How do you do an experiment to see if the atmosphere is 20% oxygen?
Grr someone please just think about the whole chemical formula for burning!! hydrocarbon + oxygen => carbon dioxide + WATER that’s the reason your crummy experiment makes the water rise, also the flame doesn’t go out because the oxygen is used up, its just the concentration of oxygen is so low and that of carbon dioxide is so great. The best experiment school lab experiment for this is as described above, get a volume of air and keep passing over heated copper as this will react with the oxygen, then you can work out the percentage difference, and hence the amount of oxygen that was present.
Don’t get taken by the old candle in a jar in a pan of water trick. That’s the one where the water level rises by 20% when the candle goes out. I believe callum and mindy and perhaps others have suggested that one. It is totally incorrect. Even when the candle goes out there is lots of O2 left in the jar, along with CO2 and water vapor. Using up O2 is not the reason the water rises. It’s due to the drop in temperature when the CO2 puts out the flame. Original flame heat the air in the jar and pushed out some air. Cooler gases, at a lower pressure are in the jar. The outside pressure is greater and pushes the water level higher. Weigh some steel wool. Place in a sealed container of air. Use a gas grill sparker to ignite the steel wool. Wen it goes out weigh it again. The iron is converted to iron (III) oxide. 4Fe(s) + 3O2(g) –> 2Fe2O3(s) Knowing the volume of the container and the gain in mass of the steel wool, you can compute a percentage of O2 in the container.
