Physical & Chemical Changes
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
Physical Changes
A physical change is a change in which no new substance is formed - only the physical appearance or state changes.
- No new substances are formed
- Chemical composition stays the same
- Usually easily reversed
| Type of change | Examples |
|---|---|
| Change of state | melting ice, boiling water, condensing steam |
| Dissolving | sugar or salt dissolving in water |
| Changing shape/size | cutting paper, crushing rock, bending metal |
| Mixing | mixing sand and salt (no new substance - can be separated again) |
Chemical Changes
A chemical change (chemical reaction) forms one or more new substances with different properties from the starting substances.
- New substance(s) are formed
- Chemical composition changes
- Usually difficult or impossible to reverse
| Type of change | Examples |
|---|---|
| Combustion | burning wood or a candle - forms CO₂, H₂O, ash |
| Rusting | iron + oxygen + water → iron(III) oxide |
| Reacting | acid + metal, neutralisation - new products (salts, gases, water) |
| Decomposition | CaCO₃ → CaO + CO₂ |
Observable Signs of a Chemical Change
- Colour change - e.g. copper turning green as it corrodes
- Gas produced - bubbling/fizzing, e.g. acid + carbonate
- Precipitate forms - a solid appears in a solution
- Temperature change - energy released or absorbed
- Light produced - e.g. combustion
- Difficult to reverse - the original substances are hard to recover
Comparing Physical and Chemical Changes
| Aspect | Physical change | Chemical change |
|---|---|---|
| New substance formed? | No | Yes |
| Reversible? | Usually yes, easily | Usually no, or only with difficulty |
| Bonds | no bonds broken or made | bonds broken and new bonds formed |
| Examples | melting, dissolving, cutting | burning, rusting, reacting with acid |
Worked Example: Physical or Chemical?
- Question: classify each change as physical or chemical, giving a reason. (a) dissolving salt in water (b) burning sugar (c) magnetising an iron nail
- Answer (a): physical - the salt still exists as Na⁺ and Cl⁻ ions in solution and can be recovered unchanged by evaporation
- Answer (b): chemical - new substances (carbon and water vapour) are formed; the change cannot be reversed
- Answer (c): physical - the nail is still iron chemically; only its magnetic property has changed
Common Mistakes
- ❌ Assuming melting/dissolving must be chemical because the substance "disappears"
- ✓ if the original substance can be recovered unchanged (e.g. by evaporating water, or cooling a melt), no new substance has formed - it is physical
- ❌ Assuming a colour change always means a chemical reaction
- ✓ some colour changes are physical and reversible (e.g. anhydrous cobalt(II) chloride turning pink when it absorbs water is itself a reversible reaction, not a permanent chemical change)
Mass is always conserved in both physical and chemical changes - see the mole and equation-balancing topics in the Stoichiometry section for how this is calculated.
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