You are right about the energy given off as
heat due to the phase change. The material inside
in its liquid form is actually what is called a
supercooled liquid. That means that at room
temperature it would rather be in the solid phase.
However, before it can change phases it needs
something solid to start growing from. This could
be a little piece of dirt or a sharp point on the
container that holds the liquid or many other
things. This is where the metal disc comes into
play. By snapping the disc, the liquid gets the
instability it needs and it rapidly changes to a
solid, giving off heat.
The material inside most heat pads is called
sodium acetate. It is very pure and non-toxic.
It is used because it solidifies at 130 Fahrenheit
(F) but is also stable enough to stay liquid
down to room temperature. This means that when it
is a liquid and you snap the disc it will solidify
and heat up to 130 F. To return to a liquid
requires all the solid to be melted again so you
will have to go up above 130 F. Here is a link to
a movie that shows the solidification in action:
heat pads in action
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