The Oxygen Content of Air
Years ago, I can remember a teacher affixing a candle to a clear pie plate, putting some water in the plate and then inverting a jelly jar over the candle. The candle went out and water was pulled up in the jelly jar as the teacher explained that the candle had burned up all of the oxygen in the air and the water rushed in to replace it. When I took High School Chemistry and learned about combustion processes I remembered that demonstration and was puzzled. Since the combustion products were Carbon Dioxide and water vapor, the demonstration should have worked in reverse. After trying it out again, I realized that the heat from the candle was expanding the air and the bubbles outside the jar contained both combustion products and air from the jar. When the air cooled, there was simply less air in the jar. I was disappointed and resolved to be more skeptical in the future. Perhaps this was a good lesson in skepticism but a poor demonstration in meteorology.
The Oxygen content of air can be demonstrated quite simply with a variant of this old standard but misleading demonstration. Instead of a candle, place a large wad of wet steel wool in the jar as in the diagram. The rust takes some time, perhaps two to three days for the Oxygen to react with the Iron, but the demonstration does produce results which are scientifically correct and can be quantitatively checked with the appropriate microbalance. Unfortunately, it is not as dramatic.