Turning things on its head

I had a great moment in science over the last couple of days.  As part of my uni studies, we had to do an assignment on constructing a Predict-Observe-Explain (POE) that we could use with our students. (For another, unplanned example of a POE that happened in my chem class have a look here)

The POE I was putting together relates to the “candle in the bell jar” experiment, where you burn a candle underneath a jar in a dish of water; as the candle burns and eventually goes out, the water from the dish moves into the jar. Sounds a little bit “whoop-de-doo”, but it works really well in person I promise! But I was having difficulty in coming up with a relevant explanation for a Year 8 class. My thought (which was based on my own teacher’s explanation when I saw this at school) was that the candle uses up the oxygen inside the jar, which drops the pressure and the higher pressure in the air outside the jar forces the water inside.

However, with a little more than a day to go before I had to both do the demonstration and hand in the assignment, I found out that I (and my old science teacher) had it completely wrong!  It turns out it’s all to do with the air inside the jar heating up because of the candle and expanding so much that it escapes from underneath the jar. Once the candle runs out of oxygen, the air that’s left inside cools down really quickly and contracts, dropping the pressure and the water gets forced in to balance it out!  Needless to say this was a stressful thing to realise, but it reminded me of how science isn’t about neat little answers, it’s about messy questions!

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