The rising water mystery, on this moment of science.

You may well have seen this experiment in school – but did you get the right explanation? It could be that some of us may have gotten the wrong story. The experiment is simple: put a burning candle in the middle of a saucer, fill the saucer with water, now put a drinking glass upside-down over the candle, so the rim of the glass is in the water. After a few seconds, of course, the flame uses up the oxygen in the glass and goes out – this we all expect. Now we are asked to look at the water level inside the glass. We notice that the water has risen a fraction of an inch in the glass – indicating that the volume of the gas inside is less than before, and traditionally, we’re told how this shows how the oxygen in the glass has been consumed. There are a couple things wrong here.

First, oxygen is about 20% fresh air and the volume of the gas in the glass doesn’t decrease by nearly as much. We have to remember that even though the flame uses up oxygen, it also makes a roughly equal volume of carbon monoxide and carbon dioxide. The same thing happens when any fossil fuel is burned, coal or oil for example.

Second, notice how the water rises in the glass quickly, not gradually, and that it rises only after the candle goes out. Now you can see what really happens when you invert a drinking glass over a candle in a saucer full of water. Heat from the burning candle makes the air under the glass expand. After the candle goes out, the gas quickly cools and contracts. Pressure inside the glass therefore falls, an atmospheric pressure pushes more water up into the glass.

This moment of science comes from Indiana University, with production support from the Office of the Provost. I’m Yael Ksander.