🧪 Chemistry · Topic 3 of 10
Start with the study notes on Physical & Chemical Changes (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
A physical change alters only the shape, size, state or appearance of a substance. No new substance is formed. Most physical changes can be reversed.
Examples:
Remember: Not every physical change is easy to undo. A cut sheet of paper or a broken glass stays broken, but no new substance has formed, so it is still a physical change.
A chemical change (or chemical reaction) produces one or more new substances with different properties. Most chemical changes cannot be reversed easily.
Examples:
A chemical change is often shown by one or more of these:
Remember: The real proof of a chemical change is that a new substance has formed. The signs above are clues.
Magnesium ribbon burns with a dazzling white light, leaving white magnesium oxide powder. Dissolved in water, this turns red litmus blue, so it is basic.
Put an iron nail in blue copper sulphate solution. The solution turns green and a brown coating of copper forms on the nail.
Iron is more reactive, so it pushes copper out of its compound. This is a displacement reaction.
Carbon dioxide turns lime water milky, because insoluble calcium carbonate forms. This is the standard test for carbon dioxide.
Example: Vinegar and baking soda fizz and give carbon dioxide, which turns lime water milky.
A burning candle shows both kinds of change: the wax melts (physical), and the wax vapour burns to form new substances (chemical).
In a chemical equation, the starting substances on the left are reactants, and the new substances on the right are products.
Rusting is a chemical change in which iron reacts with oxygen and water to form rust (hydrated iron oxide), a brown, flaky substance.
Example: Iron bridges are painted regularly, and roofing sheets are galvanised.
Examples:
Crystallisation is a physical change: the same substance simply comes out of solution as crystals. Cooling a hot saturated solution slowly and without disturbance gives large, well-shaped crystals.
Example: Growing crystals of copper sulphate from its hot solution.
| Misconception | Correct understanding |
|---|---|
| Every change that can't be undone is chemical. | Cutting paper can't be undone, but no new substance forms, so it is physical. |
| Dissolving salt makes a new substance. | The salt is unchanged; evaporating the water gets it back. |
| A burning candle shows only one kind of change. | Melting wax is physical; burning wax vapour is chemical. |
| Iron rusts in dry air. | Rusting needs both oxygen and moisture. |
| Colour change always means a chemical change. | It is a clue; the real test is whether a new substance formed. |
Is melting ice a physical or a chemical change? Why?
Physical: it is still water, only its state has changed.
Why is burning paper a chemical change?
New substances such as ash and gases form, and the paper cannot be got back.
What two things does iron need to rust?
Oxygen (air) and water (moisture).
What happens when an iron nail is left in copper sulphate solution?
The blue solution turns green and brown copper coats the nail: a displacement reaction.
What is the test for carbon dioxide?
It turns lime water milky.
Why does a candle show both kinds of change?
The wax melts (physical) and its vapour burns into new substances (chemical).
How does galvanising protect iron?
The zinc coating corrodes first, protecting the iron underneath.
Is souring of milk physical or chemical?
Chemical: bacteria change milk sugar into lactic acid, a new substance.
Key takeaway: Ask one question: has a new substance formed? If not, it is a physical change. If yes, it is a chemical change.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
A change in which no new substance is formed is called a:
Answer: A — Physical change
In a physical change only the physical properties - shape, size, state or appearance - change. The substance itself remains the same, and no new substance is formed.
Q2. easy
A change in which one or more NEW substances are formed is called a:
Answer: A — Chemical change
A chemical change (or chemical reaction) produces one or more entirely new substances with different properties from the original ones.
Q3. medium
Melting of ice into water is an example of a:
Answer: A — Physical change - only the state changes
Ice and water are the same substance in different states. Cooling the water freezes it back into ice, so the change is physical and reversible.
Q4. medium
Burning of paper is a chemical change because:
Answer: A — New substances (ash, smoke, carbon dioxide) are formed and cannot be turned back into paper
Burning produces new substances with completely different properties - ash, gases and heat. The paper cannot be recovered, so it is a chemical (and irreversible) change.
