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Class 7 Science: Changes Around Us: Physical and Chemical, 8 practice questions with answers

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These are 8 practice questions on the fifth chapter of the new NCERT Class 7 Science textbook, Curiosity, called Changes Around Us: Physical and Chemical. Each has a short answer and the textbook page it comes from. They cover telling physical from chemical changes, the lime water test on soda water, the fire triangle, combustion and rusting compared, changes that can and cannot be reversed, changes with long-term effects, comparing two changes, and erosion. For heat in nature, see our Class 7 Heat Transfer in Nature guide. Try each question before you read the answer.

These questions are our own, drafted with AI help. Every fact was checked against the page it cites by a test script, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

What the chapter says you need

Facts from the NCERT textbook (Curiosity, Grade 7, Chapter 5, printed pages 57 to 72):

Physical or chemical?

Question 1. Is each a physical or a chemical change? (a) Bending a steel spoon. (b) Ice cream melting in the sun. (c) Sharpening a pencil. (d) Cooking an egg. (e) Lighting a gas stove.

Answer: physical: (a), (b) and (c); chemical: (d) and (e). In a physical change only shape, size or state change and no new substance is formed: (a) changes shape, (b) changes state, and (c) changes size and shape. In a chemical change a new substance is formed: the chapter counts cooking among chemical changes, and the gas burning in a stove is combustion, a reaction with oxygen that forms new substances and gives out heat and light. (Physical and chemical changes, pages 59 and 60; combustion, page 62; cooking, page 68.)

Question 2. When a bottle of soda water is opened, bubbles of gas rush out. (a) How could you show that this gas is carbon dioxide? (b) In your test, what makes the liquid turn milky, and is that a physical or a chemical change?

Answer: (a) lead the gas into freshly made lime water: if it turns milky, the gas is carbon dioxide; (b) a new, insoluble white substance, calcium carbonate, so it is a chemical change. (a) Turning lime water milky is the chapter's test for carbon dioxide, so the gas from the soda can be bubbled through a little fresh lime water, for example through a bent straw. (b) Lime water is calcium hydroxide in water. Carbon dioxide reacts with it to form calcium carbonate, a white substance that does not dissolve in water, along with water: calcium hydroxide + carbon dioxide → calcium carbonate + water. The undissolved white solid makes the liquid look milky. A new substance has formed, so the change in the lime water is a chemical change. (The lime water test, pages 60 and 61.)

Burning

Question 3. The chapter's "fire triangle" lists three things that combustion needs. Which one is missing or removed in each case? (a) A lid is placed on a pan whose oil has caught fire, and the flames die out. (b) Logs are pulled out of a campfire and it dies down. (c) A newspaper lies on a table for weeks in the air and never catches fire.

Answer: (a) oxygen; (b) fuel; (c) heat to reach the ignition temperature. Combustion needs (i) a combustible substance, the fuel, (ii) oxygen, and (iii) heat to bring the fuel to its ignition temperature. (a) The lid cuts off the supply of air, and so of oxygen, which burning needs. (b) Removing the logs takes away the fuel. (c) Paper is combustible and air is all around it, but it never reaches its ignition temperature, the lowest temperature at which it catches fire. (Oxygen is needed for combustion, page 63; ignition temperature and the fire triangle, page 64.)

Question 4. (a) What is a combustible substance? Name two of the chapter's examples other than wood. (b) The rusting of an iron gate and the burning of wood are both chemical changes. Give one way in which they are alike and one way in which they differ.

Answer: (a) a substance that can undergo combustion, such as paper, cotton or kerosene; (b) alike: each forms a new substance; different: burning gives out heat and light, while rusting creeps on slowly with no flame. (a) Combustion is a chemical reaction of a substance with oxygen that gives out heat and/or light; substances that can do this are combustible. The chapter's examples are wood, paper, cotton and kerosene. (b) Both are chemical changes because a new substance forms: rusting makes iron oxide, and burning makes new substances, such as carbon dioxide from the carbon in the fuel and oxygen from the air. They differ in how they look: burning is combustion, which gives out heat and light, while a gate rusts slowly over days and weeks with no flame at all (that last point is our everyday observation, not the chapter's words). (Rusting and combustion, page 62; carbon dioxide from burning, page 63.)

Reversible, desirable, slow

Question 5. Which of these changes can be reversed and which cannot? (a) Stretching a rubber band and letting go. (b) Bending a piece of soft wire and straightening it again. (c) Burning a matchstick. (d) Boiling an egg.

Answer: can be reversed: (a) and (b); cannot: (c) and (d). A change can be reversed if we can get back the object or substance we started with. (a) The band returns to its first size. (b) The wire can be bent back to its first shape; it is still the same wire. (c) and (d) form new substances, and we cannot get the unburnt matchstick or the raw egg back. (Reversible changes, page 66; burning and cooking as chemical changes, pages 62 and 68.)

Question 6. The chapter describes two changes caused by human activity that have long-term effects on the environment. (a) What does the growing use of fuel in vehicles do? (b) How does drying paint add to the problem? (c) Give one example from the chapter of a change that can be undesirable in one situation and useful in another.

Answer: (a) it raises the amount of carbon dioxide in the air; (b) it releases substances by evaporation that pollute the air; (c) the decay of food. (a) More fuel burnt in cars, trains and aeroplanes increases the carbon dioxide in the atmosphere. (b) As paint on walls, doors and furniture dries, substances evaporate from it and cause air pollution. (c) Food decaying in storage is undesirable, but the same breakdown is useful for turning food waste into compost. (Desirable and undesirable changes, pages 66 and 67.)

Question 7. Compare two changes: a puddle drying up on a sunny day, and an iron tawa left wet in the sink overnight that has brown spots on it in the morning. For each, say (a) whether it is physical or chemical, (b) whether it can be reversed, and (c) whether it is desirable.

Answer: the puddle: physical, can be reversed, neither good nor bad; the tawa: chemical, cannot be reversed, undesirable. (a) Water evaporating only changes its state, so drying is a physical change. The brown spots on the tawa are rust, iron oxide, a new substance, so rusting is a chemical change. (b) Water vapour can be condensed back into liquid water, so the drying can be reversed; the rusted iron cannot simply be changed back. (c) The chapter gives the rusting of iron as an example of an undesirable change. A puddle drying up is just one of the many everyday changes around us. (Physical changes, page 59; rusting, page 62; reversing changes, page 66; undesirable changes, page 67.)

Question 8. (a) Why are the pebbles in a fast-flowing stream smooth and rounded? (b) What happens to the sand and soil the water carries when it reaches a still lake?

Answer: (a) the flowing water keeps wearing them down; (b) it settles to the bottom and, over a very long time, can harden into new rock. (a) Rocks and pebbles in rivers look smoother because the flowing water erodes them all the time. (b) Wind and flowing water carry broken rock, soil and sediments from place to place; this is erosion. When the water slows down, as in a lake or the ocean, the material settles, and these sediments harden over time into new rock. Most such changes take thousands of years and cannot be reversed. (Erosion, page 68.)

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Sources

All opened and read on 2 November 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 56 for this chapter.

The facts are from the textbook pages above. The questions, answers and checks are our own. This guide is general information; follow your teacher and the textbook.