false, match the following, classification, analogy, assertion and reason, very short, short, detailed answers, and HOTS questions.

I. Choose the Correct Answer (MCQ)

Q1. When a woolen yarn is knitted to get a sweater, the change can be classified as ______ (a) physical change (b) chemical change (c) endothermic change (d) exothermic change Answer: (a) physical change ✓ — Only the shape changes; no new substance is formed.

Q2. ______ of the following are endothermic changes. (a) Condensation and melting (b) Condensation and freezing (c) Evaporation and melting (d) Evaporation and freezing Answer: (c) Evaporation and melting ✓ — Both absorb heat energy from the surroundings.

Q3. The chemical change is ______ (a) water to clouds (b) growth of tree (c) cow dung to bio-gas (d) ice-cream to molten ice-cream Answer: (c) cow dung to bio-gas ✓ — New substances (methane, carbon dioxide) are formed.

Q4. ______ is an example of a periodic change (a) Earthquake (b) Formation of rainbow in sky (c) Occurrence of tides in seas (d) Showering of rain Answer: (c) Occurrence of tides in seas ✓ — Tides occur at regular, predictable intervals.

Q5. ______ is not a chemical change. (a) Dissolution of ammonia in water (b) Dissolution of carbon-dioxide in water (c) Dissolution of oxygen in water (d) Melting of polar ice caps Answer: (d) Melting of polar ice caps ✓ — Melting is a physical change; no new substance is formed.

Additional MCQs

Q6. Change of state from solid to liquid and liquid to gas is ______ (a) physical change (b) chemical change (c) combination (d) none Answer: (a) physical change ✓ — Only the state changes; the substance remains the same.

Q7. Rusting of iron is ______ (a) an irreversible chemical change (b) a reversible chemical change (c) an irreversible physical change (d) a reversible physical change Answer: (a) an irreversible chemical change ✓ — Rust is a new substance (iron oxide) and cannot be easily converted back.

Q8. Keeping a stone in sunlight for few hours is ______ (a) physical change (b) chemical change (c) neither physical nor chemical (d) combination Answer: (c) neither physical nor chemical ✓ — No change occurs to the stone.

Q9. Beating an egg to make a cake is a ______ (a) physical change (b) reversible change (c) chemical change (d) change in state Answer: (c) chemical change ✓ — Baking transforms the egg into a new substance (cake).

Q10. Large crystals of pure substances can be obtained from their solutions by ______ (a) sublimation (b) evaporation (c) melting (d) crystallization Answer: (d) crystallization ✓ — Crystallization produces pure, large crystals from a saturated solution.

Q11. Which is not a physical change? (a) crushing of a paper (b) Burning of a paper (c) making boat of a paper (d) melting of butter Answer: (b) Burning of a paper ✓ — Burning produces ash, smoke, and gases — new substances.

Q12. Cut vegetables turn brown when exposed to air, due to ______ (a) evaporation (b) oxidation (c) neutralization (d) displacement Answer: (b) oxidation ✓ — Exposure to oxygen causes enzymatic browning.

Q13. Which gas is produced when vinegar reacts with baking soda? (a) Hydrogen (b) carbon dioxide (c) carbon monoxide (d) oxygen Answer: (b) carbon dioxide ✓ — NaHCO₃ + CH₃COOH → CO₂ + H₂O + CH₃COONa

Q14. Vanaspathi is obtained from vegetable oils by addition of ______ (a) oxygen (b) hydrogen (c) carbon dioxide (d) nitrogen Answer: (b) hydrogen ✓ — Hydrogenation adds hydrogen to unsaturated oils.

Q15. The simplest method of preventing rusting is to coat iron with oil, grease, or paint because ______ (a) the layer prevents air and water from reaching the iron surface (b) the layer reacts with iron to form a protective coating (c) oil and grease are good conductors (d) paint makes iron shiny Answer: (a) the layer prevents air and water from reaching the iron surface ✓

II. Fill in the Blanks

1. Filling up a balloon with hot air is a physical change. 2. Stretching gold coin into a ring is a physical change. 3. Opening a gas cylinder knob converts liquid fuel into gaseous fuel. This is an example of physical change. 4. Spoiling of food is a chemical change. 5. Respiration is a exothermic chemical change.

