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Old 11-04-2012, 17:36   #16
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Re: Ice to keep something from freezing?

Originally Posted by Wotname View Post
Well I learn something new every day - Nitrogen ice - never heard the term before! I would have guessed it was a bad way of expressing "solid nitrogen"!

For me, water is liquid H2O, ice is solid H2O, "water ice" is a mixture of liquid H2O and solid H2O both at 0 degC and cant exist at any other temperature. I am using H2O to denote pure water as you rightly pointed out that when pure water is compounded by other substances, the numbers change.
"Nitrogen ice" i know.. you hear some crazy stuff now and again.. more often than not on Discovery channel which has become so pseudoscientific it's shocking!

Wrt the point at which elements or compounds can exist in solid liquid and gaseous states at the same time you'll be refering the the triple-point of water.. a surprisingly difficult thing to measure to accuracy in practice - but is used to set one point of the scale for international standards.

I would have written about this but as usual wikipedia has already done a good job:
Triple point - Wikipedia, the free encyclopedia

This will also help answer the question regarding the freezing point of water being effected by pressure etc. (i.e. deviation from STP)


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Old 11-04-2012, 18:02   #17
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Re: Ice to keep something from freezing?

Originally Posted by David M View Post
Wheeler is still there? Lloyd Kitazono is still there also. One in a while I take one of his oceanography classes out on the research boat and show them how the professionals do it.....LOL. He is a great guy. We have all kinds of sampling gear and electronics to impress them...

Yeah, I remember the additional energy it takes to jump from a solid to a liquid state for water and the giving up of energy in the opposite direction. I'm not sure how much it would work in the scenario you described. Maybe give Jim a call

James*Wheeler - California Maritime Academy -

Lloyd*Kitazono - California Maritime Academy -
Both Jims are still there, Buckly and Wheeler. I have nothing but the highest respect for both, even went to the "rate your professor" website about 3 months ago, to post positives to counter the negatives from current midshipmen. Since you have their link you probably saw my ratings and comments.

Also, Paul Leyda and his robust ego is still there too. Paul is just a couple years younger than me.

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Old 28-06-2012, 18:30   #18
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Re: Ice to keep something from freezing?

In case anyone's still confused about certain terminological matters, my understanding is this:

Wotname took "water ice" to signify a mixture of water and ice, (which I would prefer to call "water + ice" or "an ice/water mix")

... whereas as I understand it, Cavalier intended "water ice"* to signify "ice composed of frozen water"

(as opposed to some other liquid, CO2 being another obvious example .... although (red herring alert!) 'dry ice' a rather misleading term, given that "water ice" is dry to the touch once it gets colder than freezing)

*[on edit: perhaps "frozen water" would be a less confusing term?]

It's difficult to get a mixture of water and "water ice" below 0 deg C, but may not be impossible, given sufficient purity. I don't know. That observation is possibly another red herring in the real world situation, where AFAIK there are always nucleators to defeat supercooling.

But the point I'm trying to make is this, and I think a lot of lay people (like me) are under this misapprehension:
The belief that ice (frozen water) cannot be colder than 0 deg C is what I'm talking of, and this is a different matter.

If that were so, skiers would have never experienced "wild" powder snow, so light and dry that you leave no tracks (because it flows back into them, like paint levelling) and the aerial component either hangs like smoke or just sublimes into the air rather than falling back to earth.... (ahhh, memories!)

It's complicated by the fact that snow is a good insulator, so that snow caves are warmer than staying outside whenever temp (adjusted if necessary for wind chill) gets below zero...

but the crystals on the inside of a snowcave will soon become wet, because you're warming the air inside the cave to zero with body heat and respiration. Hence in a relatively short time, as I understand it, the superficial layer of snow insulates you from the coldness of what lies behind.

I think this, and other examples like it, is why we tend to think that ice is necessarily at freezing point, never below.

That's only true, AFAIK, when it's in the presence of liquid water - even if only a surface film, a few molecules thick.

As always, I'm open to correction on any of this, and would welcome it from anyone with the ability, time and inclination.
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Old 29-06-2012, 09:02   #19

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Re: Ice to keep something from freezing?

Bob, a sack of heated "blue goo" might be your best bet. My understanding is that blue goo, like water, undergoes a phase change when it converts from a liquid (gel) to a solid. When the molecules shift from one state to another, they bind/release more energy than a simple change in temperature would involve.

So, hot water, or heated goo. It absorbs energy all the way to the phase change point, then it gobbles a larger chunk of thermal energy in order to make the phase change. And continues absorbing energy past that as it cools.

Mysterious changes in snow are caused by sublimation, another process where individual particles of the solid matter literally "zoom off" to other places. Much like ice cubes shrinking in a self-defrosting freezer.

Of course if you stuck a mess of kittens in the box, they'd help keep it warm too.

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