Le 2011/09/05 18:50, Patrick Elliott a écrit :
> On 9/5/2011 10:47 AM, Darren New wrote:
>> On 9/5/2011 8:52, Invisible wrote:
>>> Yes, but under /normal/ circumstances, it boils at 100°C, which is why
>>> it's
>>> defined that way. :-P
>>
>> No it doesn't. At 100°C, it's in equilibrium. Just like at 0°C, it's
>> neither getting more frozen or less frozen.
>>
> Worse than that, in certain conditions you can "hyper-heat" water, and,
> I assume, probably super cool it, without changing state. In the former
> case you just need a container that has "no" places for bubbles to form.
> Not sure what you would need to do in the later case to make it happen,
> if you could.

You can have water that stay liquid down to -10°C and possibly even 
less. You need a container with very smooth surface and no particles in 
suspention. In this state, a vibration can be just enough to cause 
almost instant crystalisation.

If you slowly heat very pure water, it can bet a good bit warmer that 
100°C before it start to boil. There will be vapour escaping from the 
surface, but no ebulition, even in a open container. It's a dangerous 
situation, as any disturbance can cause explosive ebulition projecting 
scalding water everywhere.

If you slowly cool down steam and don't give it any nucleation point, 
that steam may stay totaly gaseous at relatively low temperature. You 
get hyper-saturated steam. It appens in, and around, clouds. Again, it's 
an unstable situation and even a very faint sound, or a dust particle 
can cause almost instant condensation.

But you can't prevent ice from melting as you slowly heat it.

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On Mon, 05 Sep 2011 21:12:15 -0400, Alain wrote:

> You can have water that stay liquid down to -10°C and possibly even
> less. You need a container with very smooth surface and no particles in
> suspention. In this state, a vibration can be just enough to cause
> almost instant crystalisation.

Which is actually quite cool to see - I've seen it done with beer before.

Jim

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On Mon, 05 Sep 2011 19:32:49 +0100, Orchid XP v8 wrote:

>>> Yes, but under /normal/ circumstances, it boils at 100°C, which is why
>>> it's defined that way. :-P
>>
>> Not here at 4,000 feet - "normal" circumstances here have it boiling at
>> a slightly lower temperature. :P
> 
> 96.2°C, a piffling 3.8° lower.

Which is still not 100C no matter how you slice it.  The difference in 
humidity, altitude, and other things makes cooking things that require 
precise measurements (usually baked goods) slightly different than at 
other altitudes and in other conditions.

>> IOW, it all depends on how you define "normal".  (Hence my suggestion
>> of reducing the atmospheric pressure, because here, lower pressure is
>> 'normal').
> 
> This is why the /actual/ specification is far more complicated. But for
> any sane real-world purpose, you can more or less ignore such
> technicallities.

Baking is a real-world purpose.  An awareness of the 'normal' as well as 
'current' environment are, for some things, absolutely *critical* in 
order to be successful.

Jim

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On 06/09/2011 06:25 AM, Jim Henderson wrote:
> On Mon, 05 Sep 2011 21:12:15 -0400, Alain wrote:
>
>> You can have water that stay liquid down to -10°C and possibly even
>> less. You need a container with very smooth surface and no particles in
>> suspention. In this state, a vibration can be just enough to cause
>> almost instant crystalisation.
>
> Which is actually quite cool

I see what you did there.

I've got a box of hand warmers at home. They contain a super-saturated 
solution (of what I don't know). Once you provide a nucleation point, 
the whole lot crystallises within a few seconds. It also gets quite warm 
in the process. (This is what makes it good for warming your hands.)

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On 06/09/2011 06:27 AM, Jim Henderson wrote:
> On Mon, 05 Sep 2011 19:32:49 +0100, Orchid XP v8 wrote:
>
>>>> Yes, but under /normal/ circumstances, it boils at 100°C, which is why
>>>> it's defined that way. :-P
>>>
>>> Not here at 4,000 feet - "normal" circumstances here have it boiling at
>>> a slightly lower temperature. :P
>>
>> 96.2°C, a piffling 3.8° lower.
>
> Which is still not 100C no matter how you slice it.  The difference in
> humidity, altitude, and other things makes cooking things that require
> precise measurements (usually baked goods) slightly different than at
> other altitudes and in other conditions.

