So I put my Diet Coke in the fridge as soon as I got to work this evening and I promptly forgot about it. A few hours later, I remembered the Coke and rushed to the freezer, in the hopes that I would recover the Coke before it froze and made a mess. Thankfully, I got to the freezer in the nick of time and it wasn’t frozen. However, as soon as I opened the Coke, it froze instantaneously. Why did this occur? As far as I can figure, the temperature of the Diet Coke was very near its freezing point (whatever that is, but I would guess it is pretty close to the freezing point of water). When I opened the bottle, gas was now able to escape, allowing evaporation that was not occurring before the bottle was open. This cooled the Diet Coke to its freezing point. Unfortunately, my physiscs knowledge has faded since high school, and I have no idea if I’m right. Do any Dopers care to confirm my suspicions or am I so pathetically off base as to incite ridicule and derision?
I can’t fully and authoritatively answer your question but I can venture that the fact that it was a DIET Coke–as opposed to a regular Coke–probably had something to do with what happened. During the winter, I stopped putting diet pop out in the garage because it either froze solid or exploded. I think pop with sugar in it has a lower freezing point than pop with Nutrasweet. I also think pop with Nutrasweet becomes more unstable when it’s frozen than sugared pop.
Anyway, I’m sure someone out there can fill both of us in.
This question has come up a number of times in the past in GQ.
I think it happens for a number of reasons; the freezing point of liquid is affected by the amount of dissolved gases in it, so when you release the pressure, some of the gases come out of solution and the liquid is now able to freeze.
On top of that is the possibility of supercooling; the liquid is cooled below it’s natural freezing point, but does not solidify because there are no nucleation points for the ice crystals to start; the bubbles of gas that appear when you open the cap provide nucleation points and it freezes. (if this is the case, it would also probably freeze if you simply shook it without removing the cap).
Just a note on your theory about coke & diet coke reaction to freezing cold temperature.
pour two glasses, one of coke and one of diet, drop in some ice into each one and the diet coke explodes with fizz above and beyond the level that the normal coke does, so your noting that nutrasweet has a reaction to cold and is more volatile is potentialy proven. but i was always asleep through science class when I was at school.
This was explained by my chemistry teacher that since water expands when it freezes, water under pressure will simply not freeze. When you release the pressure, it instantly freezes. I don’t know how accurate this is, but it makes sense:)
Damn if I can remember what it’s called, but I strongly suspect this law of gases has much to do with it:
The carbon dioxide gas in the Diet Coke is pretty much at or just above freezing while it’s enclosed in the can, and is at a certain pressure. Open the can, the pressure drops very rapidly, and therefore the temperature drops just as rapidly. The liquid undergoes an exothermic reaction because it’s now warmer than the gas, so it loses heat and freezes.
This is a very common event, and it’s caused by supercooling. In order for ice to form, there must be solid particles which provide “nucleation centers.” Clean water-based liquids in smooth-walled containers will often supercool rather than freezing at the usual temperature. Water can remain liquid all the way down to -20F (-30C) before it will spontaneously freeze. But if there are any centers for nucleation, freezing will occur at 32F (0C).
And if you suddenly introduce some nuclei into supercooled water, then the water will turn into slush within a few seconds.
People constantly report this “instant freezing effect” with bottled soda water and with bottled beer which has been left in the freezer. The bubbles seem to trigger the freezing, so it might be that the surfaces of nano-sized gas bubbles can provide a substrate where ice crystals can begin growing.
(No, it’s not caused by compression in the can. There is an air space, so the water isn’t being compressed.)
PS, supercooled water is common in winter. It’s called “freezing rain” or “ice storm.” Some of the raindrops are full of ice, but others are supercooled liquid. After all, there is no dirt inside, and there are no container walls to trigger crystallization. When supercooled water falls onto the frozen landscape, it builds up a layer of clear ice whose weight can destroy trees and power lines.
That’s interesting. Does a can of cola get extremely cold when you pop the tab? I don’t remember this ever happening. It would take a lot of negative heat to change a bottle of beer into a beer slurpee. However, you could be right if the surface of the microscopic bubbles are extremely cold. Even if the gas doesn’t significantly cool the bulk liquid, if it cooled the bubble surfaces that would explain why the fizz can trigger the supercooled liquid into freezing.
The “negative heat” is stored as supercooled water, as water where the temperature is five or ten degrees below the melting point. When the freezing gets triggered, the liquid changes from -10C fluid into 0C ice. Also, I don’t think you’ll see the Coke ever freeze solid. Instead it turns into slush because there wasn’t enough stored negative heat to freeze it solid. If you could supercool your Coke to -30C, then you’d probably see the whole can freeze solid when you pop the tab.
PS, I think you should be able to trigger “slush-ification” by whacking the unopened can against a tabletop. Crystal nucleation in some fluids can be triggered by sound waves or shock waves, but I don’t know if a hard whack would be enough to do it with coke.