All lessons › Challenge: Particle puzzles › Lesson 3
Australian primary science · Years 5–6 (challenge) · Lesson 3 of 3
Physical or chemical? You be the judge
Inquiry question: What is the best evidence that a new substance has formed?
Check practical materials
Observing and testing
What students learn
In a physical change, the particles stay the same and no new substance forms. In a chemical change, atoms rearrange to make new substances. Fizzing, colour change and temperature change are clues, but we need several clues and a reason before we decide.
Success criteria
- I can describe the difference between a physical and a chemical change.
- I can list clues of a chemical change, including a temperature change.
- I can explain why one clue on its own is not enough to decide.
Preparation and materials
Before this lesson: Students can sort changes as reversible or irreversible (Change detectives) and know what a compound is (lesson 2).
- Bicarbonate soda (bicarb), in a labelled cup with a teaspoon — 1 box, shared (1 level teaspoon per pair)
- White vinegar, in a labelled jug — about 1 litre (about 50 mL per pair)
- Table salt — 1 small packet (1 teaspoon per pair)
- Ice cubes — 1 tray (2 cubes per pair)
- Clear plastic cups — 39 (3 per pair)
- Tap water — about 1 litre
- Zip-lock sandwich bags — 13
- Paper towels and a tray per pair — 13
- Safety glasses, if your school has them — 26
Teacher setup
- Put bicarb, vinegar and salt in labelled containers at one station per group of tables.
- Fill a cup of ice just before the lesson.
- Try the bicarb and vinegar bag yourself first so you know how fast it fizzes.
Safety and adaptations
- Never taste anything in a science lesson, even kitchen ingredients.
- Vinegar stings if it splashes in your eyes. Wear safety glasses if you have them, and keep cups low on the table.
- Don’t seal the bicarb and vinegar bag completely: leave a gap so the gas can escape and the bag can’t burst.
- Tell the teacher straight away about any splash or spill.
Review your school policies and curriculum requirements before teaching.
How to investigate
The lesson sequence should leave pupils with evidence to interpret, not only an activity to finish.
- Write a hypothesis before each test: “If we…, then…, because…”.
- Test 1: put a teaspoon of salt in a cup of water and stir. Record clues.
- Test 2: put two ice cubes in a dry cup and hold it. Record clues after 5 minutes.
- Test 3: put a teaspoon of bicarb in a bag, add about 50 mL of vinegar, close it most of the way and feel the bag. Record clues.
- For each test, judge: physical or chemical? Give at least two pieces of evidence and say how sure you are.
Teacher modelling prompt
Hold up an ice cube in one hand. Think aloud: “It feels cold, and it is changing. Is that a chemical change? The water is still water, just liquid now. So ‘getting cold’ alone can’t prove a chemical change.” Show the class how to put a level teaspoon of bicarb in a bag, add vinegar, and close the bag most of the way.
Record and explain evidence
Recording question: What is the best evidence that a new substance has formed?
| Test | My hypothesis | Clues I noticed (gas, colour, temperature, smell, solid) | Physical or chemical? | How sure am I, and why? |
|---|---|---|---|---|
| ________________ | ________________ | ________________ | ________________ | ________________ |
Example recording headings; record actual observations, including inconclusive results.
Discussion prompt
Who has the better evidence? What would make you more sure?
Check understanding
What should pupils know? What is the difference between a physical change and a chemical change?
Explain with evidence: The ice and the bicarb bag both got cold. Why is that not enough to call both chemical changes?
Apply the idea: A cake rises and turns brown in the oven. Is baking a physical or chemical change? Give two clues.
Teacher answer guidance and saved classroom work are available through the main lesson experience where supported.