Consider the experimental apparatus shown in the photo to the left, in which a candle is suspended above boiling water. This equipment could be used to test a hypothesis about the chemical composition of the gas in the bubbles that rise from boiling water.

What would be observed if the bubbles were composed of 

(a) water, 

(b) hydrogen, or

(c) oxygen?

Strategy This problem asks you to think about what you expect to observe in an experiment and alternatives for different hypotheses. At this stage, you may need to do a little research to answer this question—find out how hydrogen gas behaves chemically in the presence of a flame. We also have to remember some basic facts about fire that we’ve seen in science classes before. To be sustained, fire requires both a fuel and an oxidizer. For a burning candle, the fuel is wax from the candle and the oxidizer is oxygen from the surrounding air.