Why Does the Water Rise?
You’ll have to watch closely...
You’ll have to watch closely and use everything that you know about air in order to explain the mystery of the rising water. You heard right! Air is the key to why the water rises in this experiment... but you'll have to do the experiment yourself to find out just how air affects the water.
- Candle and matches
- Pie pan or dish
- Juice bottle, jar, or clear vase
- Food coloring
This experiment requires the use of matches... and that means adult supervision.
- Fill a plastic cup up with water. About 9 oz. should do the trick.
- Add 2 or 3 drops of food coloring to the water. This will make the movement of the water easier to see later on in the experiment.
- Pour the water into the plate or pan and place the candle in the middle of the water.
- Light the candle.
- Cover the candle with the vase and think about what is taking place both inside and outside of the vase. What invisible thing is inside the vase? Carefully observe what happens to the water around the vase. It's bubbling! What happens to the candle flame?
- 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 flame heats the air in the vase, and this hot air expands. Some of the expanding air escapes out from under the vase — you might see some bubbles. When the flame goes out, the air in the vase cools down and the cooler air contracts. The cooling air inside of the vase creates a vacuum. This imperfect vacuum is created due to the low pressure inside the vase and the high pressure outside of the vase. We know what you're thinking, the vacuum is sucking the water into the vase right? You have the right idea, but scientists try to avoid using the term "suck" when describing a vacuum. Instead, they explain it as gases exerting pressure from an area of high pressure to an area of low pressure.
A common misconception regarding this experiment is that the consumption of the oxygen inside of the bottle is also a factor in the water rising. Truth is, there is a possibility that there would be a small rise in the water from the flame burning up oxygen, but it is extremely minor compared to the expansion and contraction of the gases within the bottle. Simply put, the water would rise at a steady rate if the oxygen being consumed were the main contributing factor (rather than experiencing the rapid rise when the flame is extinguished).
To further demonstrate how the expansion and contraction of air within a bottle can cause water to rise, try this experiment using hot and cold water:
- Use the same colored water as you used in the candle experiment.
- Fill a large jar or container with hot water - the hotter water the better. (You may need a glove or oven mitt for this experiment).
- Submerge a long narrow-necked bottle in the hot water for 15 to 30 seconds.
- Stand the heated bottle in the same fashion you did with the candle.
- You should see the same effect as before. The process is slower, but it is the same expansion and contraction you saw with the candle.
Why Does Water Rise?
July 12th, 2010
Click the thumbnail below to see the video.
Diamond - November 11, 2012
I do agree with the people on here.But i dont know much about science because i dont pay attention in class (6 grade).But i do think it was co2 and hot air that made the water rise?
Different view from my teacher
Beckie - October 23, 2011
I am a HK student studying in college and our teacher asked us about why the water rises. And he told us the answer with the fire triangle and yet, he has not said anything about the air pressure so I was confused. I believed that water should rise in a steady rate if the major factor about it is oxygen. However, our teacher did not agree and had different idea. Can you give me more evidence about it so that I can convince him, the stubborn one?
Jose Sanchez - July 28, 2010
I agree that the heat from the candle causes the gas in the bottle to expand which causes some air to escape which is what may cause the bubbles to form under the bottle. I also agree that when it begins to cool the pressure caused by the heated gas goes down. BUT when the candle burns it not only produces CO2 it also produces water vapor which I have seen condense on the side...SO not all of the O2 goes into the CO2 it goes in to Water as well which occupies less volume. Either way I think the rise in water has to do with the difference in pressure outside vs inside...like a Hg barometer
Hot air and CO2
Garry Taree Australia - December 19, 2009
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
what field of scince is this in (for ex.:chemistry,botany,geology,..ect
Welkinator Lago Vista, TX - October 20, 2009
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
Michael Pascoe Portsmouth - England - October 7, 2009
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
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.