Hand warmers are small (sometimes disposable) packets which are
held in the hand and produce heat on demand to warm cold hands.
They are commonly used in outdoor activities such as hiking and
skiing to keep the extremities warm and assist insulated
clothing. They can also be used to heat small household pets
during a power outage. Other types of warmers are available to
provide soothing heat for muscular or joint aches. Depending on
the type and the source of heat, hand warmers last between 30
minutes (recrystallisation) to 12–24 hours (platinum catalyst).
A second type generate heat through exothermic crystallisation
of supersaturated solutions and are usually reusable. These can
be recharged by boiling the warmers and allowing them to cool.
Heating of these pads is triggered by snapping a small metal
device buried in the pad which generates nucleation centers
which initiate crystallisation. Heat is required to dissolve the
salt in its own water of crystallisation and it is this heat
that is released when cr