Q5. medium
Most physical changes are:
Answer: A — Reversible - the original substance can usually be got back
Because the substance itself is unchanged, physical changes such as melting, freezing, boiling and dissolving can usually be reversed.
Q6. medium
Most chemical changes are:
Answer: A — Irreversible - the original substances cannot be got back easily
In a chemical change, new substances with new properties are formed. Reversing them is difficult or impossible by simple means - you cannot un-burn wood or un-cook an egg.
Q10. medium
Rusting of iron requires the presence of:
Answer: A — Both oxygen (air) and water (moisture)
Iron rusts only when both air and moisture are present. In dry air or in water free of dissolved oxygen, iron does not rust - which is why humid weather speeds up rusting.
Q13. medium
Stainless steel does not rust because it contains:
Answer: A — Chromium and nickel mixed with iron
Stainless steel is an alloy of iron with chromium and nickel. The chromium forms a thin protective oxide layer on the surface that stops further corrosion.
Q14. medium
Rusting occurs faster in coastal areas because:
Answer: A — The air is more humid and contains salt, which speeds up rusting
Sea air carries both high moisture and dissolved salts. Salt makes the water a better conductor, which accelerates the electrochemical process of rusting.
Q15. medium
Which change is BOTH physical and reversible?
Answer: A — Boiling of water into steam
Steam can be cooled and condensed back into water - the substance is unchanged. The other three form new substances and are chemical changes.
Q17. medium
Souring of milk (formation of curd) is a chemical change because:
Answer: A — Bacteria convert lactose into lactic acid, a new substance
Lactobacillus bacteria convert the milk sugar into lactic acid, which curdles the milk protein. Curd is a new substance with different taste and properties, and cannot be turned back into milk.
Q20. medium
When copper sulphate solution reacts with an iron nail, the blue colour fades and a brown coating appears on the nail. This is because:
Fe + CuSO4 → FeSO4 + CuAnswer: A — Iron displaces copper, forming iron sulphate and copper metal
Iron is more reactive than copper, so it displaces copper from copper sulphate. Green iron sulphate forms in solution and reddish-brown copper deposits on the nail - a displacement reaction.
Q23. medium
An insoluble solid formed when two solutions react is called a:
Answer: A — Precipitate
A precipitate is an insoluble solid that separates out when two solutions react. Its appearance is a clear indication that a chemical change has taken place.
Q25. medium
Which of these changes gives OFF heat and is therefore called exothermic?
Answer: A — Burning of a fuel
Exothermic changes release heat. Burning fuels, respiration and the reaction of quicklime with water release heat. Melting, evaporation and some dissolving processes absorb heat (endothermic).
Q27. medium
Photosynthesis in plants is a chemical change because:
Answer: A — Carbon dioxide and water are converted into glucose and oxygen - new substances
Plants use sunlight and chlorophyll to convert carbon dioxide and water into glucose and oxygen. Completely new substances are formed, so it is a chemical change (endothermic - it absorbs light energy).
Q28. medium
Digestion of food in the body is:
Answer: A — A chemical change - food is broken into simpler new substances
Enzymes break down complex food molecules into simpler ones like glucose and amino acids. New substances are formed and the process cannot be reversed, so digestion is a chemical change.
Q30. medium
Cutting a piece of paper into small bits is a physical change because:
Answer: A — It is still paper - only the size and shape have changed
The bits are still paper with the same chemical composition. Only the size and shape changed, so it is a physical change (even though you cannot easily rejoin the bits).
Q32. medium
Dissolving common salt in water is a physical change because:
Answer: A — The salt can be recovered unchanged by evaporating the water
The salt particles simply spread among the water particles. Evaporating the water leaves the same salt behind, showing that no new substance was formed.
Q34. medium
Which change is used in the kitchen and is a chemical change?
