Re: Does burning candle experiment really demonstrate oxygen consumption?

Interesting question. Just for reference: an explanation of this experiment in a book called Science Experiments & Amusements for Children by Charles Vivian.

But, let's get back to your questions. First, the candle's flame will go out once there is no more oxygen available to fuel the combustion. So this part of the experiment is OK.

Next, combustion can result in reaction products like CO2 and H2O (see p revious archive), as well as some ash and solid material containing oxides. You definitely have a point that there will be some gaseous reaction products but they are difficult to quantify. The thing is that gas is compressible (and expandable), so you won't necessarily end up with the same total VOLUME that you started with.

But this concept leads me to your comment about bubbles - this experiment is really designed for a closed system. If there are bubbles escaping from the bottom of the inverted container, then the water rise you are seeing will be the result of a vaccum in the glass bottle/jar... and the water level change will occur when the hot gases cool down.

But I think there will always be some lag between the time the flame goes out and the water rises. The flame is going to heat up the gas inside the container no matter what and the trapped gases will expand. The water is heavy and will not be able to rise immediately into the jar because it has to overcome the force of the hot gases - which will disappear when the gas cools back down.

So, while I see a number of very good points in your argument, it sounds like the experiment works best when the candle does not burn for too long inside the glass jar (so the trapped gas does not heat up too much), no bubbles are escaping, and you are a bit patient with these 1960s classics. ;)

I hope this rebuttle has helped a bit. And there are a number of other great "classics" in the book I referenced above. Enjoy!

Kieran