Fire, dance with me!
A direct view into the interior of a (rocket) engine
The popular experiment of alcohol vapors burning in a water bottle jar demonstrates several interesting physics and chemistry principles. Understanding this burning is the very first step in understanding the engineering of (rocket) engines.
Fire in a bottle
There is something special about fire that excites emotions in humans. Although, it could be that our fascination with fire is a direct consequence of not having mastered it as a child. Whatever the reason, fire attracts human attention and can stir up feelings of awe. These feelings are especially emphasized in kids, which makes fire experiments one of the most popular STEM topics.
On the other hand, parents and teachers often fear of showing fire experiments to the kids as this is considered dangerous. But, as we already discussed, you should not isolate kids completely from such low-level dangers. Instead, teach them how to protect themselves and stay safe.
In recent times, one of the most popular STEM experiments among chemistry teachers is the “whoosh” bottle alcohol explosion demonstration. It is a simple experiment, and it goes like this:
Take an empty round water bottle jar (used for water coolers) and pour a handful of isopropyl alcohol (it works with ordinary ethanol, too). I use 99% alcohol, purchased at a local electronics shop.
Close the bottle and shake it (kids love to participate in this part). Make sure that alcohol wets the bottle walls. This will fill the bottle with alcohol vapor.
Open the bottle and drain the leftover excess alcohol out.
Ignite the vapor from a safe DISTANCE!
For comparison, put a little bit of alcohol on a ceramic plate and ignite it. This will burn slowly, without any dramatic effect.
If you want to repeat the experiment, the bottle must be “ventilated” (filled with air again).
Do it with dimmed lights (in classroom) or in the evening, such that fire is visible in its full colors.
The result of vapors burning in a closed container is a (slow) explosion, which is a dramatically different situation from vapors burning above a liquid on a ceramic plate. But before you jump immediately to trying this at home, we need to talk about safety because this experiment is not a joke.
No kidding with safety here
There is seemingly a long list of precautions that you should take here, but you will see that all of them are quite simple and intuitive. However, exactly because of that people tend to skip them, which is a bad idea.
First, and the most important advice is DO NOT do this experiment if you personally do not feel comfortable about it. If fire is something that makes you highly uncomfortable, then you might not react calmly if something happens that requires your safety reaction. Second, even if you have done this experiment countless times, and you are quire relaxed about it, do not skip the safety measures.
The basic list is the following: use goggles (this is a universal advice for almost all experiments); ignite the bottle from distance (use a long stick with a burning tip); always drain the excess alcohol out (to avoid afterburning); do not use warm or hot alcohol, it should be at the room temperature to avoid excess vapor; make sure you do not have alcohol spilled over you; having leather gloves is also advisable in case you need to put off some fire (in case of a fire in the bottle, just put the lid on); use the bottle type that I mentioned as it has thick enough plastic to sustain high temperatures and pressure; if you notice grazing, frosting, or cracking on the bottle body, replace the bottle with a new one; ignite it outside where you can take safe distance (or in a big classroom if you are a teacher); if you want to make it totally safe then use a plexiglass safety shield around the bottle (in addition to safe distance).
Chain reaction
Since this burning happens very quickly, discussion and analysis of what is visible requires video recording of the burning bottle and then watching a slow-motion replay. You will notice a rich structure of flames and colors within the bottle. Understanding these details leads you to the principles of combustion, thermodynamics, and kinetics.
The overall story here is that you see alcohol burning. The isopropyl alcohol is a molecule (C₃H₈O) that consists of carbon (C), hydrogen (H), and oxygen (O). Burning is a process where alcohol reacts with the oxygen molecule (O₂) and atoms rearrange themselves such that two types of molecules are formed: carbon dioxide (CO₂) and water (H₂O), both in gaseous forms. For this reaction to happen, a free-floating alcohol molecule (i.e., in a vapor phase) must collide with an oxygen molecule in the air. Moreover, this collision must be above a certain speed to initiate this rearrangement of atoms.
