The nail bottle demonstration1 is one that many of us have conducted in our classes. To perform this demonstration, 2 – 3 mL of ethanol is placed into a plastic bottle that has two nails punctured into opposite sides of the bottle. After stoppering the bottle, a Tesla coil is touched to one of the nails. A spark jumps from one nail to the other, which initiates the combustion of vaporized ethanol inside the bottle:
We recently filmed this reaction with our high speed video camera:
I have seen this reaction performed many times, but never in slow motion like this! When I first observed this reaction in slow motion, I was amazed by the way that the initial reaction front seemed to proceed spherically outward, at least until the reaction front was impeded by the walls of the bottle. This observation made me wonder if the reaction started with a single molecular event. Upon further reflection, I noticed that the spherical reaction front in the nail bottle looked remarkably similar to the spherical shape flames take on in zero gravity. In the presence of gravity, flames rise because hot gases are more buoyant than surrounding cold air. In zero gravity, the hot gases in a flame expand, but don’t move upward because there is no buoyant force. Therefore, a flame is shaped like a tear drop in the presence of gravity but like a sphere in the absence of gravity.
So why does the initial flame in the nail bottle – which is certainly under the influence of gravity – proceed in a spherical manner? My guess is that the gases produced in the initial, most violent stage of the reaction expand so forcefully that the buoyant forces on these gases are negligible by comparison. In a sense, the initial flame front in the nail bottle simulates a flame in zero gravity!
The questions and observations I have when watching this video leads me to wonder what some of you might think after viewing this video. When you observe the nail bottle reaction in slow motion, what questions come to your mind? What observations have I have missed? Would you like to see this reaction filmed in a different way? Share your questions, comments and ideas with us!
Thanks to Kristen Lewis for help filming the demonstration.
- Shakhashiri, Chemical Demonstrations, a Handbook for Teachers of Chemistry, Volume 2, pages 216 – 219.