1
00:00:00,089 --> 00:00:02,810
I want to show you something really cool.

2
00:00:02,810 --> 00:00:05,050
Eh, well, actually it’s hot.

3
00:00:05,050 --> 00:00:07,568
But.. eh well it’s not yet.

4
00:00:07,568 --> 00:00:08,491
Here.

5
00:00:09,034 --> 00:00:10,034
This thing.

6
00:00:10,260 --> 00:00:15,980
It’s a little pouch filled with a mysterious
blue liquid and a little metal disc.

7
00:00:15,980 --> 00:00:18,664
When I squeeze that disc…

8
00:00:18,664 --> 00:00:21,193
something weird starts happening.

9
00:00:21,193 --> 00:00:24,611
There’s like a front of light blue crystal
material,

10
00:00:24,611 --> 00:00:26,160
almost like ice,

11
00:00:26,160 --> 00:00:28,263
emanating from that disc.

12
00:00:28,512 --> 00:00:29,793
And now…

13
00:00:30,019 --> 00:00:31,827
this thing is hot!

14
00:00:32,664 --> 00:00:33,981
What‽

15
00:00:33,981 --> 00:00:37,161
Yes, this is one of my favorite objects I’ve
ever encountered.

16
00:00:37,161 --> 00:00:39,664
This is a reusable hand warmer.

17
00:00:39,664 --> 00:00:42,817
Reusable? Indeed. I’ll show you how in a bit.

18
00:00:42,817 --> 00:00:47,884
But first - I’ll be frank and would not
suggest you rush out and buy some of these

19
00:00:47,884 --> 00:00:50,901
because honestly they’re not that great
at their job

20
00:00:50,901 --> 00:00:55,870
and the reusability aspect of them is both tedious and energy-intensive.

21
00:00:55,870 --> 00:01:00,110
But the physical process by which these work
is quite fascinating,

22
00:01:00,110 --> 00:01:03,902
and they do have at least one advantage compared to other options.

23
00:01:03,902 --> 00:01:07,527
Speaking of other options, here’s another option!

24
00:01:07,527 --> 00:01:10,181
This is the traditional not-reusable one.

25
00:01:10,181 --> 00:01:14,001
These products have been around for a long
while and are quite inexpensive.

26
00:01:14,001 --> 00:01:18,522
And usually have a pun-adjacent name like Hothands or
Lil’ Hotties.

27
00:01:18,522 --> 00:01:20,072
I approve!

28
00:01:20,072 --> 00:01:25,546
To use them, you simply tear open the plastic
packaging which exposes the warmer to air,

29
00:01:25,546 --> 00:01:28,410
and after a few minutes it starts producing heat.

30
00:01:28,410 --> 00:01:29,710
How do they work?

31
00:01:29,710 --> 00:01:32,398
Well, I’m afraid I’m a little rusty…

32
00:01:32,398 --> 00:01:34,123
Oh wait, right, it’s rust!

33
00:01:34,123 --> 00:01:35,235
How ironic.

34
00:01:35,235 --> 00:01:38,690
These contain a mixture of iron and other
support chemicals

35
00:01:38,690 --> 00:01:43,119
which help the iron quickly oxidize when exposed to oxygen in the air.

36
00:01:43,119 --> 00:01:46,765
This redox reaction is exothermic so it gives
off heat.

37
00:01:46,765 --> 00:01:50,255
When you use these, you literally hold onto
a little pouch of powders,

38
00:01:50,255 --> 00:01:53,802
one of which - the iron - quickly rusts, creating heat.

39
00:01:53,802 --> 00:01:59,329
Eventually all the iron is oxidized and at
that point the warmer is spent and can be disposed of.

40
00:01:59,329 --> 00:02:03,486
These are really quite interesting on their
own, but you can’t see the reaction occurring

41
00:02:03,486 --> 00:02:07,668
because it happens in this opaque pouch which
it clearly tells you

42
00:02:07,668 --> 00:02:10,077
not to open, puncture, or tear.

