How heat pack really works?

[Pawelk198604]

I recently got palm heat-pack to warm my hands when is cold outside, i works as fundraiser for charity organization, it's really cool to helps other but not on winter.

 

I got heat pack i heard that nead bent metal element to activate it and i can re-use heat pack by putting it's in boiling water than i can used it again and again.

 

I wonder how science behind this?

 

[Shpaget]

They use supersaturated sodium acetate solution that when triggered with the metal disk starts to crystallize. The crystallization releases the heat introduced by "cooking" it.

[suicidejunkie]

As a note, you can also see this with water bottles in cold weather.

If you leave pure water undisturbed and cool it slowly, you can get it a fair few degrees below zero without freezing.  It would freeze, but there is no good spot for the ice to start from.

Tap the bottle and the splashing will provide a starting point for the ice, and the whole thing will crystallize in a few seconds while getting warmer.

 

Heat pack is the same idea, but different materials with better stability and relevant temperatures.