The simple answer is that water is able to dissolve more carbon dioxide at lower temperatures.
The expanded, eli5 bot required answer is to further expound by saying that the CO2 molecules vibrate faster the more heated they are, thus they try to come out of solution more since they are higher energy. When they are low-energy, they don’t vibrate as much and so don’t shake through the dense water molecules as easily and stay trapped among them longer.
Parking the temperature aspect to one side initially; the CO2 won’t stay dissolved in the drink at atmospheric pressure – once the can is opened the previously dissolved CO2 starts to turn back into a gas which then bubbles to the surface and escapes. This is a chemical reaction.
Adding temperature into the mix, the warmer temperature means everything is moving faster (at a molecular scale)
CO2 become bubbles and escapes faster too.
Finally add to this as the drink gets warmer less CO2 can actually be dissolved (for the same pressure), so system with CO2 still dissolved is more over-saturated (even less stable) and it is more prone to forming bubbles and the gas escaping.
TLDR: CO2 doesn’t want to be in your drink at atmospheric pressure, and even less of it wants to be in a warmer drink. Everything happens faster at warmer temperatures so the CO2 escapes faster.
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