One Burning Candle Shows Two Kinds of Change at Once
Learn the one test that separates physical from chemical change, the five signs a new substance has formed, and why a candle demonstrates both at the same moment.
What is the difference between a physical and a chemical change?
The test is a single question: has a new substance been formed? In a physical change the substance stays the same and only its state, shape or size alters. In a chemical change a genuinely new substance appears, with new properties of its own.
This page covers everything in the CBSE Class 7 Science chapter on changes around us: the physical-versus-chemical test, the signs of a chemical change, reversible and irreversible changes, and the candle that manages both at once.
This page covers everything in the CBSE Class 7 Science chapter on changes around us: the physical-versus-chemical test, the signs of a chemical change, reversible and irreversible changes, and the candle that manages both at once.
How do you classify a change as physical or chemical?
Ask what you are left with at the end.
Physical changes: ice melting, water boiling, sugar dissolving, paper being folded, a wire being bent, iron being heated until it glows. In every case the substance is still ice-water, still sugar, still paper, still iron.
Chemical changes: milk turning into curd, iron rusting, paper burning, food being cooked, a banana ripening, magnesium burning to a white ash. Each leaves something new that behaves differently from what you started with.
For example, dissolving sugar in tea is physical — evaporate the water and the sugar returns. Burning that sugar is chemical — the black residue is no longer sugar and no amount of cooling brings it back.
The common misconception is that heating always means a chemical change. Heating iron makes it glow and expand, but it is still iron, so that is physical. It is the new substance, not the heat, that decides.
Physical changes: ice melting, water boiling, sugar dissolving, paper being folded, a wire being bent, iron being heated until it glows. In every case the substance is still ice-water, still sugar, still paper, still iron.
Chemical changes: milk turning into curd, iron rusting, paper burning, food being cooked, a banana ripening, magnesium burning to a white ash. Each leaves something new that behaves differently from what you started with.
For example, dissolving sugar in tea is physical — evaporate the water and the sugar returns. Burning that sugar is chemical — the black residue is no longer sugar and no amount of cooling brings it back.
The common misconception is that heating always means a chemical change. Heating iron makes it glow and expand, but it is still iron, so that is physical. It is the new substance, not the heat, that decides.
What are the signs that a chemical change has happened?
Five observations tell you a new substance has formed:
- a change in colour — a shiny iron nail turning reddish-brown
- a change in smell — milk going sour, or food turning bad
- evolution of a gas — bubbles when baking soda meets vinegar
- formation of a new substance with different properties
- release or absorption of heat and light — a burning matchstick giving out both
Seeing any of these is strong evidence of a chemical change.
For example, when you add lemon juice to milk it curdles: the colour and texture change, the smell changes, and the milk cannot be recovered — three signs at once.
Be careful with bubbles, though. Boiling water produces bubbles of steam, yet that is only water changing state — a physical change. So a sign is evidence to be weighed, not proof on its own, and you should check whether the final substance is genuinely new.
- a change in colour — a shiny iron nail turning reddish-brown
- a change in smell — milk going sour, or food turning bad
- evolution of a gas — bubbles when baking soda meets vinegar
- formation of a new substance with different properties
- release or absorption of heat and light — a burning matchstick giving out both
Seeing any of these is strong evidence of a chemical change.
For example, when you add lemon juice to milk it curdles: the colour and texture change, the smell changes, and the milk cannot be recovered — three signs at once.
Be careful with bubbles, though. Boiling water produces bubbles of steam, yet that is only water changing state — a physical change. So a sign is evidence to be weighed, not proof on its own, and you should check whether the final substance is genuinely new.
Which changes are reversible, and which are fast or slow?
A reversible change can be undone, returning the original substance. An irreversible change cannot.
Reversible: melting ice (freeze it again), folding paper (unfold it), dissolving salt in water (evaporate the water), stretching a rubber band.
Irreversible: cooking food, chopping vegetables, burning a matchstick, popping corn, curdling milk, rusting iron.
Changes also differ in speed. Fast changes include popping corn, a bursting balloon and a lightning flash. Slow changes include the ripening of a fruit, the rusting of a gate and the growth of a plant.
So chopping vegetables is irreversible but physical — you cannot rejoin the pieces, yet no new substance formed. And melting ice is reversible and physical.
That pairing is where students slip: reversible usually goes with physical and irreversible with chemical, but not always. Judge each property separately rather than assuming one from the other.
Reversible: melting ice (freeze it again), folding paper (unfold it), dissolving salt in water (evaporate the water), stretching a rubber band.
