Summary
Chapter 5 of the Class 7 Science NCERT textbook, "Physical and Chemical Changes", explains that physical changes alter shape, size, colour, or state without forming any new substance and are generally reversible, while chemical changes produce one or more new substances and are called chemical reactions.
- Distinguishing the two kinds of change — A physical change touches only properties like shape, size and state, with no new material formed and easy reversal — melting ice is the classic case. A chemical change instead creates entirely new substances that cannot be simply undone.
- Spotting a chemical reaction — Certain clues signal that a chemical change has happened: heat or light given off, a gas released, a colour shift, or a new smell. Reactions like rusting, burning magnesium and mixing copper sulphate with iron all show these signs.
- Protecting metals and purifying crystals — Rusting needs both oxygen and water, so painting, greasing or galvanising with zinc guards iron against it. Crystallisation, used to obtain pure copper sulphate from solution, is grouped as a physical change rather than a chemical one.
Key points & formulas
- 01Physical changes affect physical properties (shape, size, colour, state); no new substance forms and the change is generally reversible.
- 02Chemical changes produce one or more new substances and are also called chemical reactions.
- 03Rusting requires both oxygen and water: Fe + O₂ + H₂O → iron oxide (Fe₂O₃); it is faster in humid and coastal areas due to moisture and salt water.
- 04Rusting is prevented by coating iron with paint, grease, chromium, or zinc; coating iron with zinc is called galvanisation.
- 05Burning magnesium ribbon produces magnesium oxide (MgO), which dissolves in water to form magnesium hydroxide [Mg(OH)₂], a base.
- 06Passing carbon dioxide through lime water turns it milky because calcium carbonate (CaCO₃) forms — the standard laboratory test for CO₂.
- 07Indicators of a chemical change include heat or light given off or absorbed, sound, a colour change, a new smell, or a gas being produced.
- 08Crystallisation — obtaining pure crystals (e.g., copper sulphate) from a solution by cooling — is a physical change.
Frequently asked questions
01What is a physical change?
A change in which a substance undergoes a change in its physical properties — such as shape, size, colour, or state — is called a physical change. No new substance is formed and the change is generally reversible.
02What is a chemical change?
A change in which one or more new substances are formed is called a chemical change. It is also called a chemical reaction.
03What conditions are essential for iron to rust?
The presence of both oxygen and water (or water vapour) is essential for rusting. If either is absent, rusting does not occur.
04Why does rusting happen faster in coastal areas than in deserts?
Coastal areas have higher humidity and salt water. Higher moisture content makes rusting faster, and salt water further accelerates rust formation.
05What is galvanisation?
Galvanisation is the process of depositing a layer of zinc on iron to prevent rusting. Iron pipes used in homes to carry water are galvanised.
06What are the signs that a chemical change has taken place?
A chemical change may be accompanied by heat, light, or other radiation being given off or absorbed; sound being produced; a change in smell or a new smell; a colour change; or a gas being formed.
07What new substances are formed when magnesium ribbon burns?
Magnesium reacts with oxygen to form magnesium oxide (MgO). When magnesium oxide is dissolved in water, it forms magnesium hydroxide [Mg(OH)₂], which is a base.
08What is the standard test for carbon dioxide?
Passing the gas through lime water [Ca(OH)₂] is the standard test for carbon dioxide. CO₂ reacts with lime water to form calcium carbonate (CaCO₃), turning the lime water milky.
09What happened when an iron nail was placed in copper sulphate solution?
The blue copper sulphate solution changed to green (iron sulphate formed) and a brown deposit of copper appeared on the nail. Two new substances — iron sulphate and copper — were formed, making it a chemical change.
10What type of change is crystallisation?
Crystallisation is a physical change. Pure substances can be obtained in crystal form from their solutions without forming any new substance.
11Why is burning of wood a chemical change while cutting wood is a physical change?
Burning wood produces new substances (ash, smoke, gases) and is not reversible — it is a chemical change. Cutting wood only changes the size and shape of the wood; no new substance forms and wood remains wood — so it is a physical change.
12What gas is produced when vinegar and baking soda are mixed?
Carbon dioxide gas is produced. Vinegar (acetic acid) reacts with baking soda (sodium hydrogencarbonate) to produce carbon dioxide and other substances. The CO₂ turns lime water milky.
13Is the NCERT PDF for Class 7 Science Chapter 5 free to download?
Yes — the NCERT PDF is free and requires no sign-up.
More chapters in Science
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