43
00:02:11,049 --> 00:02:15,362
Fortunately I have scissors and a propensity
to ignore safety instructions.

44
00:02:16,063 --> 00:02:18,932
Um, don’t do this at home,

45
00:02:18,932 --> 00:02:20,655
always follow instructions

46
00:02:20,655 --> 00:02:22,822
I’m doing it so you don’t have to...

47
00:02:22,822 --> 00:02:24,269
but here we go!

48
00:02:24,269 --> 00:02:27,097
What’s in here is a very deep black powder.

49
00:02:27,097 --> 00:02:31,946
Activated carbon, aka activated charcoal is
responsible for the color.

50
00:02:31,946 --> 00:02:36,433
That charcoal isn’t part of the reaction,
instead it’s used as a source of thermal mass

51
00:02:36,433 --> 00:02:38,878
to help evenly distribute the heat produced.

52
00:02:38,878 --> 00:02:43,120
When it’s all exposed to air like this the
reaction happens quite a bit more quickly

53
00:02:43,120 --> 00:02:46,250
than it does in the warmer with its semi-permeable
membrane.

54
00:02:46,250 --> 00:02:49,282
You can actually see some steam coming from it -

55
00:02:49,282 --> 00:02:54,279
one of the support materials in here is there specifically to provide a source of water for the reaction.

56
00:02:54,279 --> 00:02:58,368
It can be any number of things, but one common
material is vermiculite,

57
00:02:58,368 --> 00:03:01,083
a silicon-based mineral that contains water.

58
00:03:01,083 --> 00:03:04,953
You can actually see some water droplets forming
on the bottom of this container,

59
00:03:04,953 --> 00:03:08,806
and they are coming from this powder and not the ambient air.

60
00:03:08,806 --> 00:03:13,777
We know that because this is getting hot,
thus ambient moisture cannot condense on it.

61
00:03:13,777 --> 00:03:17,824
Because these pouches are filled with nothing
but commonly-available substances

62
00:03:17,824 --> 00:03:20,209
they’re quite inexpensive.

63
00:03:20,209 --> 00:03:23,096
And, the materials in here are all fairly inert,

64
00:03:23,096 --> 00:03:27,126
meaning that no special consideration needs to be given to their disposal.

65
00:03:27,126 --> 00:03:30,792
It’s basically just a packet of rocks and
rust when it’s used up,

66
00:03:30,792 --> 00:03:33,315
making it Mostly Harmless.

67
00:03:33,315 --> 00:03:36,701
Although the chemical makeup of these does
vary from brand to brand

68
00:03:36,701 --> 00:03:39,265
and even between offerings within the same brand.

69
00:03:39,265 --> 00:03:44,222
The reaction is very simple, but can be tuned
to happen more quickly for faster heat,

70
00:03:44,222 --> 00:03:48,602
or more slowly for longer-lasting heat, depending
on various factors.

71
00:03:48,602 --> 00:03:53,938
So the chemical composition of these is usually
quite similar but by no means identical.

72
00:03:53,938 --> 00:03:56,242
But of course, they are one and done.

73
00:03:56,242 --> 00:03:58,583
Once you open it there’s no going back.

74
00:03:58,583 --> 00:04:03,161
It’s hard to notice through all the activated
carbon but you can definitely see a difference in color

75
00:04:03,161 --> 00:04:06,565
between spent and fresh hand warmer
contents.

76
00:04:06,565 --> 00:04:11,074
The spent one looks a bit browner, thanks
to the iron now being iron oxide -

77
00:04:11,074 --> 00:04:11,985
Rust.

78
00:04:11,985 --> 00:04:14,948
Macro shots help reveal the oxide particles.

79
00:04:14,948 --> 00:04:19,639
Once all the iron’s been oxidized, there’s
nothing you can do but throw it away.

80
00:04:19,673 --> 00:04:24,046
These, on the other hand, work through a reaction
that’s entirely reversible.

