Two kinds of change
Matter changes all the time. Ice melts, bread toasts, bikes rust and fireworks explode. Scientists sort these changes into two groups:
- A physical change alters form, size or state, but the substance stays the same.
- A chemical change produces a new substance with new properties.
Physical changes
In a physical change, the particles are rearranged or move differently, but they don’t change into different particles.
| Physical change | What happens |
|---|---|
| Melting chocolate | Solid → liquid; still chocolate |
| Crushing a can | Shape changes; still aluminum |
| Dissolving salt in water | Salt spreads out; can be recovered by evaporation |
| Boiling water | Liquid → gas; still H₂O |
| Sanding wood | Smaller pieces of wood |
| Stretching a rubber band | Shape changes temporarily |
Physical changes are often reversible, though not always (you can’t un-crush a can perfectly).
Chemical changes
In a chemical change, atoms rearrange to form new substances. The starting substances are reactants; the new substances are products.
reactants → products
Example: iron + oxygen → iron oxide (rust)
Evidence of a chemical change
| Clue | Example |
|---|---|
| Colour change | A copper roof turns green |
| Gas produced (unexpected bubbles) | Baking soda and vinegar fizz |
| Formation of a precipitate | Two clear solutions make a cloudy solid |
| Heat or light given off | A burning match |
| Heat absorbed (gets cold) | An instant cold pack |
| New odour | Rotting eggs |
| Hard to reverse | Cooked egg can’t be uncooked |
One clue alone isn’t proof. Boiling water bubbles, but that’s physical. Look for several clues.
Physical and chemical properties
| Physical properties (observed without changing the substance) | Chemical properties (how it reacts) |
|---|---|
| Colour, texture, density | Flammability (ability to burn) |
| Melting and boiling point | Reactivity with water |
| Hardness | Reactivity with acids |
| Solubility | Tendency to rust (oxidize) |
| Conductivity | Toxicity |
A chemical property can only be observed by trying to change the substance. You find out whether paper is flammable by trying to burn it.
Common chemical changes
- Combustion: burning. Methane (natural gas) + oxygen → carbon dioxide + water + energy.
- Oxidation: rusting of iron, browning of apples.
- Decomposition: hydrogen peroxide breaking down into water and oxygen.
- Fermentation: yeast converting sugar into alcohol and carbon dioxide.
- Cooking: proteins in eggs change permanently when heated.
- Photosynthesis: plants turning carbon dioxide and water into sugar and oxygen.
The particle view
| Physical change | Chemical change | |
|---|---|---|
| Particles | Same particles, rearranged or moving differently | Atoms regroup into new particles |
| Bonds | Bonds within molecules unchanged | Bonds broken and new bonds formed |
| New substance? | No | Yes |
| Mass | Conserved | Conserved |
Why it matters
Understanding chemical changes helps us:
- prevent rust by painting metal or coating it with zinc (galvanizing)
- slow food spoilage with refrigeration
- cook food safely
- design safer batteries and fuels
Common mistakes
- Treating dissolving as chemical. Sugar dissolving is physical.
- Thinking any colour change means a chemical change. Mixing paints is physical.
- Forgetting that chemical changes can absorb heat, not just release it.
Try it yourself
- Put a clean iron nail in salt water for a week. Describe and classify the change.
- Mix milk with a little vinegar. Is the solid that forms a sign of a chemical change?
- Classify: frying an egg, chopping onions, a sparkler burning, ice cream melting, bleach removing a stain.
(Answers: rust, chemical; yes, a precipitate (curds) forms; chemical, physical, chemical, physical, chemical)
Quick recap
- Physical changes alter form or state; no new substance forms.
- Chemical changes make new substances with new properties.
- Clues include colour change, gas, precipitate, heat, light and new odours.
- Chemical properties describe how a substance reacts.
- Mass is conserved in both types of change.