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Why Does the Water Rise? You’ll have to watch closely...
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Why Does the Water Rise?

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Rating:4/5 (5 Reviews)

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You’ll have to watch closely and use everything that you know about air in order to explain the mystery of the rising water.

Materials

- Candle and matches
- Pie pan or dish
- Juice bottle
- Water
- Food coloring


This experiment requires the use of matches... and that means adult supervision.
  1. Fill the pie pan 3/4 of the way with water and add 3 drops of food coloring. Place the candle in the middle of the pan.
  2. Light the candle. Think about what a candle needs to burn.
  3. Cover the candle with the jar. What invisible thing is inside the bottle? Carefully observe what happens to the water level in the bottle. What happen to the candle flame?
  4. Repeat the experiment several times until you can write down or draw a picture that explains why the water level rises.

How does it work?

The candle needs oxygen to burn. When you cover the burning candle with the jar, the flame eventually goes out as soon as all of the oxygen is used up. Since there is no more oxygen under the cup, the rest of the gases (nitrogen, argon, carbon dioxide, and others) are exerting less pressure compared to the atmospheric air. The greater atmospheric pressure on the outside of the bottle pushes the water in the pan up into the bottle.

However, there is another important factor that accounts for the rise in water level. The candle flame heats the air in the bottle, and this hot air expands. Some of the expanding air escapes out from under the bottle — you might see some bubbles. When the flame goes out, the air in the jar cools down and the cooler air contracts. The contraction of the air draws the water up into the bottle.

Hot air and CO2

Garry Taree Australia   -  December 19, 2009

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This user gave 4/5 stars


The particle counts O2 for CO2 mean there is no change in total gas particles, BUT! BUT! (Did you see the BUT)? CO2 is soluble, so it disolves and so reduces the number of gas particles present. As A science teacher I learned quickly that most of the rise is due to hot air expanding, then contracting. I was taught (perhaps dishonestly) that it was the O2 getting used up. The technique is to hold the container over the candle and discuss the predictions, in this way the air is heated and no bubbles are produced, so no pescky questions from observant skeptics. I never use this experiment without discussing the hot air aspect.

why does the water rise

domanek chicago   -  October 20, 2009

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This user gave 5/5 stars


what field of scince is this in (for ex.:chemistry,botany,geology,..ect

Befuddled

Welkinator Lago Vista, TX   -  October 20, 2009

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This user gave 5/5 stars


I too am not satisfied with the standard explanation. If it were a matter of replacing oxygen with CO2 then the water should rise gradually during combustion. If the rise were due to cooling then again the rise should be gradual as the gasses do not cool as rapidly as observed rise of water. I do agree the the ultimate answer must be related to differential pressure on the surface of the water. ...welkinator

partical size

Michael Pascoe Portsmouth - England   -  October 7, 2009

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This user gave 5/5 stars


When the candle burns, doesn't it use the oxygen to make CO2, so the number of particals in the air effectively stays the same, also, aren't CO2 particals bigger so take up more room?

Explanation not quite right

Dan Taylor Waltham, MA, USA   -  September 28, 2009

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This user gave 5/5 stars


My understanding is different than yours. I believe that every O2 burned is replaced by a CO2 so there should be no significant pressure change from burning O2. This can be seen because the water rises only as the candle is going out and the air is cooling and most of the rise happens after the candle is out all together - when no O2 is being burned. The air expansion is responsible for nearly all the water rising and the water is not "drawn" up as you describe, but pushed in from the outside by the air pressure. An interesting related experiment is to rust some steel wool in the top of the bottle which should indeed result in rising water because O2 is being chemically combined with the iron.

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