Science

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

Quick answer

A physical change alters how a substance looks or its state but makes nothing new, like ice melting. A chemical change makes new substances, like wood burning to ash and smoke.

Ages 9–14Secondary

In one sentence

A physical change changes the shape, size or state of a substance but makes no new substance; a chemical change turns substances into new substances with different properties.

Explained

Think about a candle. When you light it, two things happen at once. Near the flame, solid wax melts into liquid wax. That is a physical change: it is still wax, and it goes solid again when it cools. But the wick and the wax vapour also burn, producing carbon dioxide, water vapour, heat and light. That is a chemical change: new substances have appeared, and you cannot turn them back into wax.

candle
egg
snow

Melting wax and snow are physical changes; burning and frying an egg are chemical changes.

Physical changes include:

changes of state: melting, freezing, boiling, evaporating, condensing (see solids, liquids and gases)
changing shape or size: cutting, crushing, bending, tearing, grinding
dissolving, like salt or sugar in water, and mixing, like sand and iron filings

Many physical changes are easy to reverse (freeze the water again, evaporate the salt water to get the salt back), but not all: a smashed plate is still a physical change, because the pieces are still the same material.

Chemical changes include burning, rusting, cooking and baking, food rotting, milk going sour, fireworks, and acids reacting with bicarbonate of soda. Every chemical change is a chemical reaction. Most are hard or impossible to reverse.

How can you tell? Look for the signs of a chemical change:

Gas is given off: bubbles or fizzing (not just boiling).
Colour change that is not just mixing two colours.
Temperature change or light: the mixture gets hot or cold by itself, or glows or burns.
A precipitate forms: two liquids mix and a solid appears.
A new smell (rotten egg, burnt toast, sour milk).

These signs are clues, not proof. The real test is the question: is there a new substance?

At the level of particles, a physical change only moves particles around: they get closer together, further apart or mixed with others, but each particle stays the same. In a chemical change, the atoms rearrange into new particles. No atoms are created or destroyed, so the total mass stays the same in a closed container, in both kinds of change.

Examples

Change	Physical or chemical?	Why
Ice melting	physical	still water, just liquid; it can freeze again
Sugar dissolving in tea	physical	the sugar is still there (the tea tastes sweet)
Cutting paper	physical	smaller pieces of the same paper
An iron nail rusting	chemical	iron reacts with oxygen and water to make rust (iron oxide)
Frying an egg	chemical	the egg white turns solid and white for good
Baking soda and vinegar fizzing	chemical	bubbles of a new gas, carbon dioxide
Water boiling	physical	the bubbles are water vapour, still water
Toasting bread	chemical	new brown substances and a new smell

Common mistakes

"If you can't reverse it, it's chemical." Not always. Breaking glass and tearing paper cannot be undone, but no new substance forms, so they are physical.
"Bubbles always mean a chemical change." Boiling water bubbles because liquid water turns into water vapour: physical. Bubbles count as a sign only when a new gas is made.
"Dissolving makes the substance disappear." The particles spread out between the water particles. Evaporate the water and the solid is left behind.
Mixing up "melting" and "dissolving". Melting needs heat and one substance (ice → water). Dissolving needs a solid and a liquid (sugar in water). Both are physical.

Step by step

Ask the key questionIs there a new substance at the end, or is it the same stuff in a different form?
Look for change of state, shape or sizeMelting, freezing, boiling, cutting, crushing or dissolving point to a physical change.
Check the signsGas made, colour change, heat or light, a precipitate or a new smell point to a chemical change.
Think about getting it backCould you get the original substance back by cooling, heating or evaporating? If yes, it is probably physical.
Explain with evidenceSay what you saw and why it shows a new substance was made (or was not).

Practice questions

Physical or chemical? Explain why

Try each one, then check your answer.

