I placed a candle in a shallow water bath. I lit the candle and then placed a beaker over the candle. The candle goes out and the water rises in the beaker.
I have just done this experiment recently in my class and I have a few questions.
I explain that the water rises because as the candle consumes the oxygen, the water rises to take its place. What about the carbon dioxide that is created? Is not the same amount created as the volume of oxygen consumed keeping the water level the same?
I also notice that the water rises most rapidly at the end when the candle is going out. I would think that the water would rise fastest when the candle is burning the brightest. Why is this so?
(WAG: the cabin dioxide obviously has less volume than the oxygen that was converted to it. But I am sure someone will come up with a more technical explanation:).)
Basic combustion of a candle is some Hydrocarbon (CnHm) + some Oxygen (O2) into some CO2 and Water H2O.
The ratios depend on the particular Hydrocarbon (read as “uh, it’s been a long time since I took a Chemistry class”), but you’re definitely ending up with fewer gas molecules (CO2) than you started with (O2), since it takes an equal number of Oxygen atoms to make CO2, and some of the Oxygen atoms go into liquid water. Fewer gas molecules mean that the pressure inside the container is lower and the water is pushed up by atmospheric pressure.
After the candle goes out, you’ll also get some reduced pressure because the air inside the container was heated by the flame, and will cool off and contract.
I wouldn’t think so every molecule of O2 is replaced by one molecule of CO2. Now the CO@ molecule by itself is probably bigger (unless the O=O bonds are twice as long as the C=O bonds of which there are two). But the volume of gas is pretty much controlled by the number of molecules and not teh size of them.
The Ideal Gas Law is PV=nRT. Pressure * Volume = number of moles (proportional to number of molecules) * R (a constant) * temperature on an absolute scale.
n stays the same after the burning unless a lot of it is incomplete and you’re producing Carbon monoxide rather than CO2, but the the gas volume should go up not down. The temperature is increasing if anything, I’d think ditto there.
I doubt it. They’d go to water vapor, I’d think and that means every O2 that gets incorporated into water produces two molecules from each previous molecule – increasing the number
This may be it if you wait a while and let the air warm up before you cover the candle. I was envisioning covering the candle immediately in which case you’d get heating then cooling which would cancel.
Most of the water produced in the reaction is in the vapor phase, but it eventually condenses out. That also helps to explain why the effect is delayed, and you get the biggest effect as the flame goes out and the gas cools.
Good point. Longer term, less gas, but immediately, more.
The air immediately around the candle is going to be much hotter than surrounding air very quickly after it’s lit, so the heating will happen before you cover it. Then you get cooling once it goes out. This is probably the bulk of the noticeable change.
Wrong. Every molecule of O2 is replaced by either one molecule of CO2 or 0.5 molecules of H2O. The general formula for paraffin wax is CnH2n+2. The carbon becomes CO2 and the H becomes H2O. The H2O formed is in equilibrium governed by the vapor pressure of water in the container and a lot of it will go into the liquid phase. The gas volume definitely decreases.
So 3 O2 molecules (& -CH2-CH2-) generate 2 CO2 and 2 H2O. Initially 3 molecules of gas generates 4 molecules of gas; after H2O condenses, it’s 3 molecules going to 2. Also, heat from flame expands the gas in the beaker.
The only seal at the bottom is the water bath. With a perfect seal, their would be initially increased pressure, then decreased after cooling and condensation. (This would be the similar classic experiment carried out in a sealed metal container, which crushes under atmospheric pressure at the end of the experiment.)
With water bath seal, it is possible that the initially increased pressure may push some bubbles around the base of the beaker, which would increase the apparent volume loss at the end of the experiment. So, important to be careful not to let bubbles escape when doing water bath version.
I’ve done this experiment before and that is exactly what happened. As the candle burned, bubbles escaped from the bottom of the jar. When the flame went out, the air cooled and the water came back in. At least, that is my recollection.
My first grade teacher did this experiment and claimed it was all due to the consumed Oxygen. I loved that experiment and it was one of the things that kindled my interest in science. It was years later that I understood that her explanation was all wet.