hand warmers

[Badgerbodger]

how do the handwarmers that you boil to regenerate, work and why does clicking a metal disc activate it?

[woofgang]

http://www.wisegeek.com/what-is-a-heat-pack.htm?referrer=adwords_campaign=heatpack_ad=011421&_search_kw=hot%20packs

says this

"Another form of heat pack uses science in the form of a supersaturated solution. The heat pack itself contains a solution of sodium acetate, a special form of salt. The salt solution is supersaturated, meaning it actually has more salt molecules than it can keep in a stable liquid form. The user presses a 'trigger' made from stainless steel, causing one additional sodium molecule to start a chain reaction. As the solution begins to solidify, heat is generated. Specifically, this type of heat pack can reach 130 degrees Fahrenheit during the reaction, but not much higher. After a few hours, the reaction stops and the heat dissipates. Boiling the heat pack for several minutes will return the solution back to its supersaturated state so the entire process can begin again."

[woofgang]

you can do something similar by taking a perfectly clean heat proof jar and making a boiling solution of any pure crystalline substance (copper sulphate works well) Dissolve crustals in the water until no more will dissolve, allow to cool slightly and pout into the jar. DO NOT MOVE OR VIBRATE THE JAR. When the solution is cold if you have done it right, wqhen you tap the jar, the whole thing will start to crystallize as you watch it

[woofgang]

and with pictures!

http://www.csudh.edu/oliver/demos/supersat/supersat.htm