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Melt an ice cube and you still have water. Burn a piece of paper and you’ll never get the paper back. Those two everyday events capture the difference between a physical change and a chemical change — and learning to tell them apart is one of the first skills in chemistry.
The key question
Is a new substance formed?
- Physical change: the substance changes form, shape or state, but its chemical identity stays the same. No new substance is made.
- Chemical change: atoms are rearranged into new substances with different properties. Bonds are broken and new bonds are formed.
Physical changes
Examples:
- Melting, freezing, boiling, condensing, sublimation (all changes of state)
- Cutting, tearing, crushing, bending
- Dissolving sugar or salt in water
- Stretching a rubber band
- Magnetizing a piece of iron
- Mixing sand and iron filings (and separating them with a magnet)
Ice, liquid water and steam are all H₂O. Crushed sugar is still sugar.
Physical changes are often (not always) easy to reverse: freeze the water again, evaporate the solution to get the salt back.
Chemical changes
Examples:
- Burning (wood, gas, a candle)
- Rusting iron
- Cooking an egg, toasting bread, caramelizing sugar
- Fruit ripening and food rotting
- Digesting food
- Baking soda fizzing with vinegar
- Silver tarnishing
- Fireworks exploding
Chemical changes are usually hard to reverse. You can’t un-bake a cake.
Five signs that a chemical change has happened
None of these proves it on its own, but together they’re strong evidence:
- A gas is produced that wasn’t there before — bubbles, fizzing (and not just boiling).
- A colour change — iron turning red-brown, a copper roof turning green.
- A precipitate forms — a solid suddenly appearing when two clear solutions are mixed.
- Energy is released or absorbed — heat, light or sound; burning, glowing, an unexpected temperature change.
- A new smell — rotting food, burnt toast.
The tricky ones
Dissolving salt in water. Usually classed as physical: evaporate the water and you get the salt back unchanged. Chemists do point out that the ionic lattice breaks up and water molecules bond to the ions, so it’s a borderline case — but introductory courses call it physical.
Boiling water. Physical. The bubbles are water vapour, not a new gas — the H₂O molecules stay intact. Compare electrolysis, where bubbles of hydrogen and oxygen are new substances: chemical.
Colour change on heating. A metal glowing red-hot is physical (it cools back to the same metal). Copper turning black when heated in air is chemical: it forms copper(II) oxide.
Candle burning. Both at once! The wax melting is physical; the wax vapour burning in the flame is chemical.
Cooking an egg. Chemical. The proteins change structure irreversibly — the egg won’t turn clear again when it cools.
Frying an egg in butter and the butter melting. Melting: physical. Butter browning: chemical.
Chemical vs. physical properties
The same distinction applies to properties:
- Physical properties can be observed without changing the substance: colour, density, melting point, boiling point, hardness, conductivity, solubility. Our element pages list dozens of these for every element.
- Chemical properties describe how a substance reacts, and you can only observe them by changing it: flammability, reactivity with water or acid, tendency to rust, toxicity, acidity.
Sodium’s silvery colour and low density are physical properties. The fact that it reacts violently with water is a chemical property.
Quick answers
Is dissolving a chemical change? Generally treated as physical, because the dissolved substance can be recovered unchanged.
Is rusting a chemical change? Yes — iron and oxygen form a new substance, hydrated iron(III) oxide.
Is cutting hair a chemical change? No, physical. Dyeing or perming hair, however, involves chemical changes.
Do physical changes involve energy? Yes — melting and boiling absorb energy, freezing and condensing release it — but no new substance forms. See exothermic vs. endothermic.
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