81
00:04:24,046 --> 00:04:25,957
And that’s because…

82
00:04:25,957 --> 00:04:27,976
it’s not a reaction, exactly.

83
00:04:27,976 --> 00:04:31,982
Instead it’s our friend latent heat popping up again!

84
00:04:32,022 --> 00:04:34,615
Sometimes it prefers you call it enthalpy.

85
00:04:34,615 --> 00:04:36,789
Combine it with another fun phenomenon,

86
00:04:36,789 --> 00:04:39,181
dotiouscalifragilisticsupersaturation,

87
00:04:39,181 --> 00:04:44,929
and you’ve got a phase-change material capable of delivering heat on demand.

88
00:04:44,929 --> 00:04:49,823
What’s in this pouch is a solution of sodium
acetate trihydrate and water.

89
00:04:49,823 --> 00:04:52,929
Sodium acetate trihydrate is a crystalline
compound

90
00:04:52,929 --> 00:04:58,151
with a melting point of 58°C or about 136°F.

91
00:04:58,151 --> 00:05:02,522
Now believe it or not, I don’t
set my thermostat that high.

92
00:05:02,522 --> 00:05:07,275
It’s currently about 18°C in this room,
something like 65° F,

93
00:05:07,275 --> 00:05:09,810
and yet this remains a liquid.

94
00:05:09,810 --> 00:05:14,329
It’s a fairly viscous liquid, but liquid
nonetheless.

95
00:05:14,329 --> 00:05:18,946
That’s because this solution is both supercooled
and supersaturated.

96
00:05:18,946 --> 00:05:24,276
It contains more dissolved sodium acetate
than is ordinarily possible at this temperature,

97
00:05:24,276 --> 00:05:29,105
and more importantly the solution is below
the freezing point of sodium acetate.

98
00:05:29,105 --> 00:05:31,914
This supercooling can happen to many substances-

99
00:05:31,914 --> 00:05:37,179
you’ve probably seen one of those viral 
videos where a water bottle is frozen but…

100
00:05:37,179 --> 00:05:42,196
not, and then jostling it causes it to nearly instantly freeze.

101
00:05:42,196 --> 00:05:47,262
In that case, the water remained a liquid
below its freezing point until it was disturbed.

102
00:05:47,262 --> 00:05:49,014
It was supercooled.

103
00:05:49,014 --> 00:05:51,950
That’s more or less what’s going on right here,

104
00:05:51,950 --> 00:05:57,592
except rather than water, the substance in question is the sodium acetate dissolved in it.

105
00:05:57,592 --> 00:06:01,166
Sodium acetate is useful in this situation
for two reasons:

106
00:06:01,166 --> 00:06:03,744
one it has a rather high melting point,

107
00:06:03,744 --> 00:06:07,358
and two in solution it’s remarkably easy to supercool.

108
00:06:07,420 --> 00:06:10,074
Once it’s all melted and dissolved in a
solution,

109
00:06:10,074 --> 00:06:13,840
that solution remains liquid well below its freezing point,

110
00:06:13,840 --> 00:06:15,874
and that’s what’s happening here.

111
00:06:15,874 --> 00:06:18,554
This solution is what’s known as metastable.

112
00:06:18,554 --> 00:06:23,590
It’s in a sort of in-between state; 
not really frozen, obviously

113
00:06:23,590 --> 00:06:26,698
but not really melted, either.

114
00:06:26,698 --> 00:06:31,266
The sodium acetate is colder than its melting
point yet still molten.

115
00:06:31,266 --> 00:06:32,969
And here’s the important thing:

116
00:06:32,969 --> 00:06:36,580
when it does freeze, it will release heat energy.

117
00:06:36,580 --> 00:06:39,531
That’s our friend latent heat showing up
again.

118
00:06:39,531 --> 00:06:42,046
Just as there’s a latent heat of vaporization,

119
00:06:42,046 --> 00:06:46,099
the energy needed to get a substance to move from the liquid phase to the gaseous phase,

120
00:06:46,099 --> 00:06:48,738
there’s also a latent heat of fusion,

121
00:06:48,738 --> 00:06:51,786
the energy needed to get a solid substance to melt

122
00:06:51,786 --> 00:06:54,201
(fusion is another word for melting, see).

