quest
Hazard to Self
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Registered: 15-9-2003
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sodium acetate heating pillow
Hello guys,
yesterday I found on some gadgets site a "heating pillow",
this pillow is filled with sodium acetate solution. In this pillow there is also a metal disk, when you press the disk the pillow becomes warm (54
degree celsius in 5 seconds - as the site claims).
this pillow stays warm for about 1 hour.
for reuse you just boil the pillow in hot water for some minutes.
I know sodium acetate solution can be chilled in the freezer and when you touch it with your finger it starts to make crystals and it looks like it is
freezing instantly.
After this long prolog, my question is just - why sodium acetate solution become so warm when recrystallize and why only the metal disk making it
crystallize?
I guess the sodium acetate make chemical conections with its molecules and breaking connections with the water, and more energy is produced from
making the connections the breaking the water connections.
(if this is the case, why doesn't it recrystallize when chealed?)
thanks.
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12AX7
Post Harlot
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The chemical reaction is: a supersaturated solution of sodium acetate crystallizing into the trihydrate solid. Solids have a lot less energy than
liquids (that's why they are hard instead of moving past themselves, for one), so heat is released. How much depends on the enthalpy of that phase
change.
The quirky thing is, perhaps because of the bulky nature of the acetate ion (it has to be oriented properly to settle into a solid), sodium acetate is
able to remain supersaturated for an extended period of time, given it doesn't have anything in contact to seed crystallization. And that's where the
metal disc comes in: the shock of plinking it is evidently enough to induce crystallization. This is probably the same effect as scraping the side of
a beaker to induce crystallization when recrystallizing substances. From there, the crystals grow like a heat wave across the package.
Tim
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ScienceGeek
Hazard to Others
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Registered: 22-1-2008
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That is not quite true. It is not the shock that induces recrystallization, but instead, by bending the metal piece you create a nucleation source. In
this case, a very narrow slit is exposed by bending the metal.
This same principle goes for old soda- bottles that wont stop effervescing. Old bottles have a lot of scrapes in them 
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