Water getting hotter once frozen?

I packed a fridge with temperature sensors and took over the temperature control in order to have extra cold beer.


One of those sensors is submerged in the middle of a container containing a 4.5% seltzer so I can get an idea of the temperature inside the surrounding beer cans.


This submerged sensor is the yellow curve at the bottom:


[![Internal fridge sensors](https://i.stack.imgur.com/HKgoy.jpg)](https://i.stack.imgur.com/HKgoy.jpg)


I'm trying to explain what happened around 6:25.


I was maintaining the seltzer at -3°C and I confirmed it was not frozen. However I think I was maintaining it slightly below it's freezing point.


This morning I saw that it somehow raised in temperature abruptly and it's now in this jelly/half frozen state.


I'd like to know why this really sharp raise in temperature.
Is this how freezing works?




I'm speculating, but it's possible this is what happened:


I looked up the freezing point of seltzer water (aka club soda), and it's $\pu{-10 ^{\circ}C}$. Plus your product also contains alcohol and sugars, which would further lower the freezing point. Since you have a temperature sensor in it, I assume it's open. Thus the seltzer will gradually lose its dissolved $\ce{CO2}$, and the alcohol will gradually evaporate. As this happens, its freezing point will go up.


I'm guessing that your seltzer lost enough $\ce{CO2}$ and alcohol for its freezing point to exceed $\pu{-3 ^{\circ}C}$. When that happened, the seltzer, which was at $\pu{-3 ^{\circ}C}$, entered a "supercooled" state (meaning it was now a liquid below its freezing point), and thus started to freeze. When a substance freezes, it releases thermal energy (you have to add thermal energy to melt something; freezing is simply going in the opposite direction). That could be what caused the temperature jump.


It sounds like its freezing point is now $\pu{-1 ^{\circ}C}$, and the "half-frozen state" you are seeing is phase coexistence between ice and the remaining sugar-water solution.


I don't know why you're seeing the "jelly-like" consistency, but it could be from the dissolved sugars and malted barley left behind in the water as the ice forms.


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