123
00:06:54,201 --> 00:06:58,662
Materials in the liquid phase contain more
energy than they do in the solid phase,

124
00:06:58,662 --> 00:07:00,806
and since this is currently a liquid,

125
00:07:00,806 --> 00:07:04,561
yet it’s colder than the freezing point of sodium acetate,

126
00:07:04,561 --> 00:07:09,455
it’s got more energy in it than it really
should, in a manner of speaking.

127
00:07:09,455 --> 00:07:13,843
It managed to cool below its freezing point
without actually freezing,

128
00:07:13,843 --> 00:07:17,111
which means it hasn’t released its latent heat of fusion.

129
00:07:17,111 --> 00:07:22,276
The energy it took to get it to melt is still
here because it’s still molten.

130
00:07:22,276 --> 00:07:25,977
Pressing on or bending the metal disc, though,
will fix that.

131
00:07:25,977 --> 00:07:30,184
Remember it’s metastable, or stable but
barely.

132
00:07:30,184 --> 00:07:33,743
When the tiny cuts in the metal disc scrape
past each other,

133
00:07:33,743 --> 00:07:39,824
they create a disturbance great enough to cause a crystal of solid sodium acetate to form in the solution.

134
00:07:39,824 --> 00:07:43,676
This creates a nucleation site, sometimes
this is called seeding,

135
00:07:43,676 --> 00:07:46,049
and this breaks the metastable state.

136
00:07:46,049 --> 00:07:52,360
A chain reaction occurs and a front of crystals
quickly forms radiating away from the activation site.

137
00:07:52,360 --> 00:07:56,930
This is the sodium acetate precipitating out
of the solution and solidifying.

138
00:07:56,930 --> 00:08:00,182
In moments, the entire thing is frozen and,

139
00:08:00,182 --> 00:08:01,621
counterintuitively,

140
00:08:01,621 --> 00:08:02,942
hot.

141
00:08:02,942 --> 00:08:06,817
As the sodium acetate falls out of the solution
and crystals form,

142
00:08:06,817 --> 00:08:08,621
in other words as it freezes,

143
00:08:08,621 --> 00:08:14,949
it releases the stored latent heat of fusion
and heats up right about to its freezing point.

144
00:08:14,949 --> 00:08:19,508
It can be a little mind-bending to think of
it as freezing while also getting hot,

145
00:08:19,508 --> 00:08:21,489
but that’s precisely what happens.

146
00:08:21,489 --> 00:08:26,160
It's just… the freezing point of sodium
acetate isn’t cold like ice.

147
00:08:26,160 --> 00:08:31,399
As a matter of fact there was a weird viral
thing going around some years ago called "hot ice" -

148
00:08:31,399 --> 00:08:36,141
that was this exact substance, though
not in a pouch like you see here.

149
00:08:36,141 --> 00:08:38,927
And it generates heat extremely quickly.

150
00:08:38,927 --> 00:08:43,549
Take a look in a thermal camera and you’ll
see that the instant it solidifies, it’s hot.

151
00:08:43,549 --> 00:08:47,234
It doesn’t quite reach the freezing point
of sodium acetate trihydrate,

152
00:08:47,234 --> 00:08:48,758
but it’s pretty close.

153
00:08:48,758 --> 00:08:51,418
Just look at my hands compared to the hand
warmer -

154
00:08:51,418 --> 00:08:53,563
they’re quite a bit colder.

155
00:08:53,563 --> 00:08:55,848
Luckily, it doesn’t all freeze immediately.

156
00:08:55,848 --> 00:08:59,454
After the crystals have spread out, you can
knead this thing

157
00:08:59,454 --> 00:09:02,566
and you’ll discover that it’s pretty soft.

158
00:09:02,566 --> 00:09:06,797
It feels almost like lightly packed snow,
but warm.

