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Q & A: water volume vs temperature

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Most recent answer: 02/21/2013
Q:
How can water both expand when it freezes and expand as it heats up? Is it true that as the earth heats up the oceans could potentially heat up and the result of warmer water would be that the water would take up more space? I would think it would take up slighlty less space as it warms from ice.(This is separate from adding more water mass to the oceans from melting ice.)
- George (age 42)
Tucson
A:
The (positive) change in volume when water freezes doesn't have a necessary logical connection to how the volume changes as the liquid is cooled. However, it happens that liquid water does expand a bit as it's cooled below 4C. Above 4C it expands as it's heated. Here's some of the data: .

So yes, the first little bit of warming of ice water causes it to shrink a tiny amount. Further warming causes more significant expansion. Most of the oceans are warm enough for global warming to cause net expansion. It's already happening, and it's measurable. (see )

Mike W.

(published on 02/14/2013)

Follow-Up #1: Is there a liquid-solid continuum?

Q:
This is a great Q&A thread; the perfect find after my typical hour-long drive to work which brings me past several frozen lakes and countless fleeting musings. Today I began to think about the water of these lakes and the liquid-solid state continuum that must exist as the colder, less dense water rises to the top to freeze and its interaction with the (dare I say "warmer") water below. Conceptually, I think, there must be a "slush-zone" slightly above where the water is in equilibrium where the water is more viscous. I'm thinking of this in the same terms of the transition zones between magma and mantle. My mind wandered a bit after this, beyond the local lakes and the spring thaw to the constant cold polar regions. If my idea of a "slush zone" is valid, then, while the water itself would be ever changing the zone's presence would be constant. Does this increased viscosity make for a micro-environment of sorts? Are there organisms that thrive BECAUSE the water is thicker?
- Randy (age 37)
Illinois
A:
Randy- I made a guess as to what question you were following up. You can write back if you meant another thread.

It turns out that there really isn't a liquid-solid continuum. There's an abrupt difference, in density, hardness, electrical properties, etc. between these two different phases, the jumbled-up molecules of the liquid phase and the orderly crystalline array of the solid phase. The change between liquid and ice is thus called a "first-order phase transition". Slush is just a mixture of ice crystals in liquid water, not a real in-between state. If you leave it at some temperature for long enough, it will all turn to either ice or liquid, depending on the exact temperature.

Nevertheless, you're right that the viscosity of water does increase quite a bit as it gets colder. That's pretty typical for most liquids even if they aren't near a phase transition.

Some liquids, however, show a surprisingly sharp increase in viscosity as they cool  near to a special temperature. They turn solid-seeming without ever going through a phase transition to form actual crystals. This frozen state is called a "glass". I don't know much about magma, but it's possible that it has one of these glass transitions, without a sharp phase transition from the very viscous liquid to the hard glass, like obsidian. At the mantle-crust interface, I guess things are just stirred up a lot by all the heat flow.

I don't know whether any organisms rely on the increased viscosity below 0C. I bet they have evolved to work best at whatever viscosity they live in.  Here's a semi-related fun fact. Some fish near Antarctica live through the winter at temperatures well below the freezing point of their own blood. They're supercooled. How do they manage? They have sort of anti-nucleating agents, little proteins that bind to the surface of tiny ice crystals to prevent them from growing.

Mike W.

(published on 02/21/2013)

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