All lessons › Fire, fuel and change › Lesson 3
Australian primary science · Year 6 · Lesson 3 of 3
Break the fire triangle
Inquiry question: What happens to a flame when we take away its oxygen?
Check practical materials
Observing
What students learn
A flame needs fuel, heat and oxygen. Covering it or surrounding it with carbon dioxide cuts off oxygen and puts it out. Melting wax is reversible, while burning wax makes new substances.
Success criteria
- I can predict, then observe, what happens to a covered flame.
- I can explain my observations using the fire triangle.
- I can identify reversible and irreversible changes in a burning candle.
Preparation and materials
Before this lesson: Students can name the fire triangle and explain that burning is irreversible (lessons 1 and 2).
- Tea-light candles — 3–4
- Long matches or a long-nosed lighter (teacher only) — 1
- Baking tray with a layer of sand or damp soil (candle stands in it) — 1
- Two glass jars, one large (about 1 L) and one small (about 250 mL) — 1 each
- Bicarbonate soda (baking soda) — 2 tablespoons
- White vinegar — 1 cup
- Tall jug or large measuring jug — 1
- Fire blanket or a bucket of water, within reach — 1
- Safety glasses for the teacher — 1 pair
- Timer — 1
Teacher setup
- Check your school or department’s rules and risk assessment requirements for open flames in classrooms.
- Clear a bench away from paper, curtains and the doorway; place the tray of sand on it.
- Set students in a semicircle at least 2 metres back, so all can see.
- Have the fire blanket or bucket of water within reach.
- Print one investigation record per pair.
Safety and adaptations
- Teacher demonstration only: students do not light, touch or move candles.
- Students stay at least 2 metres back; tie back long hair if moving closer is ever needed.
- Keep a fire blanket or water bucket within reach.
- Jars get hot: let them cool before touching.
- Do not use the jar-over-water version that makes water rise; it is often explained incorrectly.
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.
- Demo 1: the teacher lights a tea light and places the large jar over it. Students time how long until it goes out. Repeat with the small jar on a fresh candle.
- Demo 2: the teacher puts 2 tablespoons of bicarb soda in the tall jug and adds vinegar. Students observe the fizzing (a chemical reaction making carbon dioxide gas).
- After the fizzing settles, the teacher tilts the jug over a lit tea light, pouring only the invisible gas, not the liquid. Students observe.
- Demo 3: students observe the melted wax pool in a candle that has been blown out. Does it go hard again?
- Pairs record predictions, observations, which side of the fire triangle was removed, and whether each change was reversible or irreversible.
Teacher modelling prompt
Light one tea light (teacher only). Think aloud: “Fuel: the wax. Heat: the flame. Oxygen: from the air. Before I cover it, everyone writes a prediction: will it go out, and how fast?” Model writing a prediction with a reason.
Record and explain evidence
Recording question: What happens to a flame when we take away its oxygen?
| Demo | Our prediction | What we observed | Side of the fire triangle removed | Reversible or irreversible change? |
|---|---|---|---|---|
| ________________ | ________________ | ________________ | ________________ | ________________ |
Example recording headings; record actual observations, including inconclusive results.
Discussion prompt
What did we observe that helps decide?
Check understanding
What should pupils know? Why does a candle under a jar go out?
Explain with evidence: Explain why a carbon dioxide extinguisher can put out a fire.
Apply the idea: Name one reversible and one irreversible change you saw today, with evidence.
Teacher answer guidance and saved classroom work are available through the main lesson experience where supported.