Additional Fill in the Blanks

6. When water is added to quicklime, there will be evolution of heat along with formation of slaked lime. 7. A lump of curd is the precipitate obtained by chemical reaction between hot milk and lemon juice. 8. Heat may be evolved or absorbed during a chemical change. 9. Salt is obtained from sea water by the process of evaporation. 10. When magnesium is burnt in air, a new substance formed is magnesium oxide. 11. Fermentation occurs in the absence of air and in the presence of micro-organisms such as yeast. 12. Catalysts are substances that speed up a chemical change without undergoing change themselves. 13. Galvanisation is a process in which zinc is coated as a layer on iron. 14. The chemical formula of rust is Fe₂O₃·H₂O. 15. Condensation happens when molecules in a gas cool down. 16. Solid substances like camphor, naphthalene heat without becoming liquid (sublimation). 17. Dissolution of glucose in water is an endothermic change. 18. In an endothermic process, the speed of the molecules is increased hence they move faster. 19. Freezing is the changing of a liquid into its solid state by cooling. 20. The rate of evaporation increases with rising temperature.

III. True or False (with corrections)

Q1. Cutting of cloth is an example of a periodic change. Answer: False. Cutting of cloth is a physical change, not a periodic change.

Q2. Taking a glass of water and freezing it by placing it in the freezer is a chemical change. Answer: False. It is a physical change — only the state changes from liquid to solid.

Q3. A bean plant collecting sunlight and turning it into bean seeds is an example of physical and non-periodic change. Answer: False. It is a chemical and non-periodic change — photosynthesis produces new substances.

Q4. If the chemical properties of a substance remain unchanged and the appearance or shape changes, it is called a periodic change. Answer: False. It is called a physical change.

Q5. Tarnishing of silver is an example of endothermic change. Answer: False. Tarnishing of silver is an exothermic chemical change — it releases energy.

Additional True/False

Q6. Nitrogen gas turns lime water milky. Answer: False. Carbon dioxide gas turns lime water milky.

Q7. Cutting a log of wood into pieces is a chemical change. Answer: False. It is a physical change — no new substance is formed.

Q8. Iron pipes coated with zinc do not get rusted easily. Answer: True. — Zinc acts as a protective layer preventing contact with air and moisture.

Q9. Iron and rust are the same substances. Answer: False. Rust is iron oxide (Fe₂O₃·H₂O), which is chemically different from iron.

Q10. The chemical name of baking soda is sodium bicarbonate. Answer: True.

Q11. Adding sugar to milk is a physical change. Answer: True. — Sugar dissolves but no new substance is formed.

Q12. Magnesium + oxygen → magnesium oxide (2Mg + O₂ → 2MgO) Answer: True.

Q13. Evaporation is a fast process and occurs only at the surface. Answer: False. Evaporation is a slow process and occurs only at the surface.

Q14. The rate of evaporation is more when the surface area is greater. Answer: True. — Greater surface area means more molecules can escape.

Q15. When lemon juice is mixed with soda water, they produce brisk effervescence. Answer: True. — Carbon dioxide gas is released.

IV. Match the Following

Set 1

Set 2

V. Classification: Physical vs. Chemical Changes

Classify the following as physical or chemical changes:

VI. Analogy

Q1. Physical Change : Boiling :: Chemical Change : ______ Answer: Burning

Q2. Wood to saw dust : ______ :: Wood to Ash : Chemical change Answer: Physical change

Q3. Forest fire : ______ change :: Change in period in a school : periodic change Answer: Non-periodic

VII. Assertion and Reason

Q1. Assertion: The explosion of a fire cracker is a physical change. Reason: A physical change is a reversible change. (a) Both true, R explains A (b) Both true, R doesn't explain A (c) A true, R false (d) A false, R true Answer: (d) A is false but R is true. ✓ — Explosion of a firecracker is a chemical change (new substances form), but a physical change is indeed reversible.