We bring this extra energy of increased seeped using fire. Fire is a situation where molecules move very fast. We measure an average speed of molecules with the quantity known as – temperature. Once the alcohol-oxygen reaction is initiated, the products of this reaction (CO₂ and H₂O) are released with a high speed (i.e., high temperature). This creates a chain reaction as they hit other molecules and give them higher speed.
Notice now that if we regulate how fast the chain reaction is unrolling, we can also control the speed of fire consuming the alcohol. In case of open fire, on a ceramic plate, only small amounts of vapor above the liquid evaporate and come into the contact with oxygen supplied constantly from the open atmosphere above the plate. This results in a slow chain reaction as we starve it of fuel and oxygen.
On the other hand, within a bottle, the entire volume is filled with a fuel-oxygen mixture. This means a huge number of molecules waiting for the chain reaction to start. Indeed, if you observe the video closely, you can notice how a blue veil (i.e., a fire front) is moving through the bottle. This is the chain reaction spreading in a form of slow-moving explosion.
An interesting addition to this story is visible in a form of fire colors. The blue fire is the reaction we just described. It means that all the fuel is transformed into CO₂ and H₂O. We call this a complete burning and it reaches temperature up to about 2000°C. Behind this hot burning front some alcohol vapor is released from the bottle walls. These molecules have enough speed (i.e. temperature) to start fire, but there is a lack of oxygen since it was mostly consumed by the complete burning. The whatever oxygen was left, now it enters new reactions, but there is not enough oxygen to produce only CO₂ and H₂O. Instead, some carbon is released in a form of very fine soot particles (carbon atom aggregates). This soot is heated so much that it glows yellow-white, reaching temperatures slightly above 1000°C.
The foggy end
High temperatures of produced gases result in a high pressure. This pressure is pushing a fast jet out of the bottle. But at the very end of burning, the temperature starts to drop, which quickly decreases the pressure so much that the outside pressure becomes bigger than the interior pressure. This results with the bottle “sucking in” the air containing fresh oxygen. A sudden increase in oxygen enhances the burning, which in turn increases the pressure. The whole cycle repeats itself a few times in a fraction of a second. The pressure oscillations create vibrations of the bottle walls. We hear these vibrations as a low “thump” sound.
The fun does not end here. If you wait for a few seconds, you will notice that the bottle starts to fill with a fog. This is really fog – the hot gases in the bottle cool down and the water vapor cannot stay in a gas phase any more. Hence, the water vapor starts to condensate into tiny liquid droplets (fog) and into moisture on the bottle walls. You will see how liquid starts to accumulate on the bottle bottom. This is water, mixed with traces of unburned alcohol and black soot.
The experiment demonstrates one extremely important technological principle - that we can extract much more energy from a fuel very fast if we ignite a mixture of oxygen and fuel vapors in a closed container. This discovery of complete burning reaction revolutionized the modern society because it led to the development of internal combustion engines.
We are surrounded today by various engines running on different types of fuel under different methods of ignition. One extreme case of engines is the rocket engine. The basic principle is the same as in the bottle experiment – fill a container with a rocket fuel vapor and an oxidizer (in some cases pure oxygen, which should be avoided in the bottle experiment as the whole bottle would explode) and then ignite it. The hot gasses are channeled into a narrow nozzle to increase the gas speed and then the jet of hot gas pushes the rocket in the opposite direction. Of course, the rocket engines have a mechanism of continuous supply of reactants within the combustion chamber. And they can sustain huge pressures. And many more details, but the key principle is visible in this simple bottle experiment.
Extra reading for teachers and STEM enthusiasts
There are many variations possible with this experiment and you can find a study on this in the educational research paper from 1999:
Fortman, J. J., Rush, A. C., Stamper, J. E., Mark J. Waner, & Jay A. Young. (1999). “Variations on the “Whoosh” Bottle Alcohol Explosion Demonstration Including Safety Notes”. Journal of Chemical Education, 76(8), 1092.
It is worth reading it to avoid mistakes and dangers that could come from the changes you introduce.
If you need a more structured classroom material, you will find variations of it on the internet (for example THIS).
Experiments with smaller plastic bottles are actually possible, but you have to put more effort into make it safe and practical for education. If you want to go down that alley, read THIS.