instant cold packs
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instant cold/heat packs
I'm not too sure whether I posted this question in the right place, I'm new to this forum and I think it's awesome!! If I have posted it in the wrong
place, I apologize in advanced.
I am interested in the chemistry behind instant heat and cold packs and how they work (the type of instant cold packs that don't need to be placed in
the freezer before use). I have read a few threads about what is actually inside these packs and how they work, but I had a couple questions:
1) Is my understanding of how instant heat/cold packs work, correct?
Heat packs:
Exothermic reaction is initiated when breaking the tab causing the temperature to be around 130 F for around 15-20 min. This reaction is created by
the crystallization process of the liquid sodium acetate/water causing it to solidify/release heat.
Cold Packs:
Endothermic reaction initiated when breaking the tab causing temperatures to be around 35 F for 10-15 min. Reaction begins when the ammonium-nitrate
tab is broken, mixing it with the water, absorbing heat from the endothermic reaction.
2) If the reaction for heat packs can be "reset" by boiling it, melting the crystals back into the liquid state, is there a way to
reset the reaction for cold packs so that they can be re-usable similar to the heat packs? (perhaps via boiling/breaking another tab to reverse the
reaction..?)
3) Is it possible to store the cold packs at room temperature (or any other temperatures other than temperatures relative to the
fridge/freezer) and still be able to activate the endothermic reaction creating the cold temperatures?
Thanks!
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elementcollector1
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Of a one-track mind, aren't you? Username - instant cold packs, first topic - instant cold packs... Makes me think something's up here. Call me
paranoid, but you're going to have to let us know what your intended use is beyond 'interested in the chemistry'. Granted, that can be a valid reason,
but there are other uses for ammonium nitrate.
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bfesser
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<strong>elementcollector1</strong>, read the numbered questions asked. Nothing suspicious, just some beginner-level confusion. Granted,
the username is odd. [edit: gender neutral]
[Edited on 14.10.13 by bfesser]
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elementcollector1
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I did, and I guess I'll answer them.
1) Your understanding is for the most part correct.
2)Cold packs can be 'reset' by taking the solution inside, boiling it back down to ammonium nitrate crystals, and setting up the same thing as in a
cold pack (a full bag of water that can be squeezed to burst, inside a pack full of crystals).
3)Cold packs can be stored at any temperature below the point where the plastic begins to degrade (usually anywhere between 150 degrees C and 300
degrees C). They can be activated at any temperature as well (given the same circumstances as previous).
Sorry for being paranoid - there are a few chemicals that immediately raise red flags of interest and affect my posts, ammonium nitrate being one of
them.
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Metacelsus
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Be careful when boiling down ammonium nitrate solutions. If you boil off all the water, you will transition directly to molten ammonium nitrate, which
will then decompose and give off dense, choking fumes (I found this out the hard way  ). Instead, boil off most of the water, cool the solution, and collect the precipitated crystals.
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instant cold packs
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ah, I can understand why this can be a sensitive topic given the specific chemicals involved. As for my username...I just used the first thing that
came on my mind? I'm really only interested on why cold packs can't be tossed into a pot of boiling water to have the reaction reset similar to heat
packs after they have been used. I just don't understand why all the cold packs I have bought in the past are either one time use or require being
placed in the freezer in order to reuse/reset the cold pack (hence not instantaneous since I can't carry it around like the heat pack and activate the
reaction so that I can use it on the spot).
Which brings me to a point which I still don't really understand.
What's the key difference between the 'resetting' (boiling) in heat and cold packs?
Why can't we just simply toss the cold pack into a pot of boiling water and upon removal, be able to use the cold pack again, instead of throwing it
away?
edit: Sorry, I wanted to add one more question:
Is there a way to reset the reaction for instant cold packs that doesn't involve taking the solution out?
If not, how come heat packs are able to reset by simply tossing the entire pack in boiling water and be ready for future use without needing to take
out the solution?
[Edited on 15-10-2013 by instant cold packs]
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bismuthate
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Let me explain, so if you threw a sealed plastic bag full of water into boiling water it will burst (don't try this). Also the Ammonium nitrate
absorbs heat when dissolved in water so you would need to get rid of the water to reuse it. It is just energy absorbed when a solvent dissolves
certain compounds.
Here, have a few wiki links.
http://en.wikipedia.org/wiki/Enthalpy_change_of_solution
http://en.wikipedia.org/wiki/Instant_cold_pack
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WGTR
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Quote: Originally posted by instant cold packs  | | I'm really only interested on why cold packs can't be tossed into a pot of boiling water to have the reaction reset similar to heat packs after they
have been used. [Edited on 15-10-2013 by instant cold packs] |
The hot packs that you are talking about use a super-cooled/super-saturated salt dissolved in water. Heating the solution dissolves the salt; but
when it cools down again the salt does not come out of solution until a nucleation point is provided (either by clicking a metal tab, or providing a
solid salt crystal). At that point the salt crystallizes quickly, releasing heat in the process.