Q2. Assertion: The process of converting liquid water to vapours by heating is called boiling. Reason: Converting water vapours to liquid by cooling is called condensation. (a) Both true, R explains A (b) Both true, R doesn't explain A (c) A true, R false (d) A false, R true Answer: (b) Both A and R are true but R is not the correct explanation of A. ✓

Q3. Assertion: Burning of wood log to charcoal is a physical change. Reason: Products of burning wood can be easily converted back to a wood log. Answer: Both A and R are false. ✓ — Burning wood is a chemical change; the products cannot be converted back to wood.

Q4. Assertion: Formation of iron oxide from iron is a chemical change. Reason: For rust to form, iron must be exposed to air and water. (a) Both true, R explains A (b) Both true, R doesn't explain A (c) A true, R false (d) A false, R true Answer: (a) Both A and R are true and R is the correct explanation of A. ✓

Additional Assertion and Reason

Q5. Assertion: Burning of a candle is considered physical as well as chemical change. Reason: Melting of wax is physical; vapourised wax burns, which is chemical. Answer: (a) Both A and R are true and R is the correct explanation of A. ✓

Q6. Assertion: No new substance is formed when water is heated to get steam. Reason: Conversion of water into steam is a physical change. Answer: (a) Both A and R are true and R is the correct explanation of A. ✓

Q7. Assertion: Cutting paper into very small pieces is an irreversible change. Reason: Physical changes are always reversible. Answer: (c) A is true but R is false. ✓ — Cutting paper is irreversible (you cannot rejoin it), but physical changes are not always reversible.

VIII. Very Short Answer Questions

Q1. State two examples of periodic changes. Answer: Seasonal changes, motion of hands of a clock.

Q2. Mention any two exothermic reactions. Answer: Burning of magnesium ribbon and burning of a candle.

Q3. Cold milk is heated and it becomes hot. Which type of change is it? Answer: Reversible physical change. — Milk can be cooled back down.

Q4. What type of change is artificial ripening of fruit? Answer: Irreversible chemical change. — New substances form and the process cannot be reversed.

Q5. What type of change is coloring of a paper? Answer: Physical change. — No new substance is formed; only appearance changes.

Q6. Growing of nails is a periodic change. Why? Answer: It occurs periodically at regular intervals.

Q7. What type of energy change is associated when ice melts? Answer: Physical change; endothermic (heat energy is absorbed).

Additional Very Short Answers

Q8. Mention few physical properties of a substance. Answer: Lustre, malleability, ductility, density, viscosity, solubility, mass, and volume.

Q9. What are crystals? Answer: Solids in their purest form that have definite geometrical shapes.

Q10. Is conversion of milk into ice-cream a chemical change? Answer: Yes, since properties of ice-cream differ from milk — new substances are formed.

Q11. Which is used as a catalyst during hydrogenation of oils? Answer: Nickel, platinum, or palladium.

Q12. Conditions needed for chemical changes? Answer: Physical contact, heat, light, electricity, and pressure.

Q13. Chemical reaction of baking soda with lemon? Answer: Sodium hydrogen carbonate + citric acid → sodium citrate + carbon dioxide + water.

Q14. Examples of endothermic process? Answer: Melting, vaporization, and sublimation.

Q15. Examples of exothermic process? Answer: Freezing and condensation.

Q16. What happens when baking soda is mixed with lemon juice? Answer: A hissing sound is heard as bubbles of carbon dioxide rise in the reaction vessel.

IX. Short Answer Questions

Q1. Distinguish physical and chemical changes.

Q2. How can a change occur in a substance? Answer: A change occurs through alteration in properties such as colour, texture, and state, often involving formation of a new substance. Changes can be brought about by physical contact, heat, light, electricity, pressure, or the action of microorganisms.

Q3. Suggest a method to collect water from sea water. Answer: Evaporation — Heat seawater so water evaporates, leaving salt behind. The water vapour can be collected by condensation on a cool surface.

Q4. Is solar eclipse a periodic change? Give your reason. Answer: Yes, it occurs after a definite interval of time and can be predicted with accuracy.

Q5. Difference between dissolution of sugar and burning of sugar? Answer:

• Dissolution of sugar: Sugar dissolves in water but remains present; evaporating water recovers the sugar. It is a physical and reversible change.
• Burning of sugar: Oxygen reacts with sugar as chemical bonds break; energy is released as smoke. It is a chemical and irreversible change.

