The Candle Elevator

Using phenomena in science classroom as a means to spark inquiry is a great way to start a lesson or even a unit. Besides the easy access to the physics and science behind the phenomenon, science content can be distributed as well as scientific processes. Students not only learn about basic principles, they also learn scientific inquiry and reasoning as well as experimental skills.

The candle elevator is another one of my favorite science magic tricks. Putting a glass on top of a burning candle, floating in some water, will put the candle out and the water rises into the glass. But why does this even happen?

This is a perfect starting point for some scientific inquiry. A popular assumption includes the thought of the candle “using up” the air inside the glass and the water wanting to fill that space. But candles don’t use up air, they convert oxygen and produce carbon monoxide and water in form of steam. This mixture takes up less space than the converted oxygen but this alone cannot be the reason for the water rising so high. So what is it then?

More important than the conversion of oxygen seems to be the process of the candle heating up the air in the glass. This makes the air expand and ultimately escape the glass. Before the glass even touches the water at all, the air in it already expands and escapes. If the air cools down again, it reduces its volume, creating a lower atmospheric pressure than outside of the glass. However, the water is not sucked into the glass, but pressed by the higher atmospheric pressure all around.

I really love this experiment because it creates the perfect opportunity for some student inquiry. Teachers can build on the natural curiosity of their students and with some impulses, challenge and guide their thought process. Also, it is a perfectly small home experiment to spark that curiosity about science.