NCERT Curiosity · Class 7 · Science
Changes Around Us: Physical and Chemical
An ice cube becomes water. A rosebud becomes a flower. A banana turns brown and smelly. What kinds of changes are these?
Key idea
Physical Changes
Folding paper, inflating a balloon, crushing chalk into powder — the material stays the same substance even though its shape or size changes.
A physical change alters only physical properties — shape, size, or state — with no new substance formed.
Key idea
Chemical Changes
Blowing breath into plain water causes no changebut blowing into lime water turns it milky.
A new substance formsCalcium hydroxide (lime water) + Carbon dioxide → Calcium carbonate (insoluble, milky) + Water.
Key idea
Vinegar and Baking Soda
Bubbles = a new gas. Mixing vinegar or lemon juice with baking soda produces fizzing — carbon dioxide gas escapes, confirmed by turning lime water milky.
Key idea
Combustion and the Fire Triangle
Combustion happens when a substance reacts with oxygen, releasing heat and often light.
Key idea
Ignition Temperature
1
What it means
The minimum temperature at which a substance catches fire.
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2
Seeing it happen
Focusing sunlight through a magnifying glass onto paper heats a small spot until it smokes and ignites.
Key idea
Burning a Candle: Both at Once
1
Physical changes
Solid wax melting, flowing up the wick, and evaporating.
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2
Chemical change
The burning of wax vapour itself, producing the flame — a genuinely new process, happening at the same time as the physical ones.
Key idea
Reversible and Irreversible Changes
1
Reversible
The original substance or object can be recovered — melted ice can be refrozen, water vapour can be condensed back.
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2
Irreversible
The original form cannot be recovered — chopped vegetables, or popcorn made from corn.
Key idea
Desirable and Undesirable Changes
1
Desirable
Milk turning to curd, fruit ripening, cooking food.
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2
Undesirable
Rusting of iron, food decay — though the same kind of change (decomposition) can be desirable elsewhere, like in a compost pit.
Key idea
Weathering and Erosion
1
Weathering
Temperature swings, root growth, and freezing water (physical) — plus reactions with water and air (chemical) — slowly break rocks down into soil.
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2
Erosion
Wind and flowing water carry away broken rock, soil, and sediment — a physical change that reshapes the land over time.
Chapter · Key terms to remember
Key Terms
Physical change
Shape, size, or state changes; no new substance.
Chemical change
One or more new substances form.
Combustion
Reacting with oxygen, releasing heat and/or light.
Chapter · Key terms to remember
More Key Terms
Ignition temperature
The lowest temperature a substance catches fire at.
Reversible change
The original substance can be recovered.
Irreversible change
The original form cannot be recovered.
Chapter · Key terms to remember
More Key Terms
Weathering
Rocks broken down by physical and chemical action.
Erosion
Broken-down material carried away by wind or water.
Questions for your notebook
Write these down, then discuss
1Which statements describe a physical change? (i) The state may or may not change. (ii) A substance with different properties forms. (iii) No new substance is formed. (iv) The substance undergoes a chemical reaction.
2State True or False, correcting the false ones: (i) Melting wax is necessary for a candle to burn. (ii) Condensing water vapour is a chemical change. (iii) Composting leaves is a chemical change. (iv) Mixing baking soda with lemon juice is a chemical change.
3Is water changing to ice, and water changing to steam, a physical or chemical change? Explain.
Project as-is — students copy the questions, then the class discusses answers together.