Additional Short Answers

Q6. How is settling of curd a chemical change? Answer: Milk's properties completely change; curd is a new product and cannot be converted back to milk, making it irreversible and chemical.

Q7. How does painting an iron gate prevent rusting? Answer: Paint acts as a protective layer, preventing iron from contacting air or moisture, both of which are necessary for rusting.

Q8. Example where heat, light, and sound are produced during a chemical change. Answer: Burning of a firecracker produces heat, light, and sound simultaneously.

Q9. Example of a physical change involving a colour change. Answer: Heating iron wire turns it red; on cooling, it returns to its original colour.

Q10. Five changes during a chemical change. 1. Change in colour 2. Change in physical state 3. Evolution of gas 4. Absorption or evolution of heat 5. Formation of a precipitate

Q11. How can we prevent rusting? Answer: Apply a coat of paint or grease regularly to prevent iron from contacting oxygen and water/vapour.

Q12. Note on catalyst. Answer: Catalysts speed up chemical changes without undergoing any change themselves. Example: yeast acts as a catalyst in fermentation of sugar.

Q13. Why are we advised not to play with fireworks? Answer: Firework explosion is a chemical change producing heat, light, sound, and unpleasant gases that pollute the atmosphere and can cause injury.

Q14. Why is evaporation not a good technique to separate salt from sea water? Answer: Soluble impurities do not get removed in the process of evaporation — only water evaporates, leaving all dissolved substances behind.

Q15. Importance of chemical changes in daily life. Answer: Medicines are prepared through chains of chemical changes; materials like plastics, soaps, detergents, perfumes, acids, bases, and salts are all made through various chemical changes.

X. Detailed Answer Questions

Q1. Explain: Digestion is a chemical change. Answer:

• The mouth physically breaks food into small pieces; mechanical digestion occurs in mouth, stomach, and small intestine
• Food is chemically changed when new, smaller substances are formed by enzymes
• Raw materials cannot be recovered in their original form
• Digestion is a permanent, irreversible change
• New substances (glucose, amino acids, fatty acids) are formed from the original food

Q2. How is the iron blade fixed into a wooden handle in tools used to dig the soil? Answer: 1. The ring in the iron blade of the spade is heated first 2. Heating causes the blade to expand, making the ring bigger 3. Metals always expand on heating, so the blade fits easily onto the handle 4. Cold water is poured on the blade, causing contraction of the expanded iron 5. Alternatively, the spade is left to cool undisturbed so it is firmly fixed

Q3. Characteristics of physical change with examples. 1. No new substances are formed — chemical properties remain unchanged. Example: ice melting forms water — same substance in both states. 2. Usually temporary and reversible — Example: heating water produces vapour; cooling vapour recovers water. 3. Chemical properties do not change — Example: melted gold has the same chemical composition as solid gold. 4. Physical properties like colour, shape, and size may change — Example: cutting vegetables or inflating a balloon change size and shape.

Q4. Characteristics of chemical change. 1. Heat, light, or other radiation may be given off or absorbed 2. Sound may be produced 3. A change in smell may occur or a new smell may be given off 4. A colour change may take place 5. A gas may be formed 6. Formation of precipitate

Q5. Indicators of a chemical change with examples. 1. Evolution of gas: Adding lemon juice to broken eggshells produces carbon dioxide bubbles. 2. Evolution of heat: Adding water to quicklime (calcium oxide) produces heat and forms slaked lime (calcium hydroxide). 3. Change of odour: Spoiled food emits a foul smell, indicating chemical change. 4. Change of colour: An iron nail kept in water turns reddish brown, indicating rusting (chemical change). 5. Formation of precipitate: Curd forms as a precipitate from the reaction between hot milk and lemon juice.

Q6. Why do water droplets form inside the plate covering cooked food? Answer: 1. Water vapour emerges from hot food and rises 2. The covering plate is cooler than the hot food 3. Energetic molecules lose energy upon touching the cooler plate 4. Molecules slow down and move closer together 5. They collect together to form liquid droplets 6. Condensation occurs when gas molecules cool down

Q7. Notes on: (i) Curdling of milk: Curdling gradually turns liquid into solid, forming clumps. Adding a few drops of curd to hot milk causes curdling in minutes. Lemon extract can also be used, though the taste and texture differ from naturally curdled milk.

