Science labs › Chemistry › Combustion
Put a jar over a candle. Why does the flame go out, and when?
Combustion lab · Last updated: 16 October 2026
You put a jar over a burning candle and watch the flame shrink and go out. They find out why it happens, when it happens and what changes if the jar is bigger. It is a classic experiment that shows what fire needs, and it explains how fire blankets and extinguishers work.
What you'll discover
- Burning needs three things: fuel, oxygen and enough heat.
- Inside a closed jar the flame uses up oxygen near it and fills the jar with carbon dioxide and water vapour, so it goes out.
- A bigger jar holds more oxygen, so the candle keeps burning for longer.
The science in plain words
Combustion is a chemical reaction in which a fuel combines with oxygen and releases energy as heat and light. Candle wax is made mostly of compounds of carbon and hydrogen. The flame's heat melts the wax, the liquid wax climbs the wick and turns into vapour, and it is this vapour that burns. The carbon and hydrogen combine with oxygen to form carbon dioxide and water vapour.
A fire needs three things at once: fuel, oxygen and enough heat to keep the reaction going. Scientists draw these as the fire triangle. Remove any one side and the fire goes out. Covering a candle with a jar cuts off the supply of fresh air. As the flame burns, the air around it holds less and less oxygen and more carbon dioxide and water vapour. The flame goes out well before all the oxygen in the jar is used up, because once the oxygen around the flame falls low enough, the reaction cannot release heat fast enough to keep itself going.
A bigger jar holds more air, so the candle burns for longer. Fire blankets and sand smother flames in the same way, by stopping oxygen reaching the fuel. Water mainly works by cooling the fuel below the temperature at which it can burn.
Where it fits
Common questions
Why does a candle go out when covered with a glass?
The glass stops fresh air reaching the flame. As the candle burns, the air inside holds less oxygen and more carbon dioxide and water vapour. When the oxygen near the flame falls too low, the flame cannot produce enough heat to keep burning and goes out.
What are the three things a fire needs?
A fire needs fuel, oxygen and heat. These three make up the fire triangle. Taking away any one of them puts the fire out: shutting off the fuel, smothering the flames to block oxygen, or cooling the fuel with water below the temperature at which it burns.
What does a candle produce when it burns?
A burning candle mainly produces carbon dioxide and water vapour, along with heat and light. If there is not enough oxygen, it also makes soot, which is tiny bits of unburnt carbon, and some carbon monoxide. Soot glowing in the flame gives it its yellow colour.
Related labs
- Chemical Reactions: Molecules must crash hard enough to react. What makes that happen more often?
- Redox Reactions: Two metals and two solutions make a battery. Where do the electrons come from?
- Carbon Cycle: When we burn fuel, where does the carbon go? And how much of it stays in the air?
See all chemistry labs or every Taz science lab.
Try it yourself
The Combustion lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.