Mine look better than these bit I don't want to spoil the surprise! A friend had mentioned these to me so I knew what they did and thought they will make really good Christmas presents.
After buying a few other gifts and some travelling clothes, I headed home and have spent the afternoon wondering how they work. (Yes, I am chemistry geek and this is chemistry in action).
In fact it is wonderfully simply chemistry in action ...
The liquid is a highly concentrated solution of a salt (e.g. sodium acetate) in water, and a bit of colouring. There is so much salt in the solution that no more of the salt will dissolve. There is also a curved metal popper. A few crystals of salt form on this metal disc. When the popper is popped the crystal are dislodged into the solution, this triggers rapid crystallisation in the solution, the solution becomes a solid and heat is released. The temperature cools slowly (approx 1 hour).
So that is an hour of nice warm hands!
And to reuse you boil them (put the heat back in) the solid will become a liquid again, ready for their next use.
It is so simple it is brilliant!
