What Is a Physical Change?
A physical change is a change to a substance that does not produce a new substance. The material is the same chemical before and after the change — only the form, shape, or state has changed.
Examples of physical changes:
- Melting ice (water → liquid water: still H₂O)
- Cutting paper smaller (still paper)
- Dissolving sugar in water (sugar is still sugar)
- Bending a metal rod (still the same metal)
- Boiling water (still H₂O)
Physical changes are usually reversible — you can often get the original substance back.
What Is a Chemical Change?
A chemical change (also called a chemical reaction) produces one or more new substances with different properties from the starting materials. The atoms are rearranged to form new bonds.
Examples of chemical changes:
- Burning wood → ash, carbon dioxide, water vapour
- Iron rusting → iron oxide (rust)
- Baking a cake → new compounds formed by heat
- Adding acid to a carbonate → carbon dioxide gas produced
Chemical changes are usually difficult or impossible to reverse.
The law of conservation of mass says that in a chemical reaction, the total mass of the products always equals the total mass of the reactants. Atoms are not created or destroyed — they are only rearranged. This was established by the French chemist Antoine Lavoisier in the 18th century.
Signs of a Chemical Change
You can often tell that a chemical change has occurred if you observe one or more of these:
- Colour change — a new substance with a different colour is formed.
- Gas produced — bubbles appear (not from boiling).
- Light or heat given off or absorbed — an energy change occurs.
- Precipitate formed — a solid appears when two solutions are mixed.
- Smell — a new, often strong odour is noticed.
- Difficult to reverse — the change cannot easily be undone.
Dissolving sugar in water is a physical change even though you can no longer see the sugar — it is still chemically sugar (sucrose, C₁₂H₂₂O₁₁) dissolved in water. You can recover the sugar by evaporating the water. If the sugar burns instead, it becomes a new substance — that is a chemical change.
Comparing Physical and Chemical Changes
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | No | Yes |
| Reversible? | Usually | Usually not |
| Energy change | Often small | Often large |
| Examples | Melting, dissolving, cutting | Burning, rusting, cooking |
Energy in Reactions
Chemical reactions involve energy:
- Exothermic reactions release heat to the surroundings (combustion, respiration).
- Endothermic reactions absorb heat from the surroundings (cooking an egg, photosynthesis).
Real world: Burning toast in a toaster
When bread is toasted too long and turns black, that is a chemical change — new carbon-rich compounds form, the bread becomes brittle and bitter, and you cannot reverse it back to soft bread. Melting butter on warm toast is a physical change: the butter simply changes state (solid to liquid) and could solidify again if cooled. Both happen on the same plate but are fundamentally different types of change.
Real world: Baking a cake
Mixing flour, eggs, and sugar is a physical mixture, but putting the batter in the oven triggers chemical changes: proteins in the egg denature and set, baking powder decomposes releasing CO₂ gas (making the cake rise), and Maillard reactions create hundreds of new flavour compounds in the golden crust. The cake cannot be "un-baked" — all signs of an irreversible chemical change.
Real world: Cutting and squeezing fruit
Cutting an orange into pieces is a physical change — the orange is still chemically the same; only its shape changed. But leaving the cut orange in the air causes oxidation (a chemical change) — the flesh turns brown as enzymes react with oxygen, forming new coloured compounds. The shape change (physical) and the browning (chemical) are completely different processes.
Where do you see this in real life? Melting ice in a drink (physical), burning a match (chemical), dissolving sugar in tea (physical), cooking an egg (chemical) — chemical changes form new substances and cannot easily be reversed, while physical changes are usually reversible.
Practice Problems
Is dissolving salt (sodium chloride) in water a physical or chemical change? Give a reason.
Show answer
A student observes that when a white powder is heated, it turns black and gives off a gas that puts out a glowing splint. Is this a physical or chemical change? Give two pieces of evidence.