While your child may not understand completely the real science behind this experiment, it is still an awesome science activity to do to spark their interest in STEM!
As always, adult supervision is required (THERE IS FIRE INVOLVED).
What you need
- Fill the dish or container with water to about 1 inch deep.
- Add food coloring into the water for better visual results (optional).
- Place a candle in the middle of the water. Make sure the water is at least an inch below the top of the candle.
- Light the candle.
- Invert the tall glass.
- Slowly and vertically, lower the glass to completely cover the candle.
- Watch the candle extinguish and water level rise.
Initially, you could see the water level rising a little bit while the candle was burning. But after the candle had gone out, the water level kept rising for almost an inch above the water level outside of the glass and stayed there.
So what do you think happened?
The first thought in most people’s minds is that the candle burned and used up oxygen. There was less oxygen left in there.
However, when the candle burned, it also gave out carbon dioxide, which was another gas that could replace the volume previously occupied by oxygen. Shouldn’t the volume have stayed the same then?
To figure out what happened, it helps to understand a little bit of chemistry.
2 O2 + C H 4 = C O 2 + 2 H 2 O
As you can see, during burning, two oxygen molecules result in one carbon dioxide molecule and two water molecules. So the air volume decreases.
But if this was the main cause, shouldn’t the water level have stopped rising after the candle fire went out?
Turns out that besides chemistry, there is also a physical aspect that causes a big change in air volume.
After the candle went out, the air cooled. Cold air is more dense and occupies a smaller volume than hot air. At the same time, water vapor (result of burning) condensed when the air cooled further reducing air volume inside the glass. Therefore, the water level rose even after the fire was out.
Another physical aspect contributing to the phenomenon was that the air surrounding the fire was initially hot before the glass was placed around it. So we started with air that was already less dense. Starting with less air than the outside added to the shrinkage in volume when the air cooled.
For the science-inclined minds, here is a detailed technical and scientific analysis of this experiment.
See, you can learn so much about science from one little experiment. Isn’t it amazing?