159
00:09:06,797 --> 00:09:10,764
The sensible heat energy released as the initial
crystallization occurred

160
00:09:10,764 --> 00:09:13,447
brings the entire volume up to the freezing point

161
00:09:13,447 --> 00:09:16,698
(which again is anything but what we would call cold)

162
00:09:16,698 --> 00:09:19,278
and some liquid solution hangs around.

163
00:09:19,278 --> 00:09:24,209
The latent heat it contains is slowly released
as more acetate falls out of solution

164
00:09:24,209 --> 00:09:25,886
and continues freezing.

165
00:09:25,886 --> 00:09:28,946
Over time these things get stiffer and stiffer,

166
00:09:28,946 --> 00:09:33,452
and by the time they’re all used up it’s quite firm and inflexible.

167
00:09:33,452 --> 00:09:37,913
Every single time I’ve used one of these
it’s a fascinating experience.

168
00:09:37,913 --> 00:09:44,007
The idea that something can just spontaneously
release energy by freezing

169
00:09:44,007 --> 00:09:45,279
is wild!

170
00:09:45,279 --> 00:09:47,605
And it’s a delightful sight to behold.

171
00:09:47,605 --> 00:09:52,965
Something about seeing the crystals propagate
like this is just amazing.

172
00:09:52,965 --> 00:09:57,983
The most recent set that I got came with a couple of
much larger heating pads,

173
00:09:57,983 --> 00:10:00,295
and it’s mesmerizing to see this occur.

174
00:10:00,295 --> 00:10:01,678
Just look at that!

175
00:10:02,718 --> 00:10:04,639
Seriously, look at that!

176
00:10:05,770 --> 00:10:07,085
Wowzers!

177
00:10:07,696 --> 00:10:11,017
One thing I’ve learned is that these aren’t
all created equal.

178
00:10:11,080 --> 00:10:13,529
I was trying to get an interdimensional portal going

179
00:10:13,529 --> 00:10:16,014
so I got these ones, too.

180
00:10:16,014 --> 00:10:20,879
So far no luck with the portal but the orange ones
are just… better.

181
00:10:20,879 --> 00:10:24,220
They produce heat for longer, and I suspect
the reason they do

182
00:10:24,220 --> 00:10:28,912
is that there’s more sodium acetate in this solution than in the blue ones.

183
00:10:28,912 --> 00:10:33,960
When these are spent, they’re incredibly
solid - practically rock-like.

184
00:10:33,960 --> 00:10:37,267
The blue ones, though, remain a bit soft
and somewhat flexible,

185
00:10:37,267 --> 00:10:39,319
even once at room temperature.

186
00:10:39,319 --> 00:10:43,011
I suspect the blue ones have more water in
them than the orange ones,

187
00:10:43,011 --> 00:10:46,815
which keeps them more flexible at the expense of runtime.

188
00:10:46,815 --> 00:10:49,372
So how are these reusable?

189
00:10:49,372 --> 00:10:51,150
Well, you may have already guessed.

190
00:10:51,150 --> 00:10:53,669
You simply need to melt them again.

191
00:10:53,669 --> 00:10:55,953
And to do that, you boil them.

192
00:10:55,953 --> 00:10:57,564
Like a weird egg.

193
00:10:57,564 --> 00:11:01,191
Bring water to a boil, add a used pad, and
keep the water simmering

194
00:11:01,191 --> 00:11:03,435
until the entire thing has melted.

195
00:11:03,435 --> 00:11:08,562
I usually go for a few minutes more because
if you don’t get every last crystal to melt and dissolve,

196
00:11:08,562 --> 00:11:11,386
the remaining crystals will
prevent it from supersaturating

197
00:11:11,386 --> 00:11:14,248
and it will simply freeze again as it cools.

198
00:11:14,248 --> 00:11:16,574
Ya know, like things normally do.

