- Age:6+
- Time:10 min
- Difficulty:Easy
- Mess level:Low
- Supervision:Yes
🧒 In one sentence
A candle changes the pressure inside the glass. When that pressure drops, the stronger air pressure outside pushes water up into the glass.
Materials for the candle and rising-water experiment
Use a stable tea light, shallow plate, heat-safe glass, and a little colored water.
- One small candle or tea light with a flat base
- One shallow, heat-resistant plate with a flat base
- One clear glass or jar that fits over the candle with room above the flame
- Water, enough to cover the plate by roughly 5–10 mm
- Optional food coloring to make the water level easier to see
- Matches or a lighter, for adult use only
Adult demonstration only
Use a real heat-safe glass or jar, never a plastic cup. Work on a clear, heat-resistant surface away from curtains, paper, loose sleeves, and hair. An adult should light the candle, lower the glass, and remain beside it until the flame is out. Do not press down on the glass, touch it while warm, or use chipped or cracked glass. Keep a nonflammable lid nearby to smother the flame if needed, and let everything cool before resetting the experiment.
What will happen when you cover the burning candle with the glass?
Make your prediction, then tap an answer to check!
Instructions
- Put the plate on a stable, heat-resistant surface. Place the tea light in the center.
- Pour in a shallow layer of colored water, about 5–10 mm deep. Keep water out of the wick and candle cup.
- Ask an adult to light the candle. Watch the flame for a few seconds.
- The adult should turn the heat-safe glass upside down and lower it slowly over the candle until its rim rests in the water. Do not press the rim hard against the plate.
- Watch the flame, the edge of the glass, and the water level. The flame should dim and go out; the water will then rise inside the glass.
- Leave the glass in place until it has cooled. The adult can then lift it and dry the work area.
What should you observe?
- The candle continues burning briefly after the glass is lowered.
- You may see a few bubbles escape beneath the rim as the gas inside warms and expands.
- The flame becomes smaller and goes out when the remaining oxygen concentration is too low to sustain combustion.
- Most of the visible water rise happens as the gas inside cools after the flame goes out.
- Moisture may appear on the inside wall because water vapor from combustion condenses as it cools.
Why does the water rise under the glass?
A candle burns through combustion. Liquid wax travels up the wick, becomes a vapor, and reacts with oxygen. Complete combustion produces carbon dioxide and water vapor while releasing heat and light. A smoky flame also produces tiny soot particles when some fuel does not burn completely.
The inverted glass traps a limited amount of air. The flame consumes some oxygen, but it does not remove all the oxygen or leave an empty space. It goes out when the oxygen concentration becomes too low for that flame to keep burning.
At the same time, the flame heats the gas under the glass. The warm gas expands, and some may escape under the rim as bubbles. After the flame goes out, the remaining gas cools and some water vapor condenses on the glass. Those changes lower the pressure inside compared with the atmospheric pressure outside.
The outside atmosphere then pushes more strongly on the exposed water than the trapped gas pushes from inside. That pressure difference drives water into the glass until the pressures balance.
Why this is not an oxygen-measuring experiment
It is tempting to say that the water simply replaces oxygen used by the candle. That explanation is incomplete: combustion creates new gases, warm gas can escape before the flame goes out, the remaining gas cools, and water vapor condenses. Because several processes change the pressure, the final water level does not measure the percentage of oxygen in air.
The result is often called a “vacuum” experiment, but the glass contains plenty of gas. Partial vacuum is shorthand for pressure that is lower than the pressure outside, not a completely empty space.
The same pressure imbalance can do something much more dramatic: in the can crush experiment, cooling steam leaves such low pressure inside a soda can that the outside air crushes it almost instantly.
🔬 Make it a real experiment
Mark the starting and final water levels and time how long the flame lasts. After every item has cooled, repeat with a differently sized heat-safe jar while keeping the candle and water depth the same. Predict what will change, record the result, and remember that jar size can affect both the available oxygen and how much the trapped gas warms.
What Will You Develop and Learn
- How combustion uses oxygen and produces new substances
- How a difference in air pressure can move water
- What scientists mean by a partial vacuum
- How to predict, observe, measure, and compare results safely
Key takeaways
- A candle flame needs oxygen; under a glass, the oxygen level eventually becomes too low to sustain combustion.
- Burning wax produces carbon dioxide, water vapor, heat, and light.
- Warm gas expands and some can escape beneath the rim. After the flame goes out, the remaining gas cools and water vapor condenses.
- Together these changes create lower pressure inside the glass than outside it.
- Higher atmospheric pressure outside pushes water into the glass until the pressures balance.
- The water level does not directly show how much oxygen was in the air, because several processes contribute to the pressure change.
Frequently Asked Questions
Why does the candle go out under the glass?
A flame needs a sufficient concentration of oxygen to keep combustion going. The glass prevents fresh air from entering, so the flame consumes oxygen until the concentration becomes too low to sustain it. Some oxygen still remains when the flame goes out.
Why does the water rise into the glass?
Several effects lower the pressure inside: heated gas expands and some escapes beneath the rim, the remaining gas cools after the flame goes out, and water vapor condenses. Higher atmospheric pressure outside then pushes water into the glass until the pressures balance.
Isn't the water rising just because the candle used up the oxygen?
That is a popular explanation, but oxygen consumption is only one contributor. Combustion also produces carbon dioxide and water vapor, while heating, escaping gas, cooling, and condensation all affect the pressure. The experiment therefore cannot be used to measure the oxygen percentage in air.
Does this experiment create a real vacuum?
No. It creates a partial vacuum, meaning the pressure inside is lower than the pressure outside. The glass still contains gas; creating a near-complete vacuum requires specialized equipment.
What do I need for the candle vacuum experiment?
Use a stable tea light, shallow heat-resistant plate, clear heat-safe glass or jar, water, and an adult-operated lighter. Food coloring makes the changing water level easier to see. Never place a plastic cup over the flame.
Is the candle vacuum experiment safe for kids?
Treat it as an adult-led demonstration rather than an independent children's activity. An adult must control the flame and glass, keep flammable items away, and let the glass cool before touching it. Use heat-safe glass, never plastic, and do not use chipped or cracked glass.
If you liked this experiment and want even more cool stuff to do, we recommend making a lava lamp or growing your own sugar crystals. For more physics you can see with your own eyes, try our inertia demonstration.
Happy exploring!
