WEBVTT - How do sodium acetate heat pads work?

0:00:00.520 --> 0:00:03.840
<v Speaker 1>Welcome to Brainstuff from how stuff works dot com where

0:00:03.840 --> 0:00:15.080
<v Speaker 1>smart happens. Hi. I'm Marshall Brain with today's question, how

0:00:15.120 --> 0:00:20.120
<v Speaker 1>do sodium acetate heat pads work? Sodium acetate heat pads

0:00:20.160 --> 0:00:23.120
<v Speaker 1>are fun to watch. You start with a pouch of

0:00:23.239 --> 0:00:27.360
<v Speaker 1>clear liquid. Bending a little metal disk inside the pouch

0:00:27.520 --> 0:00:31.720
<v Speaker 1>starts the crystallization process, and you can see the crystallization

0:00:31.800 --> 0:00:35.040
<v Speaker 1>proceed through the liquid. The liquid becomes a solid in

0:00:35.120 --> 0:00:38.040
<v Speaker 1>a few seconds and its temperature rises to about a

0:00:38.280 --> 0:00:42.159
<v Speaker 1>d thirty degrees fahrenheit. The process can be understood if

0:00:42.200 --> 0:00:46.040
<v Speaker 1>you think about water freezing. Water freezes at thirty two

0:00:46.080 --> 0:00:50.040
<v Speaker 1>degrees fahrenheit, but it's possible to super cool the water.

0:00:50.520 --> 0:00:53.800
<v Speaker 1>That is, you can get the water's temperature to maybe

0:00:53.840 --> 0:00:56.960
<v Speaker 1>five or ten degrees below the freezing point without it

0:00:57.000 --> 0:01:00.240
<v Speaker 1>crystallizing into a solid. You can sometimes do do this

0:01:00.440 --> 0:01:04.120
<v Speaker 1>using a very clean container and distilled water, so there

0:01:04.120 --> 0:01:07.560
<v Speaker 1>are no points for the water to begin crystallizing. In

0:01:07.640 --> 0:01:10.760
<v Speaker 1>this condition, if you tap on the glass, the temperature

0:01:10.760 --> 0:01:12.959
<v Speaker 1>of the water will jump up to thirty two degrees

0:01:12.959 --> 0:01:17.399
<v Speaker 1>fahrenheit and the water will solidify very quickly. A sodium

0:01:17.400 --> 0:01:21.920
<v Speaker 1>acetate heat pad contains sodium acetate and water. It turns

0:01:21.920 --> 0:01:25.440
<v Speaker 1>out that sodium acetate is very good at super cooling.

0:01:25.920 --> 0:01:29.960
<v Speaker 1>It freezes at a thirty degrees fahrenheit, but it's happy

0:01:30.000 --> 0:01:33.160
<v Speaker 1>to exist as a liquid at a much lower temperature,

0:01:33.480 --> 0:01:37.680
<v Speaker 1>and it's extremely stable as a super cooled liquid. Clicking

0:01:37.720 --> 0:01:41.200
<v Speaker 1>the disk, however, has the ability to force a few

0:01:41.280 --> 0:01:44.399
<v Speaker 1>molecules to flip to the solid state, and the rest

0:01:44.480 --> 0:01:47.760
<v Speaker 1>of the liquid then rushes to solidify as well. The

0:01:47.800 --> 0:01:50.880
<v Speaker 1>temperature of the solidifying liquid jumps up to a hundred

0:01:50.960 --> 0:01:55.040
<v Speaker 1>thirty two degrees fahrenheit in the process. When you boil

0:01:55.160 --> 0:01:58.360
<v Speaker 1>the solid, you melt it back to the liquid state.

0:01:58.680 --> 0:02:01.800
<v Speaker 1>You have to completely out every crystal in the pouch,

0:02:01.920 --> 0:02:05.280
<v Speaker 1>by the way, or the liquid will quickly resolidify. You

0:02:05.320 --> 0:02:08.880
<v Speaker 1>can repeat this cycle forever, theoretically, just as you can

0:02:08.960 --> 0:02:11.240
<v Speaker 1>freeze and melt water as many times as you like.

0:02:12.320 --> 0:02:15.120
<v Speaker 1>Do you have any ideas or suggestions for this podcast?

0:02:15.600 --> 0:02:18.240
<v Speaker 1>If so, please send me an email at podcast at

0:02:18.240 --> 0:02:20.960
<v Speaker 1>how stuff works dot com. For more on this and

0:02:21.040 --> 0:02:23.440
<v Speaker 1>thousands of other topics, go to how stuff works dot

0:02:23.480 --> 0:02:26.280
<v Speaker 1>com and be sure to check out the brain stuff

0:02:26.320 --> 0:02:28.799
<v Speaker 1>blog on the how stuff works dot com home page.