Speaker 1: Welcome to Brainstuff from how stuff works dot com where
Speaker 1: smart happens. Hi. I'm Marshall Brain with today's question, how
Speaker 1: do sodium acetate heat pads work? Sodium acetate heat pads
Speaker 1: are fun to watch. You start with a pouch of
Speaker 1: clear liquid. Bending a little metal disk inside the pouch
Speaker 1: starts the crystallization process, and you can see the crystallization
Speaker 1: proceed through the liquid. The liquid becomes a solid in
Speaker 1: a few seconds and its temperature rises to about a
Speaker 1: d thirty degrees fahrenheit. The process can be understood if
Speaker 1: you think about water freezing. Water freezes at thirty two
Speaker 1: degrees fahrenheit, but it's possible to super cool the water.
Speaker 1: That is, you can get the water's temperature to maybe
Speaker 1: five or ten degrees below the freezing point without it
Speaker 1: crystallizing into a solid. You can sometimes do do this
Speaker 1: using a very clean container and distilled water, so there
Speaker 1: are no points for the water to begin crystallizing. In
Speaker 1: this condition, if you tap on the glass, the temperature
Speaker 1: of the water will jump up to thirty two degrees
Speaker 1: fahrenheit and the water will solidify very quickly. A sodium
Speaker 1: acetate heat pad contains sodium acetate and water. It turns
Speaker 1: out that sodium acetate is very good at super cooling.
Speaker 1: It freezes at a thirty degrees fahrenheit, but it's happy
Speaker 1: to exist as a liquid at a much lower temperature,
Speaker 1: and it's extremely stable as a super cooled liquid. Clicking
Speaker 1: the disk, however, has the ability to force a few
Speaker 1: molecules to flip to the solid state, and the rest
Speaker 1: of the liquid then rushes to solidify as well. The
Speaker 1: temperature of the solidifying liquid jumps up to a hundred
Speaker 1: thirty two degrees fahrenheit in the process. When you boil
Speaker 1: the solid, you melt it back to the liquid state.
Speaker 1: You have to completely out every crystal in the pouch,
Speaker 1: by the way, or the liquid will quickly resolidify. You
Speaker 1: can repeat this cycle forever, theoretically, just as you can
Speaker 1: freeze and melt water as many times as you like.
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