Heat Packs

[Allen Crisp]

My daughter has two of these. They are plastic and filled with liquid. Each contains a metal disc and if you squeeze the disc between your fingers until it clicks, the contents of the pack crystallise and it gets hot. It cools in about an hour and you are left with a pack of cold, hard stuff. If you put it into hot water for ten minutes it gets liquid again and you can click the disc once more to heat it. What's going on?

[Octavius]

The re-heater heat pack contains sodium acetate, a form of salt, which under normal conditions in an open container will change from a liquid to a solid (freeze) at 130�F (54�C). By placing this solution in a sealed container, the solution can be cooled well below this temperature (as low as 14�F) (-10�C). Flexing the patented stainless steel "trigger" within the sealed container causes a single molecule of liquid to crystallize which starts a chain reaction causing the entire solution to change from a liquid to a solid. This phase change causes the pack to heat to approximately 130�F.

 

When the heat pack is triggered, the solution is "taught" how to crystallize, and the temperature immediately jumps back to its "freezing" point. Much in the same way the water crystals produce "heat" at 31�F, the heat pack produces an even heat flow with the crystals it produces at higher temperatures.

 

Once the unit has given off all of its heat, it is then recycled by heating it up. The easiest method is by boiling in water for six to thirty minutes, depending on the product size. The product is heated, melting the crystals back to a liquid state and then is allowed to cool below its freezing temperature. It is then ready to be activated again.

 

[slimfandango]

That's a great answer. I've had a hot water bottle for 8 years now but never figured out how to get it hot. I'll try your freezing suggestion (not sure if I understand all the crystal blarb, and I felt around for a disc, but can't find it), but I'll give it a go.

[Octavius]

slim - if your hot water bottle isn't working, take it back to Woolworths and ask for an air guitar instead.  That should keep you warm.

[Hippy]

Physicists call this phenomenon "Latent Heat of Crystalisation". Substances take in heat when changing from on state to another. This energy is stored in the compound as excited electrons. When the state change is reversed this energy is given up to the suroundings as heat once more. When a substance is liquid the molecules are distant from each other and can move around. They are reluctant to bump into each other because of the activity of the electrons and their mutual repulsion. (Remember magnetism? Like poles repel?) This takes energy. When crystalisation takes place the molecules line up in a 3D lattice, like getting the jigsaw all together, and the molecules are happy to snuggle closer together. The electons give up some of their energy as heat in the process.

 

Refrigeration relies upon this principle (Latent Heat of Evaporation) as the freon, or whatever gas is used, is compressed and becomes liquid. When this is squirted through a fine nozzle and the pressure reduced it becomes a gas and takes in latent heat. This is transferred to the condenser  (sort of big radiator at the back) where the latent heat is given up to the surroundings as the refrigerant material becomes liquid once more before being cycled around again.

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[Gef]

What a load of drivel in these answers.

[Octavius]

Gef - if you don't understand something, then ask.  Alternatively, if you have nothing constructive to say, don't say it.

[BillyBB]

Exactly how does the trigger cause a crystal to from?

[slimfandango]

Awesome answer Gef, the very best sort. If I don't understand something, I agree, it must be drivel. Anyway, must run, gotta go to Church...

Great answers here, finally learned something about crystals, thanks.