Gel hand warmers

[chris281150]

Anyone know the science / chemistry behind these items. You have a plastic 'bag' with a liquid inside it and then you 'click' a small metal disc inside the bag a few times and the liquid becomes quite hot and goes 'crystaline'. They can be revitalised and used again after 5 minutes of boiling. It says on the packet that it is a saline solution.

[Sequin]

They are very pure supersaturated solutions of substances such as Sodium Thiosulphate - if heated, more will dissolve in the water that is in the mixture than at a colder temperature. As the mixture then cools, the excess does not crystallise out again until you caule a localised disturbance which allows crystals to start forming. As the crystals form, they release heat, so warming the mixture up again