How do sodium acetate heat pads work?

Sodium acetate heat pads start out as a pouch of liquid and crystallize into a warm solid. Explore the fascinating chemistry that makes these heat pads work in this podcast from HowStuffWorks.com.

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Episode Transcript

Speaker 1 (00:00):
Welcome to Brainstuff from how stuff works dot com where
smart happens. Hi. I'm Marshall Brain with today's question, how
do sodium acetate heat pads work? Sodium acetate heat pads
are fun to watch. You start with a pouch of

(00:23):
clear liquid. Bending a little metal disk inside the pouch
starts the crystallization process, and you can see the crystallization
proceed through the liquid. The liquid becomes a solid in
a few seconds and its temperature rises to about a
d thirty degrees fahrenheit. The process can be understood if
you think about water freezing. Water freezes at thirty two

(00:46):
degrees fahrenheit, but it's possible to super cool the water.
That is, you can get the water's temperature to maybe
five or ten degrees below the freezing point without it
crystallizing into a solid. You can sometimes do do this
using a very clean container and distilled water, so there
are no points for the water to begin crystallizing. In

(01:07):
this condition, if you tap on the glass, the temperature
of the water will jump up to thirty two degrees
fahrenheit and the water will solidify very quickly. A sodium
acetate heat pad contains sodium acetate and water. It turns
out that sodium acetate is very good at super cooling.
It freezes at a thirty degrees fahrenheit, but it's happy

(01:30):
to exist as a liquid at a much lower temperature,
and it's extremely stable as a super cooled liquid. Clicking
the disk, however, has the ability to force a few
molecules to flip to the solid state, and the rest
of the liquid then rushes to solidify as well. The
temperature of the solidifying liquid jumps up to a hundred

(01:50):
thirty two degrees fahrenheit in the process. When you boil
the solid, you melt it back to the liquid state.
You have to completely out every crystal in the pouch,
by the way, or the liquid will quickly resolidify. You
can repeat this cycle forever, theoretically, just as you can
freeze and melt water as many times as you like.

(02:12):
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