Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- A physical change alters properties such as state, shape or mixing, but every substance keeps its composition.
- Common physical changes: phase changes (melting, boiling, sublimation and their reverses) and making or separating mixtures (dissolving, filtering, distilling).
- A chemical change forms new substances. Atoms are kept but rearranged into new combinations.
- Four common signs of a chemical change: heat or light, a gas, a precipitate, a colour change.
Key relationships
| Sign | Points to a reaction when… | Can be faked by… |
|---|---|---|
| Heat or light | energy is released as a new substance forms | dissolving NaOH (hot) or NH₄NO₃ (cold) |
| Gas | the gas is a new substance (e.g. CO₂ turns limewater cloudy) | boiling; dissolved gas escaping |
| Precipitate | two solutions mix and an insoluble product forms | crystals forming as a saturated solution cools |
| Colour change | a new colour appears | dilution making a colour paler |
Assumptions behind the evidence
- One observation is a clue. The deciding question is: has a substance with a different composition formed?
- A reaction can give no visible sign (strong acid + strong base: only the temperature rises).
- Mass is conserved in a closed container. An open container loses mass only if a gas escapes.
Mistakes to avoid
- “Temperature changed, so it reacted.” Dissolving and phase changes also transfer energy.
- Calling dilution a colour change. A paler shade of the same colour is not a new substance.
- Claim with no reasoning. Link the evidence to a new substance forming.
- Using reversibility as the test. Composition decides, not how easy the change is to undo.
- Saying atoms are lost when an open flask gets lighter. The gas has left the flask.
Quick self-check
- Is distilling sea water physical or chemical? (Physical: it separates a mixture.)
- Name a physical change that produces bubbles. (Boiling, or gas escaping from fizzy water.)
- A sealed bag containing baking soda and vinegar weighs 64.30 g before mixing. What does it weigh afterwards? (64.30 g: no gas can escape. Ignore the tiny buoyancy effect as the bag swells.)
- Why is a precipitate good evidence? (The solid is a substance that was not present in either solution.)
Next: practice questions.