Solution to Chemical Mystery #5: How to Burn Water

Light water on fire

In Chemical Mystery #5, I used chemistry to give the illusion that water can be lit on fire. The trick involves the use of hexane (C6H14) which is flammable, less dense than water, and immiscible with water. In the video below you can see how to set up and carry out this demonstration.

Congratulations to Andres Tretiakov who solved this mystery!

When I have performed this demonstration for my students, they are often quite shocked to see “water burn”. Students provide many explanations for how this trick works, which exposes various misconceptions that they hold. For example, I once had a student postulate that the water burns because water contains hydrogen, and hydrogen is flammable. Similarly, students have claimed that the water burns because water contains oxygen and oxygen supports combustion. Obviously both of these claims are incorrect. So in addition to teaching about density, miscibility and polarity, this demonstration has allowed me to reinforce to my students that the properties of an element change when it reacts to form compounds.

If you use this experiment in class, let me know what kinds of questions it brings up for your students. Also, let me know of any interesting twists on this experiment you devise.