The Burning Issue of a Candle

For this challenge, (7th in the eight-challenge series) I performed the following demonstration.

After the demonstration, I asked the following questions of the students:

1. Why did the water rise up the container?
2. What is the change in volume of air in the conical flask and what is the explanation?

The challenge for the Brawns and Beauties were to design and conduct an investigation to find as complete an answer to the questions as possible, supported by quantitative data and results.

I gave them ample time to design and conduct the investigation, and write up a lab report.

Notes for Teachers

First, a note about the demonstration: Using a conical flask, instead of, for instance, a glass or a measuring cylinder or a beaker, I discovered, was most effective. Because of the narrow neck, the result was most dramatic, which I am sure you noticed in the video.

There is so much you can have your students do and investigate about this demonstration. It can be as simple as you want it to be or as in-depth and complex as you make it.

The task set out in the pages F4 Magical Candle (meant for IGCSE First year Chemistry students) and M4 Magical Candle (MYP Year 4 Chemistry students), for instance, is the same but a simple one. The page with the same demonstration but for first year IB Diploma Chemistry students contains more questions, and therefore is considerably more challenging.

For instance, while IGCSE and MYP student might be expected to provide an explanation based just on what happens to air composition when combustion takes place, the explanation expected of IB Diploma students may include gas behavior and stoichiometry as well. But of course, keen and gifted IGCSE students could also attempt a more complete response.

In addition to the questions above, and those appearing in the three blog pages about the same demonstration, here are some more which my students and I came up with while discussing the challenge.

First, questions that directly relate to their challenge.

1. Why does the candle go out? Is it because all the oxygen in the flask is used up? If so why and how much is it? If not why not and how much is left in the container?
2. How much candle is burnt?
3. What’s the chemical formula of a candle?
4. What happens chemically when a candle burns?
5. What’s the duration for which the candle burns? How is that quantitatively related to the change in volume of the flask?
6. What contributes to the change in volume of air inside the flask, and how?
7. What is the mass/volume/moles of water and carbon dioxide produced? How does that contribute to — or affect the change in — the volume of air in the flask, if at all?
8. Why do bubbles of air escape from the mouth of the flask in the beginning and how does that affect the change in volume of air in the flask? (Watch this video to observe that clearly.)

Some more questions about the demonstration that could be interesting to investigate:

1. Would the results change if the size (height or thickness) of candle is changed? If so how and why?
2. Would the size of container (conical flask) affect the height to which water rises inside the container? If so how and why?
3. Would the temperature of water affect the height to which the water rises inside the container? If so how and why?
4. Would flame size affect height to which water rises? If so, how and why?
5. Would this demonstration only work with a candle or would it work with a different flame source, eg. spirit lamp? If it does, how would it be different and why?
6. What mass/volume of water rises up the container and why?
7. What is the pressure inside the container after the candle goes out?
8. Is there a relationship between the final pressure inside the container and the mass of water that rises up?
9. Is there a relationship between the pressure inside and the volume of carbon dioxide produced or the volume of oxygen used up or the duration for which the candle burns etc.?
10. What is the average velocity of the gas particles inside the container before and after the demonstration?
11. What factors affect the speed at which the water level rises up the neck of the flask?

(Incidentally, the Beauties came on top in this one!)

Additional Notes For Students

If you are a student and interested in investigating this on your own, you can do this at home using a glass and a bowl. Sure you’ll need a measuring cylinder or a cup, a marker and possibly a kitchen balance depending on how far you want to take it!

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