ChemistryICSE Class 8 · Physical and Chemical Changes

Physical Changes and Chemical Changes: Meaning, Differences, Energy and Mass (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

A physical change alters properties such as state, shape or size but forms no new substance and is usually reversible; a chemical change forms new substances with different properties and is usually irreversible, with heat or light given out or taken in. The total mass is conserved in both.

Melting ice can go back and forth with no new substance, but burning magnesium makes a new white substance that cannot turn back.

Illustration drawn for QuestionBuddy.

Understand Physical Changes and Chemical Changes Compared

Physical change.

no new substance is formed; the composition stays the same;
only physical properties (state, shape, size, appearance) change;
usually reversible by simple means;
little or no energy change (except for changes of state).

Examples: melting of ice, boiling of water, dissolving sugar or salt, breaking glass, cutting paper, bending a wire, magnetising iron, sublimation of camphor, glowing of a bulb filament.

Chemical change.

new substances with new properties are formed; atoms are rearranged;
usually irreversible;
heat, light or sound is usually given out or taken in.

Examples: burning of paper, wood, candles or magnesium; rusting of iron; cooking food; curdling of milk; ripening and rotting of fruit; digestion; photosynthesis; respiration; iron with copper sulphate solution.

$2 M g + O_{2} \to 2 M g O$ (a bright white flame and a white powder that is not magnesium).

Physical change	Chemical change
no new substance	new substance(s)
composition unchanged	composition changes
usually reversible	usually irreversible
little or no energy change (except change of state)	heat or light given out or taken in
mass of each substance unchanged	substances change, total mass conserved
melting wax, dissolving sugar	burning wax, rusting

A change can show both. A burning candle: melting of wax is physical, burning of wax vapour is chemical.

Watch for traps.

Irreversible does not mean chemical: breaking glass is irreversible but physical.
A gas given off does not prove a chemical change: boiling water gives steam, with no new substance.

Energy and mass. Many chemical changes give out heat (quicklime and water: $C a O + H_{2} O \to C a (O H)_{2}$ ); some take it in. In both kinds of change the total mass is conserved: 50 g of ice gives 50 g of water; in a chemical change the products' masses add up to the reactants' masses.

Physical Changes and Chemical Changes Compared: formulas and key facts

Physical: No new substance
Same composition
Usually reversible
Little energy change; Melting, dissolving, breaking.
Chemical: New substance
Composition changes
Usually irreversible
Heat or light involved; Burning, rusting, cooking.
Burning magnesium: $2 M g + O_{2} \to 2 M g O$; Chemical: bright light, white powder.
Quicklime and water: $C a O + H_{2} O \to C a (O H)_{2}$; Gives out a lot of heat.

How to solve Physical Changes and Chemical Changes Compared questions

Ask one question: is a new substance formed?

1Ask whether the substance after the change is the same as before.
2If a new substance forms, it is chemical; if only state, shape or size changes, it is physical.
3Check reversibility, but do not rely on it alone.
4Support with energy change and mass conservation.

Physical Changes and Chemical Changes Compared 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.

Bread and toast

Suppose bread is toasted until brown and crisp.

Is it physical or chemical?

Try it first, then show the solutionSolution

The bread changes colour, smell and taste, and cannot be turned back.
New substances form: chemical change.

Answer: Chemical.

Memory hook: Cannot untoast.

Wax in the flame

Imagine a candle: the wax near the wick melts and then the vapour burns.

Name the physical and the chemical part.

Try it first, then show the solutionSolution

Solid wax to liquid wax is melting, a physical change.
The burning of the wax vapour forms carbon dioxide and water: chemical.

Answer: Melting: physical; burning: chemical.

Memory hook: One candle, two changes.

Tricks to remember Physical Changes and Chemical Changes Compared

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

New stuff = chemical
Ask: is there new stuff? Yes: chemical.
Use it when: Quick test.
Glass trap
Irreversible does not mean chemical: broken glass.
Use it when: Exceptions.
Mass is safe
Total mass is conserved in both changes.
Use it when: Mass question.

Physical Changes and Chemical Changes Compared: solved examples

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

Example 1: Which is chemical? · 1 mark · MCQBoard level

Which of the following is a chemical change?
(a) melting of ice
(b) dissolving sugar in water
(c) rusting of iron
(d) breaking a glass

Try it first, then show the solutionSolution

Rusting forms a new brown substance (rust).

Answer: (c) rusting of iron

Example 2: Ice and iron · 2 marks · Short answerBoard level

Ice melts to water in a warm room, and an iron nail left in damp air turns brown. For each change, say whether it is physical or chemical, and whether it can be reversed easily.

Try it first, then show the solutionSolution

Melting of ice: physical; it can be reversed by freezing; no new substance. (1)
Rusting: chemical; a new brown substance forms and it cannot easily be reversed. (1)

Answer: Melting: physical, reversible. Rusting: chemical, not easily reversed.

