Does dissolution or crystallization produce heat?

1 Answer
Dec 28, 2017

Well, from first principles.....


#"The making of bonds RELEASES energy...."#

#"And the breaking of bonds REQUIRES energy."#

And since crystallization definitely represents the making of bonds, this is an EXOTHERMIC process. And the breaking of bonds is an endothermic process...and when we perform a simple recrystallization experiment, we often heat the slurry to get it up into solution, i.e. the dissolution process, the solution crystallizes, the heat will be released.

In first aid kits, you can often get hot packs, and cold packs. The hot packs are saturated or supersaturated solutions of some salt; you break a blister on the pack, and the solution crystallizes releasing heat. For cold packs, often there is a solid mass of ammonium chloride, and a blister breaks a container of water; all the ammonium chloride goes into solution, absorbing heat from the surroundings...