(ii) Fermentation: Microorganisms like yeast break down sugar solutions into alcohol and carbon dioxide. It is irreversible — alcohol cannot be turned back into sugar. Hence, fermentation is a chemical change.

XI. Higher Order Thinking Skills (HOTS)

Q1. Peeled and unpeeled banana does not look the same. Does that mean peeling banana is a chemical change? Answer: No, it is not a chemical change. We just separated the skin; there is no change in the composition of the banana. It is a physical change.

Q2. A very hot glass on putting in cold water cracks. What does this change indicate? Answer: When hot glass cools rapidly, it cools unevenly — the inside contracts while the outside remains expanded, causing cracks. It is a physical but irreversible change.

Q3. Boiling of water is a physical change; but boiling of egg is a chemical change. Why? Answer:

• Boiling of water: Water converts to steam; covering with a lid condenses steam back to water. Only the physical state changes — so it is a physical change.
• Boiling of egg: The chemical nature of the egg changes; a boiled egg's properties differ entirely from a raw egg — hence it is a chemical change.

Q4. Example justifying that physical changes can be reversible as well as irreversible. Answer:

• Freezing water into ice is a physical change that is reversible (ice can melt back to water).
• Cutting wood into small pieces is a physical change that is irreversible (small pieces cannot reform the original piece).
• Thus, physical changes can be either reversible or irreversible.

Q5. Photosynthesis is a chemical change — justify. Answer: Plants convert light energy into chemical energy; water and carbon dioxide combine to form glucose and oxygen. New chemical species are formed, confirming it as a chemical change.

Q6. Why is spoiling of food a chemical change? Answer: Microbes break down food particles; properties are lost and new ones gained. The foul smell indicates a change of odour, which is an indicator of a chemical change.

XII. Intext Activities

Activity 3: Two pans (wide and narrow) filled with hot water — wide pan cools faster. Why do we unfurl clothes to dry? Answer: Wet clothes dry faster when spread because the rate of evaporation depends on surface area. More surface area means a higher evaporation rate.

Activity 7: Why must magnesium ribbon be cleaned before burning? Answer: The layer of magnesium oxide must be removed to get the desired reaction. Magnesium burns with a dazzling flame, forming white ash (magnesium oxide).

Activity 9: Soap powder + water feels hot; glucose + water feels cold. What is the difference? Answer: Adding water to soap powder produces heat (exothermic). Adding water to glucose feels cold (endothermic).

Common Mistakes to Avoid

1. Calling all physical changes reversible — Some physical changes are irreversible (e.g., cutting paper, breaking glass). 2. Confusing boiling and evaporation — Boiling occurs throughout the liquid at a specific temperature; evaporation occurs only at the surface at any temperature. 3. Saying burning and rusting are the same — Both are chemical changes, but burning produces heat and light rapidly; rusting is a slow oxidation. 4. Thinking dissolution is always physical — Dissolving carbon dioxide in water forms carbonic acid (chemical change); dissolving sugar in water is physical. 5. Confusing endothermic and exothermic — Endothermic absorbs heat (melting, evaporation); exothermic releases heat (burning, condensation). 6. Calling melting of polar ice caps a chemical change — It is a physical change; no new substance is formed. 7. Forgetting that both air AND water are needed for rusting — Iron needs both oxygen and moisture to rust; coating prevents contact with both.

Exam Checklist

• [ ] Can define and differentiate physical and chemical changes with examples
• [ ] Can classify given changes as physical, chemical, reversible, irreversible, periodic, or non-periodic
• [ ] Can list the indicators of a chemical change (gas, heat, light, colour, precipitate, odour)
• [ ] Can explain why rusting is a chemical change and how to prevent it
• [ ] Can distinguish endothermic and exothermic changes with examples
• [ ] Can explain crystallization as a method of obtaining pure substances
• [ ] Can describe the process of galvanisation and its purpose
• [ ] Can explain why digestion is a chemical change
• [ ] Can give examples where physical changes are irreversible (breaking glass, cutting paper)
• [ ] Can write the chemical equation for rusting: 2Fe + 2O₂ + 2H₂O → 2Fe₂O₃·H₂O