Physical Changes vs Intro to Chemical Changes
A physical change alters the form or appearance of a substance without changing its chemical identity, while a chemical change produces one or more new substances with different properties. In brief: cutting paper, melting wax and dissolving sugar are physical changes, whereas burning paper, rusting iron and baking bread are chemical changes.
Learning to tell the two apart is one of the first skills in chemistry. This article introduces both types of change, gives a short set of clues to decide which one is happening, compares them in a table, works through examples and explains why borderline cases need care.
What is a physical change?
In a physical change, only the shape, size, state or arrangement of the particles changes. The substance remains the same and has the same chemical formula before and after. Cutting a block of wood into pieces, crushing a rock to powder, boiling water and mixing sand with gravel are all physical changes.
Physical changes are often easy to reverse. Ice melts to water and refreezes, and a solution of salt in water can be evaporated to recover the salt.
What is a chemical change?
In a chemical change, also called a chemical reaction, the atoms of the starting substances are rearranged to form different substances. The new substances have their own properties, which differ from those of the starting materials. When iron rusts, iron and oxygen combine to make iron oxide, a reddish-brown flaky solid that is not at all like the original metal.
Chemical changes are usually hard to reverse by simple means. You cannot take a burnt match and turn it back into an unburnt one.
What clues show a chemical change?
Observing one clue alone is not always enough, but several together are strong evidence.
- Gas production: bubbles form without heating, for example when baking soda is mixed with vinegar.
- Color change: the color changes permanently, such as a cut apple turning brown.
- New solid forming: a solid appears when two clear solutions are mixed.
- Heat or light released: a fire, a glow stick or a self-heating hand warmer.
- New odor: milk turning sour or eggs rotting.
These clues are described fully in a later article on the signs of a chemical reaction.
Physical and chemical changes compared
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance? | No | Yes |
| Chemical formula | Unchanged | Changed |
| Usually reversible? | Yes | Usually not |
| Energy change | Often small | Often noticeable |
| Examples | Melting, dissolving, cutting | Burning, rusting, cooking |
Worked example: decide the type of change
For each case, ask: has a new substance formed?
- Water freezes in a freezer. It is still H2O. No new substance forms, so it is a physical change.
- A silver spoon tarnishes and turns dark. The dark layer is silver sulfide, a new substance made from silver and sulfur compounds in the air, so it is a chemical change.
- Sugar is stirred into tea and disappears. The sugar can be recovered by evaporation, so it is a physical change.
- Heating sugar in a pan until it turns brown and smells like caramel. The sugar decomposes into new substances, so it is a chemical change.
- A copper wire is bent into a loop. Only the shape is different, so it is a physical change.
More examples
| Physical changes | Chemical changes |
|---|---|
| Tearing paper | Burning paper |
| Boiling water | Electrolysis of water into hydrogen and oxygen |
| Breaking a glass | Digesting food |
| Dissolving salt | Photosynthesis in leaves |
| Melting chocolate | Baking a cake |
| Grinding coffee beans | Fireworks exploding |
How did chemists learn the difference?
Alchemists knew many chemical changes, such as smelting ores and making dyes, but explained them with the ideas of their time. In the 1700s, Antoine Lavoisier carried out careful experiments in sealed vessels and showed that mass is conserved in chemical reactions. In the early 1800s, John Dalton explained chemical changes as the rearrangement of atoms, which gave a clear reason why new substances appear in some changes and not in others.
Which mistakes are common?
Mistake 1: Judging a change only by how dramatic it looks
Boiling water produces bubbles and steam, yet it is a physical change. A slowly rusting nail looks quiet, yet it is chemical.
Mistake 2: Assuming that dissolving is a chemical change
Dissolving sugar or salt in water is normally a physical change, because the substances can be recovered. Some metals dissolving in acid are chemical changes, because gas and new compounds form.
Mistake 3: Thinking that a color change always means a chemical change
Adding a drop of food coloring to water changes the color but not the chemistry. Only a permanent color change that results from a reaction counts.
Mistake 4: Believing that physical changes cannot be dramatic
A crushed car or an exploding balloon is dramatic but physical. The material itself remains the same.