Answer: A — Baking a cake
Baking involves reactions between ingredients that produce entirely new substances - the batter cannot be recovered. Chopping, melting and grinding change only physical form.
Q36. medium
To grow large crystals of copper sulphate, the hot saturated solution should be:
Answer: A — Cooled slowly without disturbance
Slow, undisturbed cooling gives the particles time to arrange themselves into large, well-shaped crystals. Rapid cooling or stirring produces many small crystals instead.
Q39. medium
Slaked lime is made by adding water to quicklime. This reaction:
CaO + H2O → Ca(OH)2 + heatAnswer: A — Releases a large amount of heat (exothermic)
Quicklime (CaO) reacts vigorously with water to form slaked lime, Ca(OH)2, releasing so much heat that the water may hiss and boil. It is a strongly exothermic combination reaction.
Q41. medium
Which of these is an example of a change that is chemical AND useful to us?
Answer: A — Fermentation of dough to make bread rise
Yeast ferments the sugars in dough, releasing carbon dioxide that makes the bread rise, along with new substances that give flavour. This is a useful chemical change.
Q43. medium
Which of these is NOT a chemical change?
Answer: A — Condensation of steam into water
Condensation is only a change of state - steam and water are the same substance. Burning, rusting and ripening all form new substances.
Q45. medium
A change in which energy is released as heat AND light is:
Answer: A — Burning (combustion)
Combustion is an exothermic chemical change that releases both heat and light - as seen when a candle, wood or magnesium ribbon burns.
Q47. medium
Which observation would BEST convince you that a chemical change has occurred?
Answer: A — A new substance with different properties has formed
The defining test is the formation of a new substance. Other clues - heat, gas, colour change, precipitate, smell - usually accompany chemical changes but the new substance is the real proof.
Q49. medium
The setting of cement and concrete after adding water is:
Answer: A — A chemical change - new hard compounds form and it cannot be reversed
Water reacts chemically with the cement compounds to form new interlocking crystals that harden. Set concrete cannot be turned back into cement powder and water.
Q51. medium
The formation of clouds from water vapour is:
Answer: A — A physical change (condensation)
Water vapour cools and condenses into tiny water droplets that form clouds. It is still water, so only the state has changed - a physical change.
Q53. medium
In a chemical equation, the substances on the LEFT side are called:
Answer: A — Reactants
Reactants are written on the left of the arrow and products on the right. The arrow shows the direction in which the change takes place.
Q55. medium
Silver ornaments turn black over time because silver reacts with:
Answer: A — Sulphur compounds in the air, forming black silver sulphide
Silver reacts with traces of hydrogen sulphide in the air to form a black coating of silver sulphide - a chemical change called tarnishing.
Q57. medium
Which pair correctly matches the change with its type?
Answer: A — Freezing of water - physical; Burning of wood - chemical
Freezing changes only the state of water, so it is physical and reversible. Burning wood forms ash, smoke and gases - new substances - so it is chemical and irreversible.
Q59. medium
Which overall statement about physical and chemical changes is CORRECT?
Answer: A — Physical changes alter form only and are usually reversible; chemical changes form new substances and are usually irreversible
This sums up the topic: in a physical change the substance stays the same and only its form or state alters; in a chemical change new substances with new properties are produced, often with heat, light, gas, colour change or a precipitate.
Q7. hard
Which of the following is NOT a sign of a chemical change?
Answer: A — A change in the shape or size of the substance
Changing shape or size alters only appearance - the substance stays the same, so it is physical. Gas evolution, precipitate formation, colour change, heat/light and smell are signs of chemical change.
Q8. hard
Which of these is a chemical change?
Answer: A — Rusting of iron
Rusting forms a new substance - hydrated iron oxide - which is brown, flaky and different from iron. The other three are physical changes.
Q9. hard
Rust is chemically:
Answer: A — Hydrated iron(III) oxide
Rust is hydrated iron oxide, formed when iron reacts with oxygen and moisture in the air. Its formula is often written as Fe2O3·xH2O.