I would suggest that the wild variations in oven temperatures have a 
vastly bigger impact than a 4% difference in the boiling point of water.

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On 9/5/2011 6:12 PM, Alain wrote:
> If you slowly heat very pure water, it can bet a good bit warmer that
> 100°C before it start to boil. There will be vapour escaping from the
> surface, but no ebulition, even in a open container. It's a dangerous
> situation, as any disturbance can cause explosive ebulition projecting
> scalding water everywhere.
>
Or, very fast, by heating it all more or less at the same time (i.e., 
with something like a microwave oven.

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On 06/09/2011 02:12 AM, Alain wrote:

> If you slowly heat very pure water, it can bet a good bit warmer that
> 100°C before it start to boil. There will be vapour escaping from the
> surface, but no ebulition, even in a open container. It's a dangerous
> situation, as any disturbance can cause explosive ebulition projecting
> scalding water everywhere.

Legend has it that boiling water in a microwave oven has this effect. I 
don't know of an authoritative source which can confirm or refute that...

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On 9/6/2011 1:04 AM, Invisible wrote:
> On 06/09/2011 06:25 AM, Jim Henderson wrote:
>> On Mon, 05 Sep 2011 21:12:15 -0400, Alain wrote:
>>
>>> You can have water that stay liquid down to -10°C and possibly even
>>> less. You need a container with very smooth surface and no particles in
>>> suspention. In this state, a vibration can be just enough to cause
>>> almost instant crystalisation.
>>
>> Which is actually quite cool
>
> I see what you did there.
>
> I've got a box of hand warmers at home. They contain a super-saturated
> solution (of what I don't know). Once you provide a nucleation point,
> the whole lot crystallises within a few seconds. It also gets quite warm
> in the process. (This is what makes it good for warming your hands.)
Seen those. There is also some expensive shirt that was made for 
joggers, which uses a similar principle, though, in that case, I get the 
impression they formulated the material so it crystalized faster than 
normal in cold temperatures, releasing heat, but softening faster than 
normal too, absorbing it, thus keeping you cooler, or warmer, depending 
on conditions.

Always wondered, in the case of the whole "heat pack" thing though if 
you couldn't make one that did the reverse, and was "recoolable", or 
whatever. The principle being, of course, than it is rechargable, so 
long as the solution doesn't change (often the plastic is 
semi-permiable, so loses moister over time, and stops working), so when 
you let it cool slowly, it "holds" the excess energy. When you apply 
kinetic energy to it, it instantly starts losing all of what it 
retained, and crystallizes. You would basically need to do the reverse, 
somehow, for a "cold pack", crystallizing it, so as to "lose" more heat 
than it needs in a stable state, then.. there comes the rub. Other than 
that it would liquify as it absorbed the heat, instead of crystallizing, 
like the supersaturation, I am not clear how you would manage to make it 
so it could recharge effectively.

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On 9/6/2011 2:36 AM, Invisible wrote:
> On 06/09/2011 02:12 AM, Alain wrote:
>
>> If you slowly heat very pure water, it can bet a good bit warmer that
>> 100°C before it start to boil. There will be vapour escaping from the
>> surface, but no ebulition, even in a open container. It's a dangerous
>> situation, as any disturbance can cause explosive ebulition projecting
>> scalding water everywhere.
>
> Legend has it that boiling water in a microwave oven has this effect. I
> don't know of an authoritative source which can confirm or refute that...
Uh.. Mythbusters? They tried it, and it did. lol

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> Always wondered, in the case of the whole "heat pack" thing though if
> you couldn't make one that did the reverse, and was "recoolable", or
> whatever.

Hmm. Plausibly.

I would imagine, like with other refrigeration technologies, the key is 
to find a material which has a phase change at the right temperature. 
Normal hand warmers become super-saturated solutions at very high 
temperatures. If you wanted to make something cold, you'd need something 
that becomes super-saturated at a low temperature.

I'm also not completely sure of the kinetics: a hand warmer releases 
heat quickly, and absorbs it slowly. That may or may not be useful for 
cooling purposes.

Unrelated, but I note that dissolving ammonia in water is an endothermic 
reaction. That, apparently, is how Victorian icecream was made. So I 
guess the question is, WOULD YOU EAT IT? ;-)

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