ChemistryICSE Class 8 · Physical and Chemical Changes

Signs of a Chemical Change and Classifying Everyday Changes (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

The signs of a chemical change are a change of colour, a gas given off, a precipitate, heat, light or sound produced or absorbed, and a change of smell or taste. No single sign is proof: always ask whether a new substance has been formed.

Understand Signs of a Chemical Change and Classifying Everyday Changes

Five clues that a chemical change has taken place.
1. Change in colour: blue copper sulphate solution turns green when iron is added; $F e + C u S O_{4} \to F e S O_{4} + C u$ .
2. Gas given off: baking soda with vinegar fizzes (carbon dioxide).
3. Precipitate formed: a white solid appears when barium chloride and sodium sulphate solutions mix; $B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$ .
4. Heat, light or sound produced or absorbed: burning, or quicklime reacting with water.
5. Change in smell or taste: milk turning sour, food rotting.

One clue alone is not proof. Boiling water gives off a gas (steam) but no new substance forms. Always ask: is there a new substance?

Classifying everyday changes.

Change	Type	Reason
Melting of butter	physical	only state changes
Curdling of milk	chemical	new substance (curd), sour taste
Rusting of iron	chemical	new brown substance
Folding paper	physical	only shape changes
Digestion of food	chemical	food broken into new substances
Formation of clouds	physical	water vapour condenses
Cooking an egg	chemical	cannot be uncooked
Glowing of a bulb filament	physical	stops glowing when switched off

Same material, two changes. A burning candle shows melting (physical) and burning (chemical).

Energy and the law. Energy (heat, light) is often released in chemical changes, but the total mass is conserved.

Signs of a Chemical Change and Classifying Everyday Changes: formulas and key facts

Signs of a chemical change: Colour change
Gas given off
Precipitate
Heat, light or sound
Smell or taste change; Need a new substance.
Iron and copper sulphate: $F e + C u S O_{4} \to F e S O_{4} + C u$; Blue to green; brown copper.
Barium sulphate: $B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$; White precipitate.

How to solve Signs of a Chemical Change and Classifying Everyday Changes questions

Collect the observations, then ask whether they point to a new substance.

1List what is seen: colour, gas, solid, heat or light, smell.
2Ask for each: could it be a physical change (for example boiling gives gas)?
3Decide whether a new substance has formed.
4Write the reason in a sentence: "because ...".

Signs of a Chemical Change and Classifying Everyday Changes in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Fizzing soda

Suppose a cook adds baking soda to vinegar and a lot of bubbles form.

Is it chemical? What evidence?

Try it first, then show the solutionSolution

A gas is given off, which is carbon dioxide.
A new substance forms, so it is chemical.

Answer: Chemical; gas given off.

Memory hook: Fizz means new gas.

Souring of milk

Imagine milk is left out on a hot day and smells and tastes sour.

Classify with a reason.

Try it first, then show the solutionSolution

A change of smell and taste occurred.
New substances formed (curd, acid): chemical change.

Answer: Chemical; new substances with a sour smell and taste.

Memory hook: New smell, new substance.

Tricks to remember Signs of a Chemical Change and Classifying Everyday Changes

Short memory aids. Say each one aloud once, then use it on a question the same day.

CGP-HS
Colour, Gas, Precipitate, Heat or light, Smell: five clues.
Use it when: Signs.
Clue, then test
A clue is not enough: ask if a new substance is formed.
Use it when: Trap questions.
Can you undo?
Cannot-uncook is chemical; melts-and-resets is physical.
Use it when: Quick check.

Signs of a Chemical Change and Classifying Everyday Changes: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: A sign of chemical change · 1 mark · MCQBoard level

Which of the following alone proves a chemical change?
(a) a gas is given off
(b) a new substance is formed
(c) the shape changes
(d) the substance becomes hot

Try it first, then show the solutionSolution

A gas or a temperature change can occur in a physical change; only a new substance proves a chemical change.

Answer: (b)

Example 2: Name the sign · 2 marks · Short answerBoard level

Name the sign of a chemical change in each: (a) a white solid appears when two clear solutions mix, (b) bubbles rise when a white powder meets vinegar, (c) a bright flame appears when magnesium is lit, (d) a pleasant smell turns foul in stored food.

Try it first, then show the solutionSolution

(a) Precipitate formed; (b) gas given off. (1)
(c) Light (and heat) given out; (d) change in smell. (1)

Answer: (a) precipitate (b) gas (c) light and heat (d) smell

Example 3: Why not enough? · 3 marks · Short answerHarder board level

Water boils vigorously and gives off a gas. A bulb filament glows and gets hot. (a) Are these chemical changes? (b) Explain why one clue is not enough. (c) State what must be present in a chemical change.