Q11. hard
Which of these does NOT help prevent rusting?
Answer: A — Keeping the iron object in a damp place
Dampness speeds rusting. Painting, greasing, galvanising (a zinc coating) and chromium plating all keep air and moisture away from the iron surface and so prevent rust.
Q12. hard
Galvanisation is the process of coating iron with:
Answer: A — Zinc
Galvanisation deposits a layer of zinc on iron or steel. Zinc is more reactive than iron, so it corrodes in place of the iron, protecting it even if the coating is scratched.
Q16. hard
A burning candle shows BOTH a physical and a chemical change because:
Answer: A — The wax melts (physical) and the wax vapour burns to form new substances (chemical)
The heat first melts the solid wax and turns it into vapour - a physical change. That vapour then burns in air, producing carbon dioxide, water vapour, heat and light - a chemical change.
Q18. hard
When magnesium ribbon burns in air, it produces:
2 Mg + O2 → 2 MgO (white powder)Answer: A — A dazzling white light and a white powder (magnesium oxide)
Magnesium burns with a brilliant white flame to form white magnesium oxide powder: 2Mg + O2 → 2MgO. This is a classic chemical change.
Q19. hard
When magnesium oxide powder is dissolved in water, the solution turns red litmus blue. This shows the solution is:
MgO + H2O → Mg(OH)2 (a base)Answer: A — Basic (alkaline)
Magnesium oxide dissolves in water to form magnesium hydroxide, a base. Bases turn red litmus blue. Metal oxides are generally basic in nature.
Q21. hard
The reaction between iron and copper sulphate is called a:
Answer: A — Displacement reaction
A more reactive metal displaces a less reactive one from its salt solution. Here iron displaces copper, so it is a displacement (or substitution) reaction.
Q22. hard
When carbon dioxide gas is passed through lime water, the lime water:
Ca(OH)2 + CO2 → CaCO3 (white, insoluble) + H2OAnswer: A — Turns milky due to formation of insoluble calcium carbonate
CO2 reacts with calcium hydroxide (lime water) to form insoluble calcium carbonate, which makes the solution milky. This is the standard laboratory test for carbon dioxide.
Q24. hard
When barium chloride solution is mixed with sodium sulphate solution, a white precipitate of barium sulphate forms. This reaction is:
BaCl2 + Na2SO4 → BaSO4 (white ppt) + 2 NaClAnswer: A — A chemical change involving formation of a precipitate
Two new substances are formed - insoluble white barium sulphate and soluble sodium chloride. A precipitate forming is a sure sign of a chemical change.
Q26. hard
A change that ABSORBS heat from the surroundings is called:
Answer: A — Endothermic
In an endothermic change, heat is taken in, so the surroundings feel cooler. Melting ice, evaporating water and dissolving ammonium chloride in water are examples.
Q29. hard
Which of these lists contains ONLY physical changes?
Answer: A — Melting of ice, breaking of a glass, dissolving salt in water
Melting, breaking and dissolving change only appearance or state - no new substance forms. Burning, rusting, cooking, souring and digestion all form new substances and are chemical.
Q31. hard
Which statement about physical changes is TRUE?
Answer: A — Not all physical changes are easily reversible - for example, cutting paper
Reversibility is a useful clue but not a strict rule. Cutting paper or breaking a glass is physical but not easily undone. The real test is whether a new substance is formed.
Q33. hard
When baking soda is added to vinegar, bubbling occurs. This shows a chemical change because:
NaHCO3 + CH3COOH → CH3COONa + H2O + CO2↑Answer: A — A gas (carbon dioxide) is produced - a new substance
Vinegar (acetic acid) reacts with baking soda (sodium bicarbonate) to release carbon dioxide gas, along with water and sodium acetate. The evolution of a gas signals a chemical change.
Q35. hard
Crystallisation of copper sulphate from its solution is classified as:
Answer: A — A physical change - the same substance forms crystals
The dissolved copper sulphate simply comes out of solution as crystals. No new substance is formed, so crystallisation is a physical change - and a useful purification method.
