My understanding of water is that as it loses energy it cools. But that when the water reaches the freezing point, it will sit at 0C for a time until it looses enough energy to freeze. The amount of energy needed to freeze water is impressive at 334j/g when compared to 4.2j/g needed to change the temperature by 1 degree.
Therefore if you have a gram of 50 degree Celsius water and it is losing 20 joules each hour in a freezing room, it will reach 0C after losing 210 joules which will happen in 10.5 hours. It will then need to lose 334 joules to freeze. So it will be liquid water, at 0C, for 16.7 hours until it finally becomes ice.
So lets say you have two rooms. Room A at -20C and Room B at -25C. And you put an identical quantity of water in each room. The water starts at 10C in both cases. You leave it there long enough so that the water in room A has lost enough energy to reach 0C but has lost no more than that. The water in room B has lost enough energy to have reached 0C and has then lost more energy but has not frozen yet.
The water in room B has lost more energy, correct? Is it now different? For example, if you place both glasses in Room C where the temperature is 0C and then apply an equal amount of heat to each glass for the same amount of time, will they end at the same temperature or will they end at different temperatures?
My thinking is that the water from room B must overcome the energy it has lost even though it has not yet frozen. I suspect I’m wrong but I don’t understand why. If the water from room B ends at the same temperature as the water from room A, where did the energy go from it’s earlier cooling?
Lets take my earlier example: there’s one gram of water at 50 degrees C, it is losing 20 joules each hour in a freezing room. You leave it in the room for 20.5 hours. The water lost 210 joules, attained 0C, then lost 200 more joules but has not yet frozen. You remove it from that room and apply 204.2 joules of energy to it. Does it end at 1 degree or at 48.62 degrees?
PS–this is not a homework assignment; I am in the process of installing an above ground water tank in a place with very long cold winters. I’ve been doing some reading to work out my safety margins (they are encouraging) but I am curious about freezing temperature water that has not yet frozen. I’m sure I’m somehow wrong about the “extra energy” business but if I were right (and certain masses of water at 0 required more energy to warm than other masses of water at 0, the “colder” water would act as a better coolant and could royally screw up your plumbing and septic system!) At this point I’m just really curious WHY I’m wrong.
You are assuming the water will all freeze as a homogeneous lump. Ice crystals will start to form inside the mass of cold water, and grow as the energy is further lost. Eventually all the water will be frozen.
However, if you do it very carefully you can avoid the start of ice crystal formation and actually drive the water to lower than 0C. This is supercooled water. It only needs a tiny jolt or addition of a nucleation site for the crystals to start to form, at which point it will almost instantaneously freeze all the water that can (as determined by the energy in the system.)
smacks forehead of course. It will form ice around the corners and along the inner surface of the tank and work its way to the center. Seems obvious now!
Still curious what happens right before crystallization, though. What precisely is happening just before you shake up that supercooled water, that makes them easily crystallize after the water gets shaken?
There are tons videos on youtube demonstrating water being poured out of a bottle and freezing. The link below is a nice one. You just need to get plastic bottles of water cold but not too cold.
Supercooled water is, in fact, colder than 0 Celsius. So what’s “going on” in it is that the molecules aren’t moving as much as liquid water ought to be.
That doesn’t really answer my question. I think it’s pretty safe to assume we all know that kinetic motion in the molecules decreases with lower temperature.
The question is what happens to the molecules right before crystallization. Why is it that actually adding energy to the system allows crystallization? The water is still a liquid at that point, which defies the usual claim of how freezing works (that the attraction of the molecules overcomes their kinetic energy).
My best guess based on what I’ve read is that adding energy allows the molecules of water to come in contact with impurities which provide nucleation points, which can then create crystallization. Said impurities might include the walls of the container.
I’m far from sure of this, but it does describe the sort of answer I was asking for. It describes the state of the water between the supercooled state to the point where it crystallizes.
Super-cooled water is water. It’s just water. It’s a little smaller that just-above-freezing water, which, you remember, is a little larger than slightly-warmer water.
There is a small energy barrier between water and ice (this is not unusual). Once crystalisation starts, the energy released is enough to take other water over the energy barrier (until everything gets too warm).
My guess would be that the energy is required to align the water molecules correctly. Once they are alligned correctly (in three dimensions) they snap into place (with release of energy).
Even in the complete absence of nucleating dust, agitating the water will produce pressure fluctuations. Some few molecules in some places will be jostled enough to stick to their neighbors as ice. Which sets off the crystallization cascade releasing energy as it goes.
Like the room full of mousetraps with ping pong balls, everything is poised on an energy pinnacle and is just waiting for a slight perturbation to begin the large-scale transition to a lower overall energy state.
This is exactly right. It’s also worth noting that supercooling is not just an interesting curiosity: it’s very common in nature (well, in cold climates anyway) and is a key element in how winter-hardy plants avoid freezing damage. A few pages of this book chapter should be non-paywalled and provide a nice explanation:
What’s the largest conceivable amount of supercooled water you could have? Could you have an entire swimming pool full of the stuff? What would happen if you dived into it?
It might even be easier to supercool a large quantity of water (if you had a large enough refrigeration system), because you’d get more volume for your surface area. If you had a supercooled swimming pool and dived into it, then ice crystals would begin forming around you, extending out from you to a thickness depending on just how far supercooled it was. Shortly after that, of course, the ice immediately next to you would start re-melting from your body heat.
If you were going fast enough, you’d get a “sonic boom” effect, with crystalization spreading out from the shock wave around your path. Possibly ending with you snap frozen in a solid block of ice –
–or perhaps sealing off your passige behind you, blocking your evil pursuers as you escape through an underwater tunnel–
If you were going slower, the ice would form ahead of you (that might be very uncomfortable) but perhaps you could “walk on water”, which would be pretty awesome too.
