Burning Candle/Oxygen Lab: Questions & Results

[yjk91]

Homework Statement

In class we did a lab with burning candles and water. In this way
1. Find a large beaker that can contain certain amount of water, graduated cylinders, and a candle.
2. After measuring each equipments, try to make the candle stand straight in the water filled beaker.
3. Light the candle and put the graduated cylinder on top of the candle. * The top of the beaker must be under water so oxygen cannot come in.
4. After the light goes out, record your observation.
Basically what it does is the the candle burns oxygen enabling water to rise.

So it is said that the Earth's air is made up of 20% oxygen.
Question 1: will this experiment prove this theory?
Question 2: what is going on inside the graduated cylinder?

The Attempt at a Solution

22.1519% 25.6929% 19.6203%
After 3 trials these percentage are the experimental values. 22,26,20.
SO i think the answer to Q1 is false because the numbers were different. but i can't back it up with valid evidence
Q2. I think it has to do with the pressure and volume. Since oxygen is burned up the water will rise to fill the empty space, but what about CO2? won't it fill the empty space of O2?

 

[wywong]

The difference in the 3 experimental results were due to experimental error. I would say
the theoretical value agreed pretty well with experimental data.

CO2 readily dissolves in water (to form carbonic acid) so it won't occupy space.

A major source of error of such experiment setup is that when the cylinder is put above the lit candle, it traps hot air. When the candle is out, the air gradually cools down and contracts, causing an extra rise in water level. Such an error depends on many factors, like how quickly the cylinder is lowered, air draft, how soon the water level is measured, etc.

 

[Borek]

http://www.stevespanglerscience.com/experiment/00000198

I have tried to repeat the experiment several times. From what I have seen if you use large jar and you don't allow hot air to escape, water doesn't rise substantially. So if you ask me, carbon dioxide doesn't dissolve in tap water fast enough. Could be you can try to add some NaOH to water to see the effect. Have to try it one day.