It’s now my second summer in Australia. The first wasn’t too bad but this year the frequency of days in the high-30s is a little too high for this English girl. Today’s going to hit 39°C in Melbourne, and the cooling at my place is broken. I think I’ll stay in the lab today and take advantage of the synchrotron’s steady 23°C and wait for things to chill off this evening.
But the one thing that would take the edge off this evening is one of the beers I’ve got, ah, not in the fridge. Well that puts a dampener on things.
But perhaps there is a way to speed up the cooling of my after-work-on-a-39-degree-day beer.
I could put in the freezer, but the rule of thumb for temperature control is that you want to maximise contact between your sample (the beer) and the coolant. I think I could do a little better than a few cold air molecules bumping into the sample every so often.
The standard tool of an ice bath is a super idea. Floating enough ice in water will mean that the whole thing will equilibrate for a good long while (until all the ice melts) at the melting point of the ice, 0°C. But things would speed up if you could equilibrate the mixture of ice at a lower temperature.
The trick is to make the water into a solution by dissolving something into it. It doesn’t even really matter what you dissolve into the water – though table salt is what you’re most likely to have – the effect will get stronger with the more materials added and dissolved into it. This effect is known as freezing point depression.
Try it later if you like: take 1 cup of table salt and dissolve in 3 cups of water. Add lots of ice and you should get a cooling bath for your stubbies that’s below -10°C. Hey, by the end of today you might even fancy a dunk in it yourself!
But it doesn’t have to be table salt to cause this effect (though I would only recommend using table salt at home); anything that dissolves into water will have this effect. It’s the freezing point depression effect that is thought to have formed much of the super-varied landforms we see on the icy moons of the solar system, worlds like Saturn’s Enceladus and Jupiter’s Europa.
There’s not much sunlight that reaches out to Jupiter and Saturn and the daily temperature at Europa, for instance, is a chilly -150°C. If these moons were only made of pure ice, chances are we’d not see half the weird landforms we do. So something is mixing with the ice to allow it to melt sometimes, to form the strange alien landscapes that we see on this moon.
One of the things I’ve been investigating at the synchrotron has been mixtures of sulfuric acid and water. When frozen these mixtures form a variety of solids, made up of sulfate (a sulfur atom bonded to four oxygen atoms) and water molecules.
Each of these solids form with different numbers of water molecules, and in a variety of crystal structures. These sulfate and water solids, known as hydrates, are thought to be all over the surface of Jupiter’s moon Europa, with the sulfate being spewed from Europa’s sister moon Io.
Adding sulphuric acid to water has quite a large freezing point depression effect. In fact, in the correct proportions the solution will not freeze to -73°C. This stronger effect is probably due to the sulfate molecules interacting a bit more strongly than table salt would with the water molecules. -73°C is a bit of an overkill (not to mention dangerous with high-concentrations of acid) to cool your beer but temperatures just below the surface of Europa could easily reach this point, probably higher, allowing for melting and possible volcano-like activity.
I’m taking the fact that I can only consider drinking beer ice cold as sign I’m rapidly adopting the Australian way of life. And, in future, I will endeavour to get my beers in the fridge early when hot days are forecast.