Physical vs Chemical Changes
Physical changes are reversible with no new substances. Chemical changes produce new substances and are usually irreversible.
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substances? | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Energy change | Small | Often large |
| Examples | Melting, dissolving, cutting, boiling | Burning, rusting, electrolysis, neutralisation |
Evidence a chemical reaction has occurred:
- Gas produced (effervescence)
- Precipitate formed (solid in solution)
- Temperature change (exothermic or endothermic)
- Colour change
- New smell/odour
- Light emitted
Law of conservation of mass: In any chemical reaction, the total mass of reactants = total mass of products. Atoms are rearranged but not created or destroyed.
Why mass sometimes appears to change in an open container:
- Gas escapes (e.g. burning — mass decreases as CO₂ and H₂O vapour escape)
- Gas is absorbed (e.g. magnesium burning — mass increases as O₂ from air is incorporated)
- In a closed container, mass always stays the same.
- Physical: no new substances, reversible. Chemical: new substances, usually irreversible.
- Evidence of chemical change: gas, precipitate, temp change, colour change.
- Conservation of mass: atoms rearranged, never created or destroyed.