waLLcandy
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Reusable hot packs?
I'm trying to find out what is in those reusable hot packs (hand warmers).
http://beprepared.com/product.asp_Q_pn_E_CW%20W500_A_name_E_HotSpot%E2%84%A2%20Reusable%20Hand%20Warmer
They are activated by flexing a small metal disc in a gel, which causes them to heat up. They can be "reset" by boiling them.
Does anyone know what's inside these things? Just curious...
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bahamuth
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Hydrated Sodium acetate.
The snapping of the disc induces crystallization, and boiling them again melts the crystals. Reason why they do not solidify after melting is that
saturated sodium acetate in water has the ability to supercool, that is going below the melting point of the solution and not solidify.
Any sufficiently advanced technology is indistinguishable from magic.
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waLLcandy
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Cool, thank you!
Is crystallization generally exothermic?
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Bot0nist
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"The nature of a crystallization process is governed by both thermodynamic and kinetic factors, which can make it highly variable and difficult to
control. Factors such as impurity level, mixing regime, vessel design, and cooling profile can have a major impact on the size, number, and shape of
crystals produced.
Now put yourself in the place of a molecule within a pure and perfect crystal, being heated by an external source. At some sharply defined
temperature, a bell rings, you must leave your neighbors, and the complicated architecture of the crystal collapses to that of a liquid. Textbook
thermodynamics says that melting occurs because the entropy, S, gain in your system by spatial randomization of the molecules has overcome the
enthalpy, H, loss due to breaking the crystal packing forces:
T(<sup>S</sup>liquid −<sup>S</sup>solid) > <sup>H</sup>liquid − <sup>H</sup>solid
<sup>G</sup>liquid < <sup>G</sup>solid
This rule suffers no exceptions when the temperature is rising. By the same token, on cooling the melt, at the very same temperature the bell should
ring again, and molecules should click back into the very same crystalline form. The entropy decrease due to the ordering of molecules within the
system is overcompensated by the thermal randomization of the surroundings, due to the release of the heat of fusion; the entropy of the universe
increases.
But liquids that behave in this way on cooling are the exception rather than the rule; in spite of the second principle of thermodynamics,
crystallization usually occurs at lower temperatures (supercooling). This can only mean that a crystal is more easily destroyed than it is formed.
Similarly, it is usually much easier to dissolve a perfect crystal in a solvent than to grow again a good crystal from the resulting solution. The
nucleation and growth of a crystal are under kinetic, rather than thermodynamic, control."
-Wiki
U.T.F.S.E. and learn the joys of autodidacticism!
Don't judge each day only by the harvest you reap, but also by the seeds you sow.
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waLLcandy
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So I saw that on wiki... I was hoping for a more succinct and direct answer. Forgive me if the answer is obvious.
Original question: In order to freeze something, the surroundings need to gain the energy lost by the system. Can I call this an exothermic process?
Furthermore does freezing = crystallization?
So I gather (correct me if I'm wrong):
*It takes less energy to melt a crystal than to reform it. Gliquid < Gsolid
*The crystalline solid state has a lower enthalpy than the liquid state, thus crystals are enthalpically favored. Hliquid > Hsolid
*In going from a liquid --> solid crystal, the change in entropy is negative
*Putting this together, crystal formation is spontaneous at very low temperatures, but non-spontaneous at high temperatures.
The wiki page made some comment about kinetics in crystal formation. Can anyone elaborate on how kinetics play a role?
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