Ice melting into water.
Show answer
Physical: it is still water, just a liquid now, and it can freeze again.
Toasting bread.
Show answer
Chemical: the bread turns brown and smells different. New substances form and you cannot untoast it.
Dissolving sugar in tea.
Show answer
Physical: the sugar is still there. If the water evaporates, sugar crystals are left.
An iron nail rusting.
Show answer
Chemical: iron reacts with oxygen and water to make rust (iron oxide), a new brown, flaky substance.
Cutting paper into pieces.
Show answer
Physical: smaller pieces of the same paper. It cannot be undone, but no new substance forms.
Frying an egg.
Show answer
Chemical: the proteins change for good. The clear part turns white and solid and cannot go back.
Water boiling in a kettle.
Show answer
Physical: the bubbles are water vapour (steam), which is still water.
Baking soda and vinegar fizzing.
Show answer
Chemical: the bubbles are carbon dioxide, a new gas made by the reaction.
Milk going sour.
Show answer
Chemical: bacteria turn the sugar in milk into lactic acid, giving a new smell, taste and lumps.
Crushing a drinks can.
Show answer
Physical: only the shape changes; it is still aluminium.
Fireworks exploding.
Show answer
Chemical: light, heat, sound and smoke show that new substances are being made.
Water vapour turning into drops on a cold window.
Show answer
Physical: condensation, gas to liquid. It is still water.
A copper roof slowly turning green.
Show answer
Chemical: copper reacts with substances in air and rain to form a new green layer.
A fizzy vitamin tablet dropped in water.
Show answer
Chemical: the bubbles are carbon dioxide, made when an acid and a bicarbonate in the tablet react in the water.
Salt left behind when sea water evaporates.
Show answer
Physical: the salt was dissolved in the water all along; evaporation just separates it.
A candle burning.
Show answer
Both: the wax melting is physical, the wax burning is chemical (it makes carbon dioxide and water vapour).

Word problems

Explain the evidence

Try each one, then check your answer.

Maya heats a white powder. It turns black and gives off a strong smell. Has a chemical change happened? Give two clues.
Show answer
Yes, a chemical change. Clues: a colour change and a new smell, both signs of new substances.
Leo mixes sand with iron filings, then pulls all the iron out with a magnet. Physical or chemical?
Show answer
Physical. Mixing made no new substance: both materials came back out unchanged.
Two clear liquids are mixed and a cloudy yellow solid appears. What is the solid called, and what kind of change is this?
Show answer
The solid is a precipitate, and its appearance shows a chemical change.
Sam says: "Breaking a glass is a chemical change, because you can't put it back together." Is he right?
Show answer
No. The pieces are still glass, so no new substance formed. It is a physical change, even though it cannot be reversed.
A sealed bag holding baking soda and vinegar has a mass of 120 g. The two react and the bag puffs up. What is the mass afterwards?
Show answer
120 g. The bag is sealed, so the carbon dioxide cannot escape. Atoms are rearranged, not lost, so the mass stays the same.
Why do bubbles in boiling water not prove a chemical change, while bubbles from vinegar and baking soda do?
Show answer
Boiling bubbles are water vapour, the same substance in a different state. The vinegar bubbles are carbon dioxide, a new substance.

Questions people ask

What are 5 examples of physical and chemical changes?

Physical: melting ice, boiling water, dissolving salt, cutting paper, crushing a can. Chemical: burning wood, rusting iron, baking a cake, frying an egg, milk going sour.

How do you identify a chemical change?

Look for a new substance. Signs include gas bubbles (not from boiling), a colour change, heat or light given off, a solid forming when two liquids mix, and a new smell.

How are physical and chemical changes similar?

In both, matter is not created or destroyed, so the total mass stays the same in a closed container. Both also often involve energy, such as heat being taken in or given out.

Is dissolving a physical or chemical change?

Usually physical. Sugar or salt in water is still sugar or salt, and you can get it back by evaporating the water. Some things that seem to dissolve, like a fizzy tablet, are actually reacting chemically.

Is cooking a physical or chemical change?

Most cooking is chemical: baking, frying, toasting and roasting make new substances. Some steps are physical, like melting butter or boiling water.

Can a chemical change be reversed?

Most cannot be reversed easily. A few can be reversed by another chemical reaction, as when a rechargeable battery is charged, but you cannot simply cool or dry the products to get the starting substances back.

Quick quiz

Physical and chemical changes

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Question 1 of 5

Which of these is a chemical change?

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