Try it first, then show the solutionSolution

(a) No, both are physical. (1)
(b) A gas or heat/light can occur without a new substance (steam is water; the filament stays tungsten and stops glowing when off). (1)
(c) A new substance with new properties. (1)

Answer: (a) no (b) no new substance (c) a new substance

Example 4: Classify the changes · 4 marks · Long answerHarder board level

Classify as physical or chemical, with a reason: (a) melting of butter, (b) digestion of food, (c) folding paper, (d) formation of clouds.

Try it first, then show the solutionSolution

(a) Physical: only state changes. (1)
(b) Chemical: food is broken into new substances. (1)
(c) Physical: only shape changes. (1)
(d) Physical: water vapour condenses. (1)

Answer: (a) physical (b) chemical (c) physical (d) physical

Example 5: The nail experiment · 4 marks · Case-basedHOTS / competency

An iron nail is kept in blue copper sulphate solution for half an hour. The nail gets a brown coating and the solution turns pale green. (a) Name two signs of a chemical change here. (b) Name the brown coating. (c) Write the equation. (d) After, a student weighs everything; what happens to the total mass?

Try it first, then show the solutionSolution

(a) Colour change of the solution and formation of a new solid on the nail. (1)
(b) Copper. (1)
(c) $F e + C u S O_{4} \to F e S O_{4} + C u$ . (1)
(d) It stays the same (the whole system is weighed; conservation of mass). (1)

Answer: (a) colour change, deposit (b) copper (c) as above (d) unchanged

Where students lose marks in Signs of a Chemical Change and Classifying Everyday Changes

The slips examiners see most often, and what to write instead.

Mistake: Treating one clue as proof
A gas or heat can appear in a physical change.
Wrong working
"A gas is given off, so it is chemical."
Right working
"A new substance (CO₂) forms, so it is chemical."
Do this: Always name the new substance.
Mistake: No reason given
Marks are for the reason.
Wrong working
"Rusting is chemical."
Right working
"Rusting is chemical because a new brown substance forms."
Do this: Add "because".
Mistake: Mixing sublimation and burning
Camphor subliming is physical.
Wrong working
"Camphor vanishing is chemical."
Right working
"Camphor sublimes: physical."
Do this: Check the substance after the change.

Exam tips

Give a reason with every classification.
Name the new substance in chemical-change answers.
Use equations for iron/copper sulphate and barium sulphate.
Quote the five signs.

Practice questions on Signs of a Chemical Change and Classifying Everyday Changes

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
What change is seen when iron is added to blue copper sulphate solution?
Hint
Colour.
Answer
The blue colour turns green and a brown deposit of copper forms.
Q2 · 2 marks · Short answerHarder board level
Why is cooking an egg a chemical change but boiling water a physical change?
Hint
New substance and reversibility.
Answer
Cooking an egg forms new substances and cannot be reversed; boiling water gives steam, which is still water and can condense back.
Q3 · 3 marks · Short answerHarder board level
A student heats a white powder; it fizzes and gives a gas that turns lime water milky, leaving a different white solid. Classify with reasons.
Hint
Gas, new solid.
Answer
Chemical change: a gas (carbon dioxide) is given off and a different solid is left, so new substances formed.
Q4 · 3 marks · Short answerHarder board level
Give two examples where heat and light are given out in a chemical change, and one example where heat is taken in.
Hint
Burning, cooking.
Answer
Burning of magnesium (bright light) and burning of a candle give heat and light. Cooking needs heat to be taken in.
Q5 · 4 marks · Long answerHOTS / competency
A student burns wax and sees melting, a flame and a black deposit on a cold spoon held in the flame. Identify the physical and chemical changes and explain the deposit.
Hint
Limited air.
Answer
Melting of wax is physical. The flame and burning of wax vapour is chemical. The black deposit is soot (carbon), formed because part of the carbon in the wax does not burn completely in a limited supply of air. It is a new substance, so this too is a chemical change.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Five signs of a chemical change?Show answer
Colour, gas, precipitate, heat/light/sound, smell or taste.
2.Is melting butter chemical?Show answer
No, physical.
3.Is curdling chemical?Show answer
Yes.
4.What does iron do to blue CuSO₄?Show answer
Turns it green; copper deposits.
5.Is cloud formation physical?Show answer
Yes.

Signs of a Chemical Change and Classifying Everyday Changes: frequently asked questions

What are the signs of a chemical change?

Change of colour, a gas given off, formation of a precipitate, heat, light or sound produced or absorbed, and a change of smell or taste.

Is a gas always a sign of chemical change?

No; boiling water gives steam without any new substance. Only the formation of a new substance proves a chemical change.

How do you classify a change quickly?

Ask whether a new substance has formed. If yes, it is chemical; if only state, shape or size changed, it is physical.