Irreversible: cooking food, chopping vegetables, burning a matchstick, popping corn, curdling milk, rusting iron.
Changes also differ in speed. Fast changes include popping corn, a bursting balloon and a lightning flash. Slow changes include the ripening of a fruit, the rusting of a gate and the growth of a plant.
So chopping vegetables is irreversible but physical — you cannot rejoin the pieces, yet no new substance formed. And melting ice is reversible and physical.
That pairing is where students slip: reversible usually goes with physical and irreversible with chemical, but not always. Judge each property separately rather than assuming one from the other.
How does a burning candle show both changes at once?
Watch a lit candle carefully and two different things are happening in the same moment.
The heat from the flame melts the solid wax near the wick into liquid wax, which runs down and then solidifies again lower on the candle. Melting and solidifying are physical changes — the wax is still wax, and you could collect and remelt it.
Meanwhile, at the wick, wax vapour is burning, reacting with oxygen and producing heat, light, carbon dioxide and water vapour. That is a chemical change — the burnt wax is gone for good and cannot be recovered.
So the same object demonstrates both: physical change in the melting wax, chemical change in the burning wick.
This is why the candle is the standard example in this chapter. It proves the two kinds of change are not opposites that exclude each other — they can occur side by side in one process.
The heat from the flame melts the solid wax near the wick into liquid wax, which runs down and then solidifies again lower on the candle. Melting and solidifying are physical changes — the wax is still wax, and you could collect and remelt it.
Meanwhile, at the wick, wax vapour is burning, reacting with oxygen and producing heat, light, carbon dioxide and water vapour. That is a chemical change — the burnt wax is gone for good and cannot be recovered.
So the same object demonstrates both: physical change in the melting wax, chemical change in the burning wick.
This is why the candle is the standard example in this chapter. It proves the two kinds of change are not opposites that exclude each other — they can occur side by side in one process.
Exam tip
Exam tip: justifying your answer, not just naming it
Writing "chemical change" alone earns almost nothing. The mark is in the reason.
Answer in two parts: name the change, then say whether a new substance formed and how you know. Curdling of milk is a chemical change, because a new substance with a different taste, smell and texture is formed and the milk cannot be recovered.
And when a question offers a tempting sign, address it. If asked about boiling water, say explicitly that bubbles appear but they are steam, so the water is unchanged and the change is physical. Naming the trap is what shows the examiner you understood the test rather than pattern-matching on bubbles.
Answer in two parts: name the change, then say whether a new substance formed and how you know. Curdling of milk is a chemical change, because a new substance with a different taste, smell and texture is formed and the milk cannot be recovered.
And when a question offers a tempting sign, address it. If asked about boiling water, say explicitly that bubbles appear but they are steam, so the water is unchanged and the change is physical. Naming the trap is what shows the examiner you understood the test rather than pattern-matching on bubbles.
Did you know
Why can't a chemical change usually be reversed?
In a physical change the substance is only rearranged, so reversing the conditions restores it — cool the water and the ice returns.
In a chemical change the original substance has become something else entirely. Getting it back would mean undoing that formation, which ordinary cooling or reheating cannot do. That is why a burnt chapati never becomes dough again, however long you leave it.
In a chemical change the original substance has become something else entirely. Getting it back would mean undoing that formation, which ordinary cooling or reheating cannot do. That is why a burnt chapati never becomes dough again, however long you leave it.
Key takeaways
Physical and chemical changes: quick revision
- The test is whether a new substance is formed: physical changes alter only state, shape or size, while chemical changes produce something new.
- Signs of a chemical change are a change in colour or smell, evolution of a gas, formation of a new substance, and release or absorption of heat and light.
- Bubbles alone prove nothing — boiling water bubbles but stays water.
- Reversible changes can be undone (melting ice, folding paper); irreversible ones cannot (cooking, burning, curdling), and changes may also be fast or slow.
- A burning candle shows both kinds together: melting wax is physical, burning wax at the wick is chemical.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Signs of a chemical change are a change in colour or smell, evolution of a gas, formation of a new substance, and release or absorption of heat and light.
- Bubbles alone prove nothing — boiling water bubbles but stays water.
- Reversible changes can be undone (melting ice, folding paper); irreversible ones cannot (cooking, burning, curdling), and changes may also be fast or slow.
- A burning candle shows both kinds together: melting wax is physical, burning wax at the wick is chemical.
You will remember all of this far better after answering five questions on it than after reading it twice.