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Speaker 1: Welcome to Brainstuff from how stuff works dot com where

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Speaker 1: smart happens. Hi. I'm Marshall Brain with today's question, how

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Speaker 1: do sodium acetate heat pads work? Sodium acetate heat pads

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Speaker 1: are fun to watch. You start with a pouch of

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Speaker 1: clear liquid. Bending a little metal disk inside the pouch

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Speaker 1: starts the crystallization process, and you can see the crystallization

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Speaker 1: proceed through the liquid. The liquid becomes a solid in

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Speaker 1: a few seconds and its temperature rises to about a

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Speaker 1: d thirty degrees fahrenheit. The process can be understood if

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Speaker 1: you think about water freezing. Water freezes at thirty two

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Speaker 1: degrees fahrenheit, but it's possible to super cool the water.

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Speaker 1: That is, you can get the water's temperature to maybe

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Speaker 1: five or ten degrees below the freezing point without it

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Speaker 1: crystallizing into a solid. You can sometimes do do this

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Speaker 1: using a very clean container and distilled water, so there

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Speaker 1: are no points for the water to begin crystallizing. In

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Speaker 1: this condition, if you tap on the glass, the temperature

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Speaker 1: of the water will jump up to thirty two degrees

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Speaker 1: fahrenheit and the water will solidify very quickly. A sodium

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Speaker 1: acetate heat pad contains sodium acetate and water. It turns

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Speaker 1: out that sodium acetate is very good at super cooling.

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Speaker 1: It freezes at a thirty degrees fahrenheit, but it's happy

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Speaker 1: to exist as a liquid at a much lower temperature,

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Speaker 1: and it's extremely stable as a super cooled liquid. Clicking

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Speaker 1: the disk, however, has the ability to force a few

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Speaker 1: molecules to flip to the solid state, and the rest

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Speaker 1: of the liquid then rushes to solidify as well. The

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Speaker 1: temperature of the solidifying liquid jumps up to a hundred

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Speaker 1: thirty two degrees fahrenheit in the process. When you boil

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Speaker 1: the solid, you melt it back to the liquid state.

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Speaker 1: You have to completely out every crystal in the pouch,

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Speaker 1: by the way, or the liquid will quickly resolidify. You

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Speaker 1: can repeat this cycle forever, theoretically, just as you can

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Speaker 1: freeze and melt water as many times as you like.

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Speaker 1: Do you have any ideas or suggestions for this podcast?

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Speaker 1: If so, please send me an email at podcast at

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Speaker 1: how stuff works dot com. For more on this and

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Speaker 1: thousands of other topics, go to how stuff works dot

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Speaker 1: com and be sure to check out the brain stuff

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