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.

The discovery of the quantum dot

Louis Brus talks about his discovery of colloidal quantum dots in 1980s.

One of the truisms of science is that the basic research scientists who invent something are not the best judges of where it’s useful.

Aluminium ion clock

The scientists at the National Institute of Standards and Technology (NIST) created a new optical clock of unprecedented precision.

The clock, which is based on a single aluminium ion, could remain accurate to within one second over 3.7 billion years. The previous record was held by a clock with one mercury ion, which was good to one second in 1.7 billion years.

My, these are some mind-boggling figures.

I thought that one ²⁷Al⁺ ion should not take much space. But then, they needed another “logic ion”, ²⁵Mg⁺. And a vacuum chamber. And two lasers. (Not three lasers, as in earlier model which used Al⁺/Be⁺ pair, so I presume the new clock is more compact.) I couldn’t find in the preprint what are the dimensions of the whole contraption. However, the NIST press release features the photo of one of the authors, Chin-wen Chou, together with the famous clock. The caption says that

The ion is trapped inside the metal cylinder (center right).

Not exactly wristwatch size but easily fits in Big Ben.

Below minus forty

Cold snap, you say? I do remember one New Year’s Day when the temperature in the Moscow region dropped below −40°. (Celsius or Fahrenheit? In this particular case, it’s the same: −40 °C = −40 °F = 233.15 K.) On 1 January 1979, we woke up in the (late) morning only to discover that we are stuck without any tea or hot food. We couldn’t switch on the gas cooker, which was running on propane/butane mixture. Why? As explained in this useful guide,

When the temperature of the liquids falls below its current boiling point the pressure inside the canister will no longer drive vapour out.

The boiling point of butane is −0.5 °C and that of propane is −42 °C; the temperature outside was below −42 °C, and the gas canisters were outside!

My mum called for a friendly neighbour who was better equipped than us. We kids did not worry too much about a guy warming up a gas canister with a blow torch. Anyway, everything went just fine and we had a wonderful day.

Magnetic monopoles in spin ice

Although the magnetic monopoles were postulated by Paul Dirac in 1931, the existence of these particles remains an open problem. This article surveys the recent breakthrough discoveries concerning magnetic monopoles within spin ice materials, dysprosium titanate (Dy₂Ti₂O₇) and holmium titanate (Ho₂Ti₂O₇).

Colour-changing mechanophores

The recent Nature publication shows that one can literally see a mechanically-induced ring-opening reaction.

Previously, we have shown with dissolved polymer strands incorporating mechanically sensitive chemical groups — so-called mechanophores — that the directional nature of mechanical forces can selectively break and re-form covalent bonds. We now demonstrate that such force-induced covalent-bond activation can also be realized with mechanophore-linked elastomeric and glassy polymers, by using a mechanophore that changes colour as it undergoes a reversible electrocyclic ring-opening reaction under tensile stress and thus allows us to directly and locally visualize the mechanochemical reaction. We find that pronounced changes in colour and fluorescence emerge with the accumulation of plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the ring-opening reaction is an activated process.

I guess we have to introduce a new ChEBI role: mechanophore.

Iron stars

According to Freeman Dyson, in rather unimaginable 10¹⁵⁰⁰ years from now, and in case proton decay does not happen, most of nuclei will either fuse or decay into iron. This will leave the universe inhabited by “cold spheres of pure iron”. I think it is cool, even if I won’t live that long to see it. However, I came across a report of recent (2006) observation of ‘iron star’ with NASA’s Spitzer Space Telescope. I don’t think these objects are the same as Dyson’s iron stars though, just the next best thing.