Well, it snowed here in Colorado this week so that means it is officially winter. That means it is time to present to you the power of cold. More specifically the power of cold liquid nitrogen rocketing a bucket into the air and frightening a bunch of high school students. What better use of science is there?
Explanation after the jump…
This week’s explosion is actually the simplest one we’ve covered so far. There’s no combustion nor detonation. All we have here is a phase change.
Phase Change Shmase Change
Yeah phase changes can be boring. Like watching ice melt. However when something goes from liquid to gas very very very quickly there can be some… interesting results. In this case we have liquid nitrogen (simply N2). The boiling point of liquid nitrogen is 77 Kelivn (also known as −196 °C or −321 °F. Brr). In the video they fill up a simple soda bottle with liquid nitrogen, put the cap on tightly, drop it in water and place a bucket on top of it and move away quickly. As ambient temperature is somewhere around 60 °F the liquid nitrogen rapidly becomes gaseous nitrogen.
It’s time to do a little mathemagic to understand what happens next. What we are going to do is determine how much pressure is in the bottle when all of the liquid nitrogen has changed to gaseous nitrogen. If this brings back bad memories of high school feel free to skip to the end – I won’t be offended! So, let’s assume that it is a one liter bottle and it is filled with liquid nitrogen. The density of liquid nitrogen is 0.808 grams/mL. There are 1000 mL in one liter so that means there are 808 grams of nitrogen in the bottle. If I divide by the atomic weight of molecular nitrogen (28 g/mol) then we end up with 28.86 moles of molecular nitrogen. Now we go to the ideal gas formula:
Remember we are trying to find P, the pressure. We know that the volume (V) is 1 liter, we know that the number of moles (n) is 28.86, we’ll say that the temperature (T) is 60 °C (289 Kelvin) and we know that R is just a constant (0.08206 liter atmosphere per mole kelvin). Plugging in the numbers and solving for the pressure gives us a pressure of 684 atmospheres.
As the ambient pressure at sea level is approximately 1 atmosphere, then the pressure inside the bottle is over 600 times larger than the pressure outside the bottle. The cheapo plastic soda bottle definitely cannot stand that kind of pressure. The bottle quickly bursts and all of the nitrogen gas under pressure comes roaring out with enough force to rocket the bucket sitting on top of the bottle into the air!
And that’s the power of cold.
Wrap it Up
I apologize for the lack of other informative posts this week. I am working on getting a paper submitted soon and various other projects. My home computer died this week as well so making the figures for the next post have been difficult. In the meantime let me know if you have any questions about explosions (or anything energy related) and I will answer them in this space. Stay warm!