Supercool

Once again, Summer is upon us. And what could be more satisfying on a hot Summer day than cold beer? I’ll tell you what: really cold beer.

In any good Spanish bar, they will serve you beer in a frozen glass, or copa fría. Here’s a pair of well-chilled beer tankards:

When you pour cold beer in copa fría, it will form ice crystals:

How much ice is formed, depends on temperature and beer strength. In general, the stronger the drink, the lower the freezing point.

On a number of occasions, I was chilling beer in the freezer. Then taking it out, opening the bottle and pouring it into the glass. There are four experimentally observed outcomes.

1. Beer is liquid both in a bottle and in a glass
2. Beer is liquid in a bottle but gets frozen as poured in a glass
3. Beer gets solid in a bottle so no way to pour it in a glass
4. Beer bottle explodes in the freezer

Scenario B is the most interesting one. (Scenario D is also interesting, but I won’t recommend it.) This is how the resulting beer slush looks like:

Now I saw a number of articles on the web where they explain this phenomenon with supercooling. I should say that I am not satisfied with this explanation. Why “supercooled” beer is not getting frozen in the bottle, even if I shake it, but forms slush once outside? When I put a bottle of (non-fizzy) rosé in a freezer, it either stays liquid (and remains liquid upon opening and pouring) or develops fine crystals of ice (which stay as they are upon opening both in a bottle and in a glass). On one occasion, a forgotten in a freezer bottle of rosé got frozen solid. (According to The Academic Wino, “the freezing point of table wine is –5 °C”, and my freezer goes down to –18 °C.)

To explain what happens, we don’t need to bring supercooling in. We just have to keep in mind that our drink is an aqueous solution. And that alcohol is only one of many solutes there. Of them, the most important are sugars and carbon monoxide. (Some beers, such as Guinness, contain dissolved dinitrogen as well as CO₂.)

For dissolution to take place, the overall change of free energy should be negative, but the heat may be either absorbed or released. The dissolving of sugar in water is an endothermic process. The increase in temperature results in an increase in solubility. The reverse process, precipitation (often in form of crystallisation), is exothermic.

On the contrary, the dissolving of gases in water is exothermic. The increase in temperature results in an decrease in gas solubility. The reverse process, gas evolution, is endothermic. So as soon as the bottle is opened, the gas starts to escape and the temperature drops — in our example, below the freezing temperature. Sometimes, it drops so rapidly that beer freezes in the bottleneck. And while I am on it: as this video shows, dissolving of alcohol in water is also exothermic. Not that it changes much in our beer glass experiments.

¡Salud!

More photos of beer @ Shutterstock.