199
00:11:16,574 --> 00:11:19,162
But, assuming it did melt completely,

200
00:11:19,162 --> 00:11:23,004
as it cools down it’ll sneakily slip right past its freezing point

201
00:11:23,004 --> 00:11:25,889
without releasing its latent
heat of fusion.

202
00:11:25,889 --> 00:11:30,924
It’ll save that for later, providing an
intriguing and effective source of portable heat.

203
00:11:30,924 --> 00:11:34,817
By the way, in normal circumstances, for instance with
plain water,

204
00:11:34,817 --> 00:11:40,111
when it cools to the freezing point it will effectively pause getting colder as it freezes.

205
00:11:40,111 --> 00:11:42,691
No matter how cold the temperature is around
it,

206
00:11:42,691 --> 00:11:47,320
a given quantity of water will just stay at the freezing point until it’s all frozen.

207
00:11:47,320 --> 00:11:50,777
Only then, after it's released all the latent
heat of fusion,

208
00:11:50,777 --> 00:11:53,027
can it get below its freezing point.

209
00:11:53,027 --> 00:11:55,516
That’s how latent heat normally behaves,

210
00:11:55,516 --> 00:11:59,028
although the rules are bent in all sorts of circumstances.

211
00:11:59,028 --> 00:12:02,625
The biggest trouble with these is that, well,

212
00:12:02,625 --> 00:12:04,426
they don’t work all that well.

213
00:12:04,426 --> 00:12:06,808
Initially they work fantastically!

214
00:12:06,808 --> 00:12:10,497
It is near-instant heat at its full intensity,

215
00:12:10,497 --> 00:12:13,728
which simply does not happen with the rusty ones,

216
00:12:13,728 --> 00:12:16,436
or even an electric heating pad for that matter.

217
00:12:16,436 --> 00:12:21,570
But their heat output starts falling rapidly
after only about 10 minutes.

218
00:12:21,570 --> 00:12:25,408
In fact, there’s a significant measurable
difference after only five minutes.

219
00:12:25,408 --> 00:12:28,834
You’ll get noticeable heat out of them for
maybe a half hour

220
00:12:28,834 --> 00:12:33,080
but by then it’s already pretty tepid, and also quite stiff.

221
00:12:33,080 --> 00:12:34,547
[creaky sounds as warmer is flexed]

222
00:12:34,547 --> 00:12:39,136
Now as I said this does seem to depend somewhat
on the warmer’s acetate concentration,

223
00:12:39,136 --> 00:12:43,419
but more important than that is its overall mass
and how you use it.

224
00:12:43,419 --> 00:12:46,181
These tiny ones came with the blue set.

225
00:12:46,181 --> 00:12:47,034
Shocker.

226
00:12:47,034 --> 00:12:50,223
And they’re… next to useless.

227
00:12:50,223 --> 00:12:53,327
You get mere minutes of heat before they’re
used up.

228
00:12:53,327 --> 00:12:57,783
There’s just too much surface area for the
volume of solution inside.

229
00:12:57,783 --> 00:13:02,016
The larger orange pads, though, last the longest
of any of these.

230
00:13:02,016 --> 00:13:07,906
Again, though, that comes at the expense of
it being a literal rock by the time it’s all over.

231
00:13:07,906 --> 00:13:10,605
And the other greatest factor is how you use it.

232
00:13:10,605 --> 00:13:15,847
If you leave it exposed to air, say if you’re
simply holding on to it with bare hands,

233
00:13:15,847 --> 00:13:21,082
it will lose its heat rapidly and whatever liquid
acetate remains will freeze faster.

234
00:13:21,082 --> 00:13:23,546
Therefore it gets used up more quickly.

235
00:13:23,546 --> 00:13:28,089
If it’s somewhat insulated, though, this
is delayed and it will last longer.

236
00:13:28,089 --> 00:13:34,084
The trouble there is, especially in the
beginning, these are borderline uncomfortably hot against bare skin.

237
00:13:34,084 --> 00:13:38,299
And of course, well, eventually you’ll have
a weird rock in your gloves,

238
00:13:38,299 --> 00:13:40,211
if you could even fit it in there.

