Physical vs chemical change
Physical: nothing new. Chemical: new substance. Look for the give-aways.
Physical change. A change of state, shape, or form WITHOUT producing a new substance.
- Melting (ice → water), boiling (water → steam), dissolving sugar in tea.
- The substance can usually be RECOVERED by reversing the change (re-freezing the water; evaporating the tea to get sugar back).
- No new chemical bonds formed or broken (in most cases).
Chemical change (chemical reaction). Produces ONE OR MORE NEW SUBSTANCES with different properties from the starting materials.
- Burning, rusting, baking a cake, photosynthesis.
- Bonds are broken and formed. Atoms rearrange.
- Often hard to reverse (would need another chemical reaction).
Evidence of a chemical change.
- Gas evolved (bubbles, fizz, smell).
- Precipitate formed (insoluble solid appears in solution).
- Temperature change (warms or cools).
- Colour change (esp. when no dilution effect).
- Light or sound (e.g. magnesium burning brightly; sodium-water fizzing).
- Often a new smell.
Tip. Even one of these signs strongly suggests a chemical change. Two or more is essentially conclusive.
- Physical: no new substance.
- Chemical: new substance with new properties.
- Evidence: gas, precipitate, , colour, light/sound, smell.
- Atoms rearrange in chemical changes; bonds break/form.