Combustion: what does a fire need?
Cover a burning candle with jars of different sizes and time how long it burns, then put the flame out by removing oxygen, heat or fuel.
Core concept
Burning needs fuel, oxygen and heat (the fire triangle); take away any one and the fire goes out
Topic: Chemical reactions
Combustion (burning) is a chemical reaction in which a fuel reacts with oxygen: A gas in the air that living things need to breathe and that fire needs to burn. All key words and gives out heat and light. A candle's wax burns to make carbon dioxide: A gas we breathe out. Plants take it in to make their food. All key words and water vapour. A fire needs three things, shown as the fire triangle: fuel, oxygen and heat. If any one is removed the fire goes out. Air is about 21% oxygen. Under a closed jar the candle uses up some of the oxygen and goes out when too little is left, so a bigger jar, holding more air, lets it burn for longer.
What you will learn
- State that burning needs fuel, oxygen and heat.
- Show that a candle burns longer when more air is available.
- Explain how fire extinguishers and fire safety work.
Before you start
- Know that air is a mixture that contains oxygen.
Materials
- Candle on a heatproof tile
- Glass jars of 250 mL, 500 mL and 1000 mL
Equipment
- Candle snuffer, matches and a stopwatch
Procedure
- Cover the candle with the 250 mL jar and record whether it is burning after 5 s and after 20 s.
- Repeat with the 500 mL and 1000 mL jars and find which burns longest.
- Leave the candle open and record.
- Put the flame out three ways: with a snuffer, with water and by pinching the wick.
How this simulation is simplified
- Air is 21% oxygen; the candle goes out when the oxygen in the jar falls to 16%.
- The candle uses the same amount of oxygen each second, so burn time is proportional to the jar's volume.