How do iron powder heat packs work?
Reusable heat packs are filled with a supersaturated solution of
sodium acetate and water. They operate on the principle of
"supercooling". Supercooling is the principle of cooling a liquid
below the point at which it normally "freezes", or crystallizes
into a solid. Many liquids can be supercooled. We're all familiar
with the thermal properties of water. Exploring the idea of
supercooling water will give us better insight into the concept and
how it works in sodium acetate heat packs.
Water typically freezes at 32F (0C). As heat is removed from
water, the temperature will decrease to the 32F freezing point. It
will remain at 32F until it is completely crystallized into ice, at
which point the temperature of the ice begins to fall.
Many of us were first exposed to supercooling in this common
high school physics experiment: Fill a very clean glass container
with pure, distilled water. Slowly decrease the temperature of the
water without physically disturbing the container. It is common to
be able to reduce the temperature of the water to well below the
32F (0C) freezing point of water without it actually changing state
into ice if we're careful not to provide any nucleation points for
crystals to form on. It isn't unusual to be able to reduce the
temperature of the water to 20F without it actually freezing. The
water in this state is "supercooled" by twelve degrees. That is, it
exists as a liquid that is twelve degrees colder than the freezing
point.
If we then provide a nucleation point -- a site upon which
crystals can begin to form -- the temperature of the water does
something truly fascinating. It bounces immediately up to the 32F
freezing point until it is all converted to ice. It then begins to
cool down below the freezing point. A nucleation point can be
formed by dropping a grain of sand into the supercooled water, or
by simply tapping the side of the glass container. Sometimes the
experiment is concluded by pouring the supercooled water out of the
container. It turns instantly to ice as it pours.
The supersaturated sodium acetate used in reusable heat packs
has the property of being very stable in a supercooled state. The
"freezing" point of the supersaturated sodium acetate mixture is
about 140F. It is very happy, however, to exist as a liquid at room
temperature or colder if no nucleation sites are provided upon
which the crystallization process can begin. Once the sodium
acetate solution does begin to freeze, it behave the same way
supercooled water does: the temperature jumps immediately to the
freezing point until all of solution is converted to solid
crystals. Since the freezing point of the sodium acetate solution
is 140F, the solution jumps to that temperature until it is
crystallized.
Reusable sodium acetate heat packs are usually "activated" by
bending a metal disk that is provided inside the plastic pouch
containing the solution. When the disk warps, a nucleation site is
provided for the sodium acetate crystals to form on. The heat pack
immediately jumps in temperature to 140F.
A sodium acetate heat pack can be "reset" by placing it in
boiling water until all of the crystals have melted back into a
liquid. Even a single remaining crystal will provide a nucleation
site for new crystals to form on, so the melting process must be
thorough. This cycle can be repeated indefinitely. In practice,
reusable heat packs typically last until the plastic pouch becomes
damaged.
