Weird Question

Malicious Wraith · Post by **Malicious Wraith** » Fri Feb 13, 2015 7:43 pm

You sciency folks.

When standing water freezes because of atmospheric cooling:
A. does the center of the water freeze first
B. does the water instead freeze from the part of the water closest to the lower temperature, and "radiate" inwards?
C. is there some weird sciency factor that controls how water freezes?
D. what am I even asking?

cashto · Post by **cashto** » Fri Feb 13, 2015 9:20 pm

Science: try it and see.

But generally, nothing cools at a uniform rate. Some part of the water will be coldest first -- usually the edges. That will be the part that freezes first.

Also because ice is less dense than liquid water, it will be transported upwards by buoyant forces as it freezes, which is why lakes etc. freeze over on top. In other liquids, ice sinks to the bottom after freezing.

Also, water can be supercooled: cooled below its freezing point and still remain a liquid if there are no nucleation sites (e.g. dust or impurities in the water).

Adaven · Post by **Adaven** » Fri Feb 13, 2015 9:42 pm

cashto wrote:QUOTE (cashto @ Feb 13 2015, 04:20 PM) Science: try it and see.

I remember as a child being fascinated by partially frozen ice cubes in our freezer. They generally froze from the outside in, but with a thicker layer on top. I'm not sure I ever got one out of the tray fully intact. More often they would crack and the water run out, leaving a hollow icy shell.

dusanc · Post by **dusanc** » Fri Feb 13, 2015 10:57 pm

Adaven wrote:QUOTE (Adaven @ Feb 13 2015, 10:42 PM) I remember as a child being fascinated by partially frozen ice cubes in our freezer. They generally froze from the outside in, but with a thicker layer on top. I'm not sure I ever got one out of the tray fully intact. More often they would crack and the water run out, leaving a hollow icy shell.

I did manage to get one out by slowly pouring hot water over upside down tray so they thaw where in contact with tray.

Ahhh childhood....

Post by **zombywoof** » Fri Feb 13, 2015 11:29 pm

Malicious Wraith wrote:QUOTE (Malicious Wraith @ Feb 13 2015, 11:43 AM) You sciency folks.

When standing water freezes because of atmospheric cooling:
A. does the center of the water freeze first
B. does the water instead freeze from the part of the water closest to the lower temperature, and "radiate" inwards?
C. is there some weird sciency factor that controls how water freezes?
D. what am I even asking?

a) no
b) yes... ish.
c) yes, if by "weird sciency factor" you mean "where the energy is leaving the system and how energy is conducted throughout the system.

For example: lakes freeze from top to bottom because generally the air temperature is much cooler than the ambient ground temperature. Because of the way water mixes, the ice will also be generally thicker at the edges than it will in the center. This is because when energy is taken from the water in the center and given to the air (to bring the air and water in thermal equilibrium), at the edges where the water is shallower there's much less water so it loses energy faster.

Generally because of that, the center of the water does NOT freeze first, though you can in some regards control this based on the shape of your water container. For example, imagine something that's like a bundt cake pan, but fully submerged. The ice will form in the "center" (near the raised part of the pan) before it forms on the "edges" of the pan.

It also depends on what the standing water is in. If you had a cylindrical container raised off the ground and made of, say, copper (or silver), the ice would form on the sides of the container before it even appeared on the top. Believe it or not you've seen that happen before: pour a cold beer into a frozen mug.

This brings us to D:

Water freezing (or any liquid freezing (or gasses deposing or condensing)) is a process by which the energy level of the average molecule (or atom in cases of monatomic gasses/liquids) drops below a certain point and intermolecular forces can keep the pile of molecules from moving. Essentially what's happening is energy is *leaving* the body of water.

Much like electricity, heat can be conducted. How quickly it's conducted depends on the material, and generally heat conduction is closely correlated with electrical conduction... which is to say that the more electrically conductive a material is, the more easily it conducts heat. This is a rule of thumb and not necessarily true; however.

Other things can affect conduction as well. You might have noticed that taking a spoon of hot soup and blowing on it will lower the temperature a lot faster than simply leaving the soup to sit on your spoon.

One of the cool things about systems in science (especially thermo) is that the larger they get, the more arbitrarily you can divide them. In the example of a lake that's shaped like an inverted omega (lowercase or uppercase) the edges are obviously more shallow than the center. We can "subdivide" large lakes into two groups then: one is a shallow ring shaped like a washer, and the other is a big cylinder that fits into the "hole" of the washer. Like electricity (and car traffic for that matter) the wider the opening available the faster things can get in and out. How "wide" the opening is is a function of how much surface area our standing body of water has with whatever's taking the energy out of it fastest (in most cases, the open air). How "much" energy there is (which needs to be removed in order to freeze the lake) is a function of the body of water's volume. Therefore, sections which have a higher surface area/volume ratio will freeze faster.

This also gets a bit more complicated as water is much worse than ice at conducting thermal energy. If you have thick enough ice and blow across its surface, you'll just tend to make the ice colder rather than freeze more water.

Hope that's illuminating!

EDIT: if you're preparing for a thermo exam in college, the information above is going to get you pooped on by the professor. It's not technical, it's not 100% accurate, and is merely presented as a general overview of how the process works. It's suitable for high school courses and below... MAYBE court if you're trying to explain the process to a jury.

MrChaos · Post by **MrChaos** » Sat Feb 14, 2015 1:51 am

Wasting your time here bud go googling. Try NIST university of Illinois Urbana (iirc) . Also go hunting commercial websites. .. it works for me sometimes. A British steam trap manufacturer, at least that is how I knew them has an incredible comprehensive series of papers on turbo machinery for example.

Malicious Wraith · Post by **Malicious Wraith** » Sat Feb 14, 2015 8:53 am

Wasn't a serious thing I needed to know.
I was pulling a partially frozen water bottle out of the trunk of my car, and I was like, "This is cool. I wonder what makes this happen. I bet the guys on FAO know."

MrChaos · Post by **MrChaos** » Sat Feb 14, 2015 3:05 pm

Malicious Wraith wrote:QUOTE (Malicious Wraith @ Feb 14 2015, 03:53 AM) Wasn't a serious thing I needed to know.
I was pulling a partially frozen water bottle out of the trunk of my car, and I was like, "This is cool. I wonder what makes this happen. I bet the guys on FAO know."

The problem originated with your inability to ask the question you wanted answered. The root seems to be in your thinking that how a wate bottle freezes directly corelates to fresh and salthough water bodies behave. The things you do b not know are the road blocks tof the answer you seek.

dusanc · Post by **dusanc** » Sat Feb 14, 2015 3:17 pm

MrChaos wrote:QUOTE (MrChaos @ Feb 14 2015, 04:05 PM) The problem originated with your inability to ask the question you wanted answered. The root seems to be in your thinking that how a wate bottle freezes directly corelates to fresh and salthough water bodies behave. The things you do b not know are the road blocks tof the answer you seek.

MrC got a smartphone.

Post by **Raveen** » Sat Feb 14, 2015 5:19 pm

Crystalisation is a fascinating and poorly understood topic. Generally water will start to form ice crystals around a nucleus of some sort. So in your bottle that could be a speck of dust or an imperfection on the side of the bottle. Once the first crystal starts to grow others will form around it.

It is perfectly possible to have supercooled water which is below freezing but still liquid. A singularly terrifying example of this (to highway engineers at least) is freezing rain where the water falls as a liquid and instantly freezes as soon as it hits the ground.