A couple of recent threads (“exploding soup” and “things you shouldn’t microwave”) led me to try and read a little bit about nucleation. I came across the following statement in the Wikipedia article on nucleation:
There is a very similar statement made in the Wiki article on freezing except it’s phrased as something like, “water freezes at −42° C if it contains no nucleators”. However, another site (admittedly not the most authoritive looking) says:
(emphasis added).
Which is correct? If it’s the “no nucleator” explanation, how “pure” must it be? Would distilled water exhibit any depression of the freezing point (cf. 0° C)?
imho, the only important factor is nucleation. Cooling rate is important if one wants an amorphous glass (which still looks frozen). E.g. if there is no nucleators, it doesnt matter how fast one cools to -40.
I would think it’s quite difficult to rapidly chill the sample without disturbing it - and nucleation can occur around disturbances/pressure gradients, as well as physical objects such as grains of dust. I think that might be the reason for the ‘chilled at the right rate’ thing.
There’s a science experiment popular with kids – which admittedly I’ve never tried – where you take a bowl of water with a stirrer in it, place it in a larger bowl filled with ice and rock salt, and leave it undisturbed for a while. Eventually, the salt-ice mixture in the larger bowl will supercool the inner bowl of ice water – in other words, cool it to a temperature below 32 F. Apparently, at that point, if you jiggle the stirrer, the bowl of water will freeze more or less instantly – probably showing the phenomenon Mangetout mentions.
Most of these “supercooled water” experiments that show up in videos are actualy supersaturated sodium acetate solutions. Fore example here. I think that supercooling water is actually very difficult.
I’ve seen it done many times in person, with bottled water with the cap still sealed, and where the person with the bottle drank it immediately afterwards. The fellow in the cubicle next to me keeps drinking water in the freezer of his office refrigerator, and more often than not, when he takes a bottle out, it’s still liquid, but freezes immediately on being shaken.
Well I certainly understand that is possible. I guess I can’t imagine how to reliably do it. Every time I stick water in the freezer it comes out frozen. I definitely take your word for it though.
Well, one big step toward doing it reliably would be to measure the appropriate temperature range. Your freezer is probably too cold for the water to stay liquid.
They probably got lucky with their freezer temp, but you should be able to replicate it reliably by fiddling with your freezer thermostat.
I’ve had urine samples in conical storage tubes supercool on me when I’ve put them in a -20C cold room. They’re clear and liquid when I first see them, but when I invert the tube, the urine instantaneously freezes into an opaque popsicle.
Honestly, if I weren’t so afraid of my wife finding a mess in the freezer, I would be less willing to take your word for it. As things stand, I am quite satisfied with what I have been told, and I like my freezer on the cold side so that I can cool my beer quickly.
Seriously though, I trust you guys on this one. I’ve just never witnessed it myself so I always thought it was fake.
I doubt that. -42 C is much colder than your average freezer. The real problem is the nucleation sites – if you container has even a microscopic scratch or some spec of dirt, then there’s a nucleation site for water to start freezing from at the normal freezing point.
Almost everyone believes pure water freezes at exactly 0 °C. They are incorrect. The latestest measurements would suggest pure VSMOW water freezes at 0.000089 °C (ITS-90). This is so close to 0 °C that, for virtually all applications, the freezing point temperature of pure water may be assumed to be precisely 0 °C.