Q37. hard
Which of these is a COMBINATION reaction?
Answer: A — Two or more substances combine to form a single new substance
In a combination (synthesis) reaction, reactants join to form one product - for example, magnesium plus oxygen giving magnesium oxide, or quicklime plus water giving slaked lime.
Q38. hard
Which of these is a DECOMPOSITION reaction?
CaCO3 --heat--> CaO + CO2Answer: A — Heating calcium carbonate to give calcium oxide and carbon dioxide
In decomposition, a single compound splits into two or more simpler substances, usually on heating or with electricity. Limestone breaking into quicklime and carbon dioxide is a classic example.
Q40. hard
Which statement correctly distinguishes physical from chemical change?
Answer: A — In a physical change composition stays the same; in a chemical change composition changes
The key difference is composition. A physical change alters only the form or state; a chemical change alters the actual substances present, producing new ones.
Q42. hard
Galvanised iron sheets do not rust even if scratched because:
Answer: A — Zinc is more reactive and corrodes first, protecting the iron
Zinc is above iron in the reactivity series, so it corrodes preferentially, sacrificing itself to protect the iron. This is called sacrificial protection and works even where the coating is broken.
Q44. hard
Ripening of fruit is a chemical change because:
Answer: A — Starches change into sugars and new compounds form, altering taste and colour
During ripening, enzymes convert starch into sugars and break down pigments and acids, producing new substances that change taste, colour and smell. An unripe fruit cannot be got back.
Q46. hard
In the reaction Zn + H2SO4 → ZnSO4 + H2, the gas evolved can be tested by:
Answer: A — Bringing a burning splint near it - it burns with a 'pop' sound
Hydrogen gas burns with a characteristic 'pop' sound when a lighted splint is brought near it. This is the standard laboratory test for hydrogen.
Q48. hard
Which of these changes is reversible even though it involves a change of state?
Answer: A — Sublimation of camphor and its re-condensation
Camphor sublimes into vapour and can be re-deposited as solid camphor on a cool surface - the same substance throughout, so it is a reversible physical change. Burning camphor is chemical.
Q50. hard
Which change is used to obtain crystals of a pure substance from an impure solid?
Answer: A — Dissolving, filtering and crystallising - all physical changes
The impure solid is dissolved, the insoluble impurities are filtered out, and the hot saturated solution is cooled so pure crystals form. Every step is a physical change, so the substance stays chemically the same.
Q52. hard
Why is a chemical change also called a chemical REACTION?
Answer: A — Because substances react with one another to produce new substances
In a chemical change the starting substances (reactants) interact and rearrange to form products with new properties. This interaction is what we call a reaction.
Q54. hard
Corrosion is best described as:
Answer: A — The slow eating away of a metal by air, moisture or chemicals
Corrosion is the gradual chemical attack on a metal surface by its environment. Rusting of iron is the most familiar example; silver tarnishing and copper turning green are others.
Q56. hard
Copper vessels develop a green coating because copper reacts with:
Answer: A — Moist air containing carbon dioxide, forming basic copper carbonate
Copper reacts slowly with moisture, oxygen and carbon dioxide in air to form a green layer of basic copper carbonate. This is why copper vessels are cleaned with tamarind or lemon before use.
Q58. hard
Which of the following would you use to slow down rusting of an iron bridge?
Answer: A — Regular painting of the exposed surfaces
Paint forms a barrier that keeps air and moisture off the iron. Salt water, humidity and scratches all expose more iron and speed up rusting.
Q60. hard
Ozone in the upper atmosphere breaks down into oxygen while absorbing harmful ultraviolet rays. This process is:
Answer: A — A chemical change - one substance changes into a different substance
Ozone (O3) and oxygen (O2) are different substances with different properties, so the conversion is a chemical change. It is also useful: it shields the Earth by absorbing ultraviolet radiation.