This classic experiment looks like magic — a candle goes out and water rises inside an upside-down jar. Many people think the water rises because the flame “uses up all the oxygen,” but that explanation is not quite right. This modern version shows the real science behind what’s happening: expanding and contracting air.
Materials
A shallow pan or dish
A short candle
Matches or a lighter (adult use only)
A small piece of aluminum foil
Water
A large, clear glass jar or bottle
(Adult supervision is required for the candle portion.)
Set-Up & Procedure
1. Prepare the Candle
Place a square of aluminum foil in the center of the pan.
Light the candle and drip a little wax onto the foil.
Press the candle into the melted wax so it stands up securely.
2. Add Water
Pour about 1 inch of water into the pan.
3. Cover With the Jar
Light the candle.
Carefully place the jar upside down over the candle, resting the rim in the water.
4. Observe
Watch closely — several things happen in order:
Air bubbles escape from the jar.
The candle flame shrinks and then goes out.
Water rises inside the jar, continuing even after the flame is gone.
This order is the key to understanding what’s really happening!
The Real Science — Explained Clearly
Most people were taught that the water rises because the flame “uses up the oxygen,” but that doesn’t match what we see. Here’s what’s actually happening:
️ Step 1: Hot Air Expands
When the jar is first placed over the candle:
The flame heats the air inside the jar.
Warm air expands, taking up more space.
This pushes some air out into the water — that’s why you see bubbles.
➡️ If oxygen loss were the cause, water would rise immediately. But it doesn’t.
❄️ Step 2: Cooling Air Contracts
Once the air hits the cooler glass inside the jar:
It begins to cool down.
Cool air contracts and takes up less space.
This creates a partial vacuum (lower pressure).
Water is pushed upward into the jar by the higher air pressure outside.
➡️ The rising water begins while the candle is still burning.
Step 3: The Candle Goes Out
As the jar fills with cooler air and water rises:
The flame gets less oxygen.
The candle flame shrinks and goes out.
The air cools even more without the flame’s heat.
➡️ The fastest rising of the water happens after the flame is out — proving that oxygen consumption is not the main cause.
✔ Correct Explanation:
The water rises because of temperature changes, not oxygen loss.
Hot air expands → escapes the jar
Air cools → contracts
Outside air pressure pushes water up into the jar
This is a perfect demonstration of air pressure, not “vanishing oxygen.”
Optional Extensions
For Younger Students
Compare “warm air rises, cool air sinks.”
Draw before/after pictures of the experiment.
For Older Students
Measure how high the water climbs.
Try jars of different sizes.
Chart the sequence of events: bubbles → shrinking flame → rising water.
