I bought a Diet Pepsi from the work cooler and brought it to my desk. Sat down, opened it, took a sip, and set it down. Was reading through my email when I noticed movement out of the corner of my eye. A good minute after having opened it, my soda was suddenly bubbling over. I quickly put a napkin under it, and then watched, fascinated. It bubbled rather energetically - enough to spill out of the neck, but not with the force of shaken bottle syndrome. Eventually, it stopped bubbling over, so I took a sip, and set it back down. As soon as I set it down, bubbles began making their way rapidly to the top, for no apparent reason. By the time I took the next sip, the soda was halfway to flat.
I have no clue why this would happen. Especially with such a long delay after opening. Any ideas?
WAG, purhaps it was partially frozen? I can recall the same thing happening when my Pepsi was left in the freezer a bit long. Maybe the cooler at your office was particularly cold?
I can tell you from experience that diet sodas freeze at a temperature a few degrees above that of regular sodas and presumably water. I worked in a grocery store and there was an unheated part of our back room. We had to be careful to make sure no diet sodas were stored back there in the winter. We had one occasion where a whole pallet of diet Coke froze solid after being left for the weekend. It wasn’t too messy until it warmed up, then it just oozed out of the cracked and split bottles, growing stronger as it warmed up.
Cold liquids can hold more dissolved gas than warm liquids. A can of Diet Pepsi at, say, 2°C may be able to keep all the CO[sub]2[/sub] it contains in solution. But when it warms up, it can no longer do so, and some of the gas escapes as bubbles. It’s likely that the Pepsi was temporarily supersaturated as it warmed – it contained more dissolved CO[sub]2[/sub] than it can normally contain at a given temperature. In order for the bubbles to escape, they need something to form on - a scratch in the container, or perhaps a sharp jolt. I think what most likely happened is that eventually the Pepsi warmed up so much it could no longer maintain its supersaturation, and bubbles started to form (possibly because it was moved suddenly, or possibly because the liquid ‘found’ a scratch inside the can, or because of dust from the air falling into the can). Once the bubbles started to form, all the surplus CO[sub]2* was released at once, until the carbon dioxide within the can reached the saturation point again. This resulted in an unusually vigorous release of bubbles – though not as much as if you had shaken or dropped the can.
Seeing as the soda wasn’t frozen - it was cold, but not very much so (stored in one of those open-air refrigerator type things), Roches’ explanation sounds the most likely. I was reminded of the “exploding hot water from the microwave” phenomenon when it happened, so I guess it makes sense that this is what happened with my soda, just on a smaller (and less dangerous) scale.
I’m not sure if this is relevant, but if I don’t say it now, I fear my entire chemistry class will have been for naught.
They don’t actually put carbon dioxide in sodas. They put in carbonic acid, which can only exist under high pressure and/or low temperature. When you open the bottle, pressure decreases and the acid molecules split into those you want in a soda: water and carbon dioxide. So maybe, because of a very low temperature in one small area within the bottle, the acid didn’t break apart completely. I find this unlikely, but possible.
The difference is just terminology - CO2 dissolved in water immediately forms carbonic acid. But it’s not that they pour acid into the bottles - the bottlers pump carbon dioxide at high pressure into the filled soda bottles, if I understand the process correctly (after all, you would have a hard time storing big tanks of carbonic acid, given its tendency to go flat.)