Changes Around Us: Physical and Chemical — Mastery

Q1. In Activity 5.2B a student inflates a balloon and then lets the air escape. Getting the uninflated balloon back tells us that this change is best classified as

• A. Chemical, because air left the balloon
• B. Combustion, because the balloon collapsed
• C. Erosion of the balloon surface
• D. Physical, because the balloon material is unchanged

Q2. Which of the following best distinguishes a chemical change from a physical change?

• A. Formation of one or more new substances
• B. A change in the shape of the object
• C. Use of heat in the process
• D. The change being slow

Q3. The chapter says a chemical reaction can be represented in short form as a chemical equation. Which option below correctly represents the burning of magnesium ribbon according to?

• A. Magnesium + Oxygen → Magnesium oxide + Heat + Light
• B. Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
• C. Vinegar + Baking soda → Carbon dioxide + Other substances
• D. Iron + Oxygen + Moisture → Iron oxide

Q4. Melting of ice cubes, the first item in Table 5.1, is classified by the chapter as

• A. A physical change
• B. A chemical change
• C. A combustion reaction
• D. Erosion

Q5. Chopping of vegetables in Activity 5.1 is best classified as

• A. An irreversible physical change
• B. A reversible chemical change
• C. Combustion
• D. Chemical weathering

Q6. Making popcorn from corn is, on the chapter's logic

• A. A chemical change that is irreversible
• B. A physical change that is reversible
• C. Combustion of corn
• D. Erosion of corn

Q7. Identify the correct reactant–product pairing for the lime-water reaction shown in

• A. Reactants: calcium hydroxide and carbon dioxide → Products: calcium carbonate and water
• B. Reactants: calcium carbonate and water → Products: calcium hydroxide and carbon dioxide
• C. Reactants: magnesium and oxygen → Products: magnesium oxide and water
• D. Reactants: vinegar and baking soda → Products: lime water and calcium

Q8. Which of the following is the chapter's stated chemical test to confirm whether a gas is carbon dioxide?

• A. Pass the gas through fresh lime water; if it turns milky, the gas is CO2.
• B. Hold a glowing splinter near the gas; if it relights, the gas is CO2.
• C. Smell the gas; CO2 has a strong garlic-like smell.
• D. Burn the gas with a matchstick; CO2 burns with a yellow flame.

Q9. When baking soda is added to vinegar in Activity 5.4, the student observes a fizzing bubbling sound. The fizzing is best explained as

• A. Carbon dioxide gas being released from the reaction
• B. Baking soda physically dissolving in vinegar
• C. Heat being produced by the mixture
• D. Air escaping from the test tube

Q10. Activity 5.4 asks the student to repeat the activity by replacing vinegar with water and adding baking soda. The expected result, by the chapter's logic, is

• A. No bubble formation, because no acid–base reaction occurs
• B. Vigorous fizzing identical to vinegar
• C. Lime water connected to the second tube turning milky
• D. A new white substance settling at the bottom

Q11. Which earlier Class 7 chapter is cross-referred for details on rusting?

• A. Exploring Substances: Acidic, Basic, and Neutral
• B. Heat Transfer in Nature
• C. The World of Metals and Non-metals
• D. Exploring Forces

Q12. Rusting of iron is explicitly classified as

• A. An undesirable change
• B. A desirable change
• C. A purely physical change
• D. A reversible change

Q13. Question 4(iii) of the chapter says: 'A chemical process in which a substance reacts with oxygen with evolution of heat is called ____, and this is a ____ change.' Fill in the blanks correctly.

• A. Combustion; chemical
• B. Rusting; physical
• C. Condensation; chemical
• D. Erosion; chemical

Q14. From the following list, identify the set that contains only combustible substances as named wood, paper, cotton, kerosene, glass, water, sand.

• A. Wood, paper, cotton, kerosene
• B. Wood, glass, sand, water
• C. Cotton, water, kerosene, sand
• D. Glass, water, sand only

Q15. A teacher tells her Class 7 students that a fire cannot start if even one component of the fire triangle is missing. From this principle, which of the following is the best reason a glass tumbler placed upside down on a burning candle extinguishes the flame?

• A. Oxygen — the second leg of the fire triangle — gets used up inside the tumbler.
• B. The fuel (wax) gets removed.
• C. The ignition temperature of wax suddenly rises.
• D. Carbon dioxide pushes the flame above the tumbler.

Q16. In Question 8 of the chapter (the 'Eco-friendly Prithvi' story), Prithvi chops vegetables and cuts fruits in the kitchen, and later the peels decompose in a clay pot to form compost. Pick the correct classification pair for these two events.

• A. Both physical changes
• B. Both chemical changes
• C. Chopping is chemical; decomposition is physical
• D. Chopping is physical; decomposition is chemical

Q17. Question 3(i) of the chapter asks: 'Melting of wax is necessary for burning a candle. (True/False)'. The chapter-correct response and reason is

• A. True; only solid wax can burn directly.
• B. True; wax must melt to react with oxygen.
• C. False; the candle cannot burn at all if wax melts.
• D. True; the molten wax rises up the wick, evaporates, and the vapour is what actually burns.

