The Mystery of the Burning Water: An Update

burning solutions

Tom Kuntzleman loves to share chemical mysteries1 and that inspired me to create a list of mysteries that are appropriate for the main topics covered in IB Chemistry. In this blog post I'd like to share some detail about how I modified the mystery of the burning water.

Just last week I showed my students the burning water mystery. Needless to say, the demo generated some good discussion and remains a hit with my students. But I wanted to take things a bit further before I shared the "answer" with them. After waiting a day without giving them any clues, I brought out samples of two "mystery" liquids (hexane and ethanol). I then showed the students that they both burn. In the picture below, hexane is on the right, burning with a bright orange flame; ethanol is on the left, burning with a much lighter blue flame.

After that I placed about 1 mL on top of a flask full of water. And lo and behold, only one of the samples burned!

This bit of additional demonstration was meant to really get them thinking about the physical properties needed for a liquid to burn on water. At this point I hadn't given away the answer - but instead connected the observations to what the students had learned about how intermolecular forces affect observable properties - such as solubility.

And with that, a few of the students started tossing around ideas and deduced that it could be an alkane that is burning, because it won't dissolve into the water and thus would float on the water. This led to others realizing the flammable liquid that didn't burn on the water was an alcohol because it would have dissolved/dispersed into the water due to hydrogen bonding and thus did not catch on fire.

Are there any additional mysteries you've used? Or small variations to the mysteries posted here in ChemEd X?

1: Browse through Tom Kuntzleman's Blog to find more mysteries.

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Comments 2

Tom Kuntzleman's picture
Tom Kuntzleman | Wed, 10/19/2016 - 09:34

Lowell:  Love your modification on this experiment. Great idea. It's a win in my book anytime we get students to connect observations to intermolecular forces!

Lowell Thomson's picture
Lowell Thomson | Fri, 10/21/2016 - 07:53

Thanks Tom. It was your set of mysteries that inspired this in the first place. Next I'll be looking at your Kool-Aid, Cotton and IMF activity.

In a separate class I actually had the students propose ideas for how to test the mystery. We then spent about 20 minutes trying things. It was a fantastic opportunity to get the students "thinking like a scientist."