Of course, you should never serve champagne at room temperature, champagne.fr recommends 8 to 10 C. But an interesting paper by Liger-Belair and coworkers reckons that you can actually form (tiny) ice crystals if you pop a bottle of luke warm champagne. Surprisingly, the warmer the champagne the colder the expanding gas of the champagne pop gets.
The gas in the headspace of champagne is not air but is mostly carbon dioxide carbon dioxide produced by fermentation. Now how much carbon dioxide dissolves in the liquid champagne depends on temperature. The warmer champagne is, the less carbon dioxide dissolves. So in cold champagne a lot of the carbon dioxide is dissolved, then little is left in the headspace, and so the pressure is relatively low. But with warm champagne, less dissolves and so here is more gas in the headspace and so a higher pressure. So you get a bigger pop with warmer champagne.
This results in something counterintuitive, that the warmer the champagne is, the colder the pop of expanding air becomes. When the cork shoots out, the carbon dioxide originally in the headspace follows the cork out. As it does so the gas expands and cools — rapidly expanding gases expand because they do work as they push against the atmosphere and this energy has to come from somewhere, it comes from the thermal energy of the gas, which then cools. This rapid expansion should be roughly adiabatic (i.e., so fast that the carbon dioxide basically takes up no thermal energy from the surrounding air as it expands), and then the temperature after expansion, TFINAL, scales as roughly TFINAL/THEAD = (pATMOS/pHEAD)2/7. So, for a initial bottle temperature of THEAD and assuming that the pressure in the bottle is eight times atmospheric pressure, pATMOS/pHEAD =1/8 as Liger-Belair and coworkers do, we get a cooling of roughly 100 K, or 100 C, of the gas that expands out of the bottle.
This is enough to take air from room temperature of 20 C down to about –80 C. At this temperature any water in the expanding gas will freeze very rapidly to form tiny crystals. The gas coming out of the champagne bottle will contain both water and alcohol from the champagne.
The surprising thing here is that the warmer the champagne, the higher the pressure in the bottle, the greater the expansion and so the greater the cooling, and so the lower the final temperature. And that the temperature gets low enough to form ice. As it is a small volume of gas it will warm up pretty quickly back to room temperature.