I'm not sure if there is an analogous reaction for a cold pack, however. Some salts will heat water up when they are dissolved into it, others will
cool it down. Ammonium nitrate happens to provide a strong cooling effect when dissolved into water. The only way that you can "store" this
cooling effect for later use is to keep the ammonium nitrate and the water isolated from each other. This is why the water is stored in a little
plastic bag inside the cold pack. If you wanted to reuse the ammonium nitrate, you would need to cut the bag open and boil the water (carefully)
back out of the solution.
[Edited on 16-10-2013 by WGTR]
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Wizzard
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WGTR - You could not boil the water out without decomposing most of the ammonium nitrate!
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confused
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you could evapourate it to obtain the crystals, that way the ammonium nitrate won't decompose
[Edited on 16-10-2013 by confused]
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vmelkon
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I don't see what the problem is. He just asked about how a cold pack works. He didn't ask where to buy some.
Now, most cold packs are either Urea or a mix of calcium and ammonium nitrate.
"Cold packs can be stored at any temperature below the point where the plastic begins to degrade (usually anywhere between 150 degrees C and 300
degrees C)."
What? The water in it will boil.
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WGTR
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Thank you for pointing this out.
If you want dry ammonium nitrate, boiling it will not get you there without a lot of decomposition, as you say. But for the purpose of a cold pack,
boiling out the water should still get ammonium nitrate dry enough to work. Or maybe not. In any case you have to control the temperature to keep it
from going too high.
Anyway, I think what the OP wants to know is if there is a way to "recharge" a cold pack without opening the bag, similar to the way the super-cooled
salt solutions work in hot packs. I don't know of one, personally, but maybe I'm missing something.
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bfesser
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| Quote: Originally posted by bismuthate | | Let me explain, so if you threw a sealed plastic bag full of water into boiling water it will burst (don't try this). |
Yes, but these heating/cooling packs contain solutions, not pure water.
<a href="http://en.wikipedia.org/wiki/Boiling-point_elevation" target="_blank">Boiling-point elevation</a> <img
src="../scipics/_wiki.png" />
<a href="http://en.wikipedia.org/wiki/Colligative_properties" target="_blank">Colligative properties</a> <img
src="../scipics/_wiki.png" />
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instant cold packs
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It kind of all makes sense to me now after reading all the links and explanations. So I guess the main point why instant cold packs can't be 'reset'
in the same way as the hot packs, is that once the chemical is 'mixed' into the solution, we can't simply 'dissolve' it in the same way we would for
the salts in the hot packs to reset the reaction.
Cool, sorry for any trouble I might have caused haha but thanks for all the help everyone!!
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bismuthate
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bfesser I thought that the heat would cause the air in the bag to expand therefor popping it. Also would AN raise the boiling point that much? (I
guess it depends on the amount of AN). Usually a pot of boiling water is quite a bit over 100 degrees C.
[Edited on 16-10-2013 by bismuthate]
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stromglas
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hi everyone. not really qualified to have much of a say but that rarely stops me....if the bag was modified to have a tyvek type outermembrane(allows
water vapour to pass but is waterproof) the the item could potentially be sold without water, water (instuctions include how much to add) could be
added through a screwtop opening. then, after use it could be left out in the sun/dessicator box and in theory the water vapour should be able to
exit through the membrane leaving you with a reusable bag of AN crystals??
Shoot me down if this is missing a whole bunch of realworld science knowledge.
long time listener first time poster
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elementcollector1
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| Quote: Originally posted by stromglas | hi everyone. not really qualified to have much of a say but that rarely stops me....if the bag was modified to have a tyvek type outermembrane(allows
water vapour to pass but is waterproof) the the item could potentially be sold without water, water (instuctions include how much to add) could be
added through a screwtop opening. then, after use it could be left out in the sun/dessicator box and in theory the water vapour should be able to
exit through the membrane leaving you with a reusable bag of AN crystals??
Shoot me down if this is missing a whole bunch of realworld science knowledge. |
Quote from Wiki: | Quote: | | Water vapor can pass through Tyvek (highly breathable), but not liquid water. |
It would work, but take an awfully long time - unless you heated it without affecting the Tyvek somehow. I might have some Tyvek (I was given a
whitesuit by a Boy Scout counselor for the Composite Materials badge), so I could run some tests and find out how feasible this is.
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