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Ice melts, milk turns to curd, a nail rusts, paper is torn. Class 7 science asks you to sort changes like these, and students who rely on "can it be reversed?" get a surprising number wrong. Chapter 5 of the new NCERT book uses a better test. This guide explains it, works through examples, and shows where the reversibility shortcut fails.
What does Chapter 5 of Class 7 Curiosity cover?
Changes Around Us: Physical and Chemical is Chapter 5 of NCERT's *Curiosity* for Grade 7 (first edition April 2025, reprint 2026-27). It covers:
Physical changes: folding paper, letting air out of a balloon, crushing chalk, changes of state.
Chemical changes: lime water turning milky, vinegar and baking soda, rusting, burning magnesium.
Combustion, the need for oxygen, ignition temperature and the fire triangle.
A candle: physical and chemical changes in one process.
Reversible and irreversible changes, and desirable and undesirable ones.
Slow natural changes: weathering of rocks and erosion.
How do you tell a physical change from a chemical change?
Ask one question: is a new substance formed?
If no, and only physical properties like shape, size or state change, it is a physical change.
If yes, it is a chemical change, and it happens through a chemical reaction that can be written as an equation.
Signs that a new substance may have formed include a new colour, gas bubbles, a new smell, a solid appearing in a liquid, or heat and light being given out. They are clues, not proof, so always name the new substance if you can.
Worked examples
Example 1: breathing into lime water. Tumbler A has tap water, tumbler B has lime water. Blow into each through a straw. A just bubbles. B turns milky, and later a white solid settles. New substance? Yes: carbon dioxide from your breath reacts with lime water to form calcium carbonate, which does not dissolve, plus a little water. Chemical change. This is also the chapter's test for carbon dioxide.
Example 2: sorting six changes. Use the two questions, new substance and reversible, for each:
Melting ice: no new substance, reversible. Physical.
Tearing paper: no new substance, not reversible. Physical.
Burning magnesium ribbon: new substance (magnesium oxide, a white powder), not reversible. Chemical.
Rusting of iron: new substance (iron oxide), not reversible. Chemical.
Vinegar and baking soda: new substance (carbon dioxide gas), not reversible. Chemical.
Boiling water: no new substance, reversible by condensing the steam. Physical.
Notice items 2 and 3. Both cannot be reversed, but only one is chemical. That is why the new-substance question comes first.
Example 3: a burning candle. The wax melts, rises up the wick and turns to vapour; these are physical changes. The vapour burns, forming new substances including carbon dioxide; that is a chemical change. So a candle shows both in one process.
Example 4: the fire triangle. Combustion needs three things: a fuel, oxygen and enough heat to bring the fuel to its ignition temperature. A candle under a glass tumbler goes out because its oxygen supply stops. Wrapping a person whose clothes are on fire in a blanket (not a synthetic one) works for the same reason.
Why do students find this topic hard?
Because the idea of a "new substance" is harder than it looks. Ahtee and Varjola (1998) asked students from 7th grade to university to describe a chemical reaction. Only 14% of 7th graders gave a response with at least one sign of sound understanding, such as atoms being reorganised or properties changing, and even among first-year university chemistry students the figure was 59%. Students also had trouble using the words substance and atom. So if this feels slippery, you are in good company.
The mistakes to watch for:
"If you cannot reverse it, it is chemical." Tearing paper and chopping vegetables are physical.
"Bubbles always mean a chemical change." Blowing air into water makes bubbles with no new substance.
"Dissolving sugar is chemical because the sugar disappears." No new substance forms; the sugar is still there, which you can tell by the taste. The chapter even asks whether you are sure, which is a good habit.
"Burning a candle is only chemical." Melting and evaporating wax are physical.
"All chemical changes are bad." Curd, cooking and compost are desirable chemical changes.
How do weathering and erosion fit in?
Weathering is the breaking down of rock by physical changes, like temperature changes, roots and freezing water in cracks, and by chemical changes, like the iron in black basalt rock forming red iron oxide. Erosion, rock and soil carried away by wind and water, is a physical change. Both are slow and take place over thousands of years.
A one-week practice plan
Day 1: List ten changes in your kitchen and apply the two questions to each.
Day 2: With an adult, mix vinegar and baking soda and pass the gas into lime water, as in the chapter.
Day 3: Draw the fire triangle and write one way to remove each side.
Day 4: Answer the chapter questions from memory, then check.
Day 5: Explain to a parent why a candle shows both kinds of change.
The lime water and baking soda reactions connect to our guide on acidic, basic and neutral substances in Class 7, and in Class 10 you will write these as balanced equations, as our post on chemical reactions and equations shows. If "new substance" still feels vague, the Fermi Concept Library has every concept in chemistry with its own lesson.
This evening, pick five changes you see at home and decide, for each, whether a new substance formed.