Q18. A Class 7 student says, 'Whenever I see bubbles in a liquid, a chemical change must be happening.' Using the chapter's evidence, which response is the most accurate correction?

• A. Agree — bubbles always mean a chemical reaction.
• B. Disagree — bubbles are only seen in physical changes.
• C. Disagree — bubbles in tap water (Activity 5.3, tumbler A) form without any new substance, so bubbles alone are not the test; formation of a new substance is.
• D. Agree — only the colour of bubbles tells us whether the change is chemical.

Q19. A Class 7 teacher in Patna wants her students to truly understand the difference between physical and chemical change. From the chapter's pedagogy, which sequence is the most effective for one classroom period?

• A. Read the In a Nutshell box aloud, then dictate definitions for students to copy.
• B. Show a chart of definitions and ask students to memorise it for the test.
• C. Predict what will happen in Activity 5.3, then perform the activity, then explain why lime water in tumbler B turns milky while tap water in tumbler A does not.
• D. Ask students to write essays on physical and chemical changes at home.

Q20. In Question 10, four set-ups (a–d) bubble a gas through lime water. (a) Vinegar + baking soda, (b) Lemon juice + vinegar, (c) Vinegar + common salt, (d) Lemon juice + baking soda. By the chapter's reasoning the lime water turns milky in

• A. (a) only — because vinegar alone produces CO2.
• B. (b) and (c) — because lemon juice is acidic.
• C. (c) and (d) — because common salt is needed.
• D. (a) and (d) — because both pair an edible acid with baking soda, which releases CO2.

Q21. Question 4(i) of the chapter reads: 'Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to ____, and this is a ____ change.' The correctly filled pair from the chapter is

• A. Erosion; physical
• B. Combustion; chemical
• C. Condensation; physical
• D. Rusting; chemical

Q22. While a candle is burning, the wax around the wick melts and a small pool of liquid wax forms at the top. According to of the chapter, this melting of wax is

• A. A chemical change, because heat from the flame creates a new substance.
• B. A chemical change, because the wax reacts with oxygen as it melts.
• C. Combustion of wax in the liquid state.
• D. A physical change, because solid wax becomes liquid wax with no new substance formed.

Q23. When liquid wax that has dripped from a burning candle cools on the side of the candle, it hardens back into solid wax. The chapter treats this solidification of wax as

• A. A chemical change, because heat has been lost.
• B. Combustion in reverse.
• C. A reversal of erosion.
• D. A physical change, because only the state of the wax changes back from liquid to solid.

Q24. The 'Know a Scientist' box says that in the nineteenth century, Michael Faraday delivered a series of lectures called Chemical History of a Candle. Faraday believed the candle was a perfect object to introduce scientific study because, through it, he could discuss

• A. The history of Indian chemistry only.
• B. The methods of preparing wax in factories.
• C. The biography of inventors of the candle.
• D. Differences between various physical and chemical processes like melting, vapourisation, and combustion.

Q25. Compared with most physical changes, the chapter says chemical changes are typically

• A. Faster but always reversible.
• B. Limited to a change of state only.
• C. Caused only by sunlight.
• D. Difficult to reverse back to the original substance, because new substances have been formed.

Q26. In Table 5.2 (Activity 5.8) the chapter answers 'No' to whether the original state can be brought back for which of the following changes?

• A. Melting of ice cubes only
• B. Boiling of water only
• C. Both melting of ice cubes and boiling of water
• D. Chopping of vegetables and making popcorn from corn

Q27. Consider the fine sand collecting on riverbeds and in lakes. This sand

• A. Is created chemically by river water reacting with itself.
• B. Falls from the sky as part of rainfall.
• C. Grows on the riverbed like a plant.
• D. Is rock pebbles, soil, and sediments that erosion has moved and that settle when the speed of the water decreases.

Q28. Asha confuses the ignition temperature of paper with its melting point. Using the chapter's definitions, the most precise distinction is that the ignition temperature is

• A. The temperature at which a solid becomes a liquid.
• B. The maximum temperature a substance can reach before cooling down.
• C. The temperature at which liquid water boils to steam.
• D. The minimum temperature at which the substance catches fire — relevant to combustion, not to a change of state.

Q29. A Class 7 student in Bhopal asks the teacher, 'If burning paper catches my book, what is the best first action?' Using the chapter's fire-triangle reasoning and the Science and Society box, which is the best response?

• A. Wait for the paper to burn out completely; the book may survive.
• B. Blow hard on the flame so it spreads to a smaller area.
• C. Pour kerosene on the book to confuse the fire.
• D. Smother the flame with a thick cotton cloth or a non-synthetic blanket to cut off the supply of oxygen, since removing any one leg of the fire triangle stops combustion.

Q30. Iron gates in a school are painted every year. From the chapter's account of rusting the most likely scientific reason for painting them is to

• A. Make the rust a darker brown.
• B. Increase the ignition temperature of iron.
• C. Convert the iron into a non-metal.
• D. Block air and moisture from reaching the iron surface, so the chemical reaction that forms iron oxide (rust) is prevented.