Chemical reactions · Contrast
Chemical change vs physical change
Two changes can look identical — the same heat, the same colour change, the same disappearing solid — and only one of them makes something new. Which question separates them every time?
Start here
Melt a chocolate bar and it comes back. Fry an egg and nothing brings it back.
Both changes were caused by heat. Both changed how the thing looked, felt and poured. In one of them you still have chocolate. In the other you no longer have egg white — you have something else that happens to be sitting where the egg white was.
What is the difference that actually matters?
Whether a new substance was made. Melted chocolate is chocolate — same substance, different state. Fried egg is not egg white with the temperature turned up; it is a different substance with different properties — firm where the old one was runny, white where it was clear. Everything else — the colour, the smell, the bubbling, whether it can be undone — is a clue. This is the question.
In a physical change, the same substances are still there. They have moved, changed state, or mixed — and the particles themselves are untouched.
In a chemical change — a chemical reaction — at least one new substance is made. The atoms are all still there, but they have been joined up differently, and what you have now behaves differently from what you started with.
Your turn · three pairs, side by side
Each pair looks alike. In each pair, one is a reaction and one is not.
Commit on both halves of a pair before you run any tests on it. Then read the evidence and see which of your two verdicts survives.
Pair 1 · both involve a candle and a flame
Wax melting
The wax near the flame turns to a clear liquid and runs down the side.
Does it come back?
It sets again as it cools, into wax.
New properties?
Liquid wax and solid wax burn the same way, smell the same and weigh the same.
Mass
The wax that ran down weighs what it weighed before.
Physical change.
A change of state. Same substance, different arrangement of the same particles.
Wax burning
The wick burns with a yellow flame and the candle gets shorter.
Does it come back?
Nothing you can do to the air above the candle turns it back into wax.
New properties?
Carbon dioxide and water vapour come off. Neither is waxy, neither burns.
Mass
The candle loses mass — and the products, if you could weigh them all, weigh more than the wax did, because oxygen joined in.
Chemical change.
The wax has been turned into different substances. The atoms are all still there, joined up differently.
Pair 2 · both make a solid vanish into a liquid
Salt stirred into water
The crystals disappear and the water stays clear.
Does it come back?
Evaporate the water and the salt is back, to the gram.
New properties?
None that are new. The salt is still salt — it just cannot be seen while it is dissolved.
Mass
Unchanged. 100 g of water plus 10 g of salt weighs 110 g.
Physical change.
Dissolving is mixing at the particle scale. Nothing new was made — which is exactly why you could get it back.
Marble chips in acid
The chips fizz vigorously and slowly disappear.
Does it come back?
Evaporate the liquid and you get a different white solid, not marble chips.
New properties?
The gas given off puts out a lit splint and turns limewater cloudy. Marble does neither.
Mass
The open flask gets lighter, because the gas leaves. Seal it and the mass does not change at all.
Chemical change.
Marble and acid have become a gas, a salt and water. "It dissolved" is the same phrase describing a completely different event.
Pair 3 · both change colour, both take time
Ice left in a warm room
The clear block goes cloudy at the edges and becomes a puddle.
Does it come back?
Put the puddle in the freezer and you have ice again.
New properties?
Ice and water are the same substance. Both are water.
Mass
The puddle weighs what the block weighed.
Physical change.
Melting. The particles have not changed at all — they have only broken out of a fixed arrangement.
An iron nail left outside
The grey nail turns orange-brown and flakes.
Does it come back?
Not by cooling or drying. A steelworks can get iron back from rust, with a furnace and carbon.
New properties?
Rust is soft, crumbly and not magnetic in the way iron is. Iron is strong and grey.
Mass
The nail gets heavier. The extra mass came out of the air.
Chemical change.
Iron and oxygen have become iron oxide, a new substance with new properties — including being useless as a nail.
Six changes. A solid vanished into a liquid in two of them and only one was a reaction. Something got hot in two of them and only one was a reaction. Colour changed in two and only one was a reaction. Every clue you can see appears on both sides of the line. The only question that sorted all six was whether the substance you ended with was a different substance.
