- Age:6+
- Time:10 min
- Difficulty:Easy
- Mess level:Low
- Supervision:Yes
The Science Behind the Candle Vacuum Experiment
Let’s talk about the candles. Why does a candle burn? Well, you know that it won’t start burning by itself - we must supply some energy. And that’s precisely what we do when we use a lighter or matches to light the candle. The chemical reaction that occurs is called combustion. Wax from the candle reacts with oxygen to produce energy - heat and light.
As a product of that reaction, carbon dioxide forms as well as some steam and of course smoke. Smoke is actually a bunch of small carbon particles that didn’t burn in the process, mixed with the steam.
Okay, so we learned that to sustain fire we need oxygen. When we cover our candle with the glass we are cutting off the oxygen supply. When the air in the glass is depleted of oxygen - lights out!
But why did the water rise? Heat from the candle made the air inside the candle quite hot and hot air expands to take more space. When the flame is out, the air starts to cool and that means it’s shrinking again. That produces a partial vacuum - the area of low pressure. The pressure outside of the glass (atmospheric pressure) is now higher than that in the glass. So water inside the glass quickly rises to make up the difference. Equilibrium!
Materials
A candle, a plate, a glass and some colored water are all you need.
All you need is some water, a glass, a plate, a candle, and a lighter.
- Lighter
- Candle
- Water (with some food coloring for the better effect)
- Plate
- Glass
Instructions for the Candle Vacuum Experiment
For step-by-step instructions on how to conduct this experiment watch the video at the beginning of the article or continue reading...
👨👧 Adult supervision needed
This experiment uses an open flame, so an adult should light the candle and stay close the whole time. Keep hair, sleeves, and anything flammable well away, and have the water right there - it's part of the experiment and doubles as a way to put the flame out.
- Put the candle in the middle of the plate and light it
- Pour the colored water (around 2 dl) on the plate so it surrounds the candle
- Cover the candle with the glass
- Watch carefully!
What will happen when you cover the burning candle with the glass?
Make your prediction, then tap an answer to check!
What Will You Develop and Learn
- Chemistry principles
- What are the atmospheric pressure and partial vacuum
- How to use the scientific method
Key takeaways
- A candle flame needs oxygen to keep burning - cover it and it soon goes out.
- Burning is a chemical reaction called combustion: wax plus oxygen gives off heat and light.
- The flame heats the trapped air, which expands; once the flame dies the air cools and contracts.
- The shrinking air makes a partial vacuum (low pressure) inside the glass.
- Higher atmospheric pressure outside then pushes the water up into the glass until the pressures are equal.
Frequently Asked Questions
Why does the candle go out under the glass?
A flame needs a steady supply of oxygen to keep the combustion reaction going. Once you trap the candle under the glass, it uses up the available oxygen and no fresh air can get in, so the flame quickly goes out.
Why does the water rise into the glass?
Mostly because of cooling air. The flame heats the air inside the glass and makes it expand; when the flame goes out, that air cools and contracts, creating a partial vacuum. The higher atmospheric pressure outside then pushes water up into the glass until the pressures are equal.
Isn't the water rising just because the candle used up the oxygen?
That's the popular explanation, but it only accounts for a small part of the rise. The oxygen that's consumed is largely replaced by carbon dioxide and water vapor, so the volume change from burning is minor. The big effect is the hot air cooling and shrinking after the flame dies.
Does this experiment create a real vacuum?
No - it creates only a partial vacuum, meaning the pressure inside the glass drops a little below the pressure outside. A true (complete) vacuum would need special pumps to remove nearly all the air.
What do I need for the candle vacuum experiment?
Just a candle, a plate, a glass, a lighter, and some water. Adding a few drops of food coloring to the water makes the rising level much easier to see.
Is the candle vacuum experiment safe for kids?
Yes, with adult supervision. The only hazard is the open flame, so an adult should light and watch the candle. The surrounding water is right there to put the flame out if needed.
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.