Example 3: The candle · 3 marks · Short answerHarder board level

A candle burns. (a) Identify one physical and one chemical change in the process. (b) Give the evidence for the chemical change. (c) Name the products.

Try it first, then show the solutionSolution

(a) Physical: wax melts; chemical: wax vapour burns. (1)
(b) Light and heat are given out, and new substances form. (1)
(c) Carbon dioxide and water vapour. (1)

Answer: (a) melting/burning (b) light and heat (c) CO₂ and H₂O

Example 4: Mass and change · 3 marks · Short answerHarder board level

50 g of ice melts completely. (a) What is the mass of water? (b) In burning, 4.8 g of magnesium forms 8.0 g of oxide: how much oxygen combined? (c) What does this show about mass in physical and chemical changes?

Try it first, then show the solutionSolution

(a) 50 g. (1)
(b) $8.0 - 4.8 = 3.2$ g. (1)
(c) Mass is conserved in both kinds of change. (1)

Answer: (a) 50 g (b) 3.2 g (c) conserved

Example 5: Irreversible but physical · 4 marks · Case-basedHOTS / competency

A student says: "Breaking a glass is irreversible, so it is a chemical change." (a) Is she right? (b) What test should she use instead? (c) Classify: cooking an egg, sublimation of camphor, a bulb filament glowing. Give reasons.

Try it first, then show the solutionSolution

(a) No: breaking glass changes only the shape and size; no new substance forms. (1)
(b) Ask whether a new substance forms. (1)
(c) Cooking an egg: chemical (cannot be uncooked, new substances). (1)
Camphor sublimation and bulb filament glowing: physical (same substance, glow stops when switched off). (1)

Answer: (a) no (b) new substance (c) egg: chemical; camphor and filament: physical

Where students lose marks in Physical Changes and Chemical Changes Compared

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

Mistake: Irreversible = chemical
Breaking glass is irreversible but physical.
Wrong working
"Cutting paper cannot be undone, so chemical."
Right working
"Cutting paper is physical: no new substance."
Do this: Use the new-substance test.
Mistake: Gas given off = chemical
Boiling water gives steam, no new substance.
Wrong working
"Boiling water is chemical because gas forms."
Right working
"Boiling is physical: steam is water."
Do this: Check the substance.
Mistake: Saying mass changes in burning
Total mass is conserved.
Wrong working
"Mass is lost when wood burns."
Right working
"Mass of ash is less because gases escape; the total is conserved."
Do this: Include the gases.

Exam tips

Define both changes with a clear new-substance statement.
Give reasons with each classification.
Use the table of differences for 3-mark questions.
Mention energy and mass when asked for more.

Practice questions on Physical Changes and Chemical Changes Compared

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
Give two examples each of physical and chemical changes.
Hint
Think of everyday changes.
Answer
Physical: melting wax, dissolving sugar. Chemical: burning paper, rusting iron.
Q2 · 2 marks · Short answerHarder board level
Why is the dissolving of sugar in water a physical change, although the sugar seems to disappear?
Hint
Can it be recovered?
Answer
No new substance forms; the sugar is still there (the water tastes sweet) and can be recovered by evaporating the water.
Q3 · 3 marks · Short answerHarder board level
When quicklime is added to water the mixture gets hot and hisses. Classify the change, write the equation and state the energy change.
Hint
New substance and heat.
Answer
Chemical change: $C a O + H_{2} O \to C a (O H)_{2}$ ; a new substance forms and a lot of heat is given out (exothermic).
Q4 · 3 marks · Short answerHarder board level
Classify with a reason: (a) forming clouds, (b) curdling of milk, (c) digestion of food.
Hint
New substance?
Answer
(a) Physical: water vapour condenses, same substance. (b) Chemical: curd is a new substance with a sour taste. (c) Chemical: food is broken into new substances.
Q5 · 4 marks · Long answerHOTS / competency
A piece of paper is (i) folded, (ii) torn, (iii) burnt. Classify each change and state, for the last, how the mass of the ash and gases compares with the mass of the paper and oxygen used.
Hint
Mass is conserved.
Answer
(i) and (ii) are physical changes: only shape and size change. (iii) is chemical: new substances (ash, carbon dioxide, water vapour) form. The total mass of ash and gases equals the mass of paper plus the oxygen used.

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.Physical change forms?Show answer
No new substance.
2.Chemical change is usually?Show answer
Irreversible.
3.Rusting is?Show answer
Chemical.
4.Is breaking glass chemical?Show answer
No, physical.
5.Mass in both changes?Show answer
Conserved.

Physical Changes and Chemical Changes Compared: frequently asked questions

What is the difference between a physical and a chemical change?

A physical change forms no new substance and is usually reversible; a chemical change forms new substances, is usually irreversible and involves heat or light.

Is dissolving sugar in water a chemical change?

No; it is a physical change because no new substance is formed and the sugar can be recovered by evaporation.

Is mass conserved in a chemical change?

Yes. The total mass of the products equals the total mass of the reactants (law of conservation of mass).