Key fact
A chemical change makes at least one new substance. A physical change does not — the same substances are still there, in a different state or a different place.
Reference · the two columns
What each kind of change does and does not do
- New substancesNo new substance.At least one new substance, with different properties.
- The particlesThe particles are unchanged — only their arrangement or spacing.The atoms are the same atoms, joined up differently.
- Reversing itUsually easy to reverse, and the same mass comes back.Usually hard to reverse, and never by cooling it down.
- Where you meet itMelting, boiling, dissolving, breaking, mixing.Burning, rusting, cooking, acid on marble, respiration.
Say it as one sentence
Build the comparison, not two descriptions
Melting wax and burning wax. Pick one clause for each half — then read what a linked comparison has to do that two separate statements do not.
First half
Second half
Your sentence
That is the sentence. Both halves name the change, both give the reason, and the second half answers the first — the wax is still wax against the wax is now something else. An examiner is looking for the link, and "whereas" is where it lives.
Every clause you could pick is true. What decides the marks is whether the two halves are about the same thing: a comparison has to say what is different, not describe two changes and leave the reader to spot it. "It can be undone" and "it gives off heat and light" are both true and neither is the difference — one is a consequence and the other happens in physical changes too.
A sentence that would get full marks
Melting wax is a physical change because the wax is still wax and cools back to a solid, whereas burning wax is a chemical change because the wax is turned into carbon dioxide and water, which cannot be turned back into wax by cooling.
Think again
“If you can undo it, it is physical. If you cannot undo it, it is chemical.”
This rule gets the right answer most of the time, which is exactly what makes it dangerous. Commit before you read on.
Smash a glass and you cannot undo it — and no new substance was made, so it is a physical change. It is broken glass, which is glass. Meanwhile a rusty nail can be turned back into iron: a steelworks does exactly that to iron ore, which is mostly rust, and rusting is unquestionably a chemical change.
Reversibility is about how hard the change is to undo, and that is a fact about energy and equipment, not about what happened to the substances. It is a useful hint and it is not the test. Ask what you have now, not whether you can get back.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What single thing tells you that a change is a chemical change?
Rung 2 · The one that catches people
Salt dissolving in water and marble fizzing in acid are both described as "it dissolved". Which pair of verdicts is right?
Rung 3 · Explain
A student heats sugar in a pan. First it melts to a clear syrup; then it turns brown, smells of caramel and sets hard. They say "the whole thing was one change". Explain how many changes happened, which kind each was, and what evidence separates them.
Rung 4 · Take it somewhere new
A cook says "cooking is just heating food up". Take two examples from a kitchen — one physical change and one chemical — and use them to explain why that sentence is wrong, and why it is easy to believe.
Key note
A chemical change makes at least one new substance, with properties the starting materials did not have. A physical change makes none — the same substances are still there, melted, dissolved, broken or moved. Bubbling, colour change, heat and irreversibility are clues that appear on both sides of the line, so none of them is the test. The test is: what have you got now?
Going further
Some changes are genuinely awkward to place, and that is not a flaw in the definition — it is what happens at a boundary. Dissolving salt in water is physical: the salt is still there and evaporating gets it back. Dissolving marble chips in acid is chemical: the marble is gone, replaced by a gas, a salt and water. Both are described in everyday speech as "it dissolved", and the word is doing two completely different jobs.
Cooking is the case worth arguing about. Boiling a potato is mostly physical — heat and water soften it. Browning a steak is chemical: hundreds of new substances form between the amino acids and sugars on the surface, which is why it smells of something the raw meat did not. Whipping cream is physical, and so is whipping it too far: the fat droplets clump together into butter, and nothing new has been made. The same pan can host both kinds of change in the same minute.
Before this lesson
Next in this unit
At GCSE this becomes
- Reaction profiles, bond breaking and bond making, and why a chemical change involves an energy change every time.
Where to next
- Next: Reactions rearrange atoms
- Previous: Proving something is pure
Mixtures and separation
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