239
00:13:40,211 --> 00:13:43,852
Lastly, there’s the energy needed to melt
them for re-use.

240
00:13:43,852 --> 00:13:46,054
This is extremely variable, mind you,

241
00:13:46,054 --> 00:13:50,778
but they need a fairly long time in near-boiling water to be reset.

242
00:13:50,778 --> 00:13:53,273
It takes a fair bit of energy to heat water,

243
00:13:53,273 --> 00:13:57,001
especially the quantity needed to reset these large packs.

244
00:13:57,001 --> 00:14:00,044
And when you get such poor runtime out of
them,

245
00:14:00,044 --> 00:14:02,223
it seems almost a futile exercise,

246
00:14:02,223 --> 00:14:07,126
especially when you consider the miniscule fraction of the energy used to reactivate these

247
00:14:07,126 --> 00:14:11,281
that they are actually able to capture and store for later release.

248
00:14:11,281 --> 00:14:16,449
Therefore I consider these to be more of a
novelty than a truly useful object.

249
00:14:16,449 --> 00:14:19,859
That said, I will give them one bit of genuine
praise.

250
00:14:19,859 --> 00:14:25,387
The heat they generate is distinctly different
from that of these disposable things.

251
00:14:25,387 --> 00:14:31,256
Being a large, dense, solid hot thing feels more
therapeutic to me,

252
00:14:31,256 --> 00:14:37,307
and I think for something like applying heat for pain relief these are pretty decent, if short-lived.

253
00:14:37,307 --> 00:14:40,162
Still, if that’s a thing you regularly need…

254
00:14:40,162 --> 00:14:43,148
I would advise getting an electric heating
pad.

255
00:14:43,148 --> 00:14:48,322
These are neat, but tedious and boiling them
gets old fast.

256
00:14:48,322 --> 00:14:50,402
They’re also not cheap.

257
00:14:50,402 --> 00:14:54,010
This set of 8 hand warmers was $25.

258
00:14:54,010 --> 00:14:57,232
That can buy a LOT of disposable hand warmers,

259
00:14:57,232 --> 00:15:03,949
in fact these were 74 cents a pair and can be obtained for even less 
when bought in bulk.

260
00:15:03,949 --> 00:15:07,748
Again - this is just some iron and various
rocks.

261
00:15:07,748 --> 00:15:13,049
I find it pretty hard to justify these reusable
gel packs in a lot of circumstances, honestly,

262
00:15:13,049 --> 00:15:18,250
as they fit an incredibly small niche and
require far more energy and effort to re-use

263
00:15:18,250 --> 00:15:21,381
than I think makes sense, at least to me.

264
00:15:21,381 --> 00:15:26,089
But, they are very useful at demonstrating
the concept of latent heat.

265
00:15:26,089 --> 00:15:31,366
There’s hidden heat energy in here because
substances in a higher phase of matter

266
00:15:31,366 --> 00:15:35,984
have more energy in them thanks to the difference
in intermolecular forces between the phases.

267
00:15:35,984 --> 00:15:41,824
It’s the process of adding that extra energy
that gets stuff to melt or vaporize.

268
00:15:41,824 --> 00:15:44,378
When you come back down the ladder of phases,

269
00:15:44,378 --> 00:15:48,779
that energy gets released as substances condense or solidify.

270
00:15:48,779 --> 00:15:53,928
In my eyes, there’s nothing that demonstrates
this quite so effectively as these things.

271
00:15:53,928 --> 00:15:56,196
Take a liquid at room temperature,

272
00:15:56,196 --> 00:15:59,297
see it instantly fall from the liquid phase to the solid,

273
00:15:59,297 --> 00:16:02,146
and feel the heat it releases as it does so.

274
00:16:02,146 --> 00:16:06,257
It’s a remarkably tangible demonstration of latent
heat,

275
00:16:06,257 --> 00:16:08,057
and yes I know you can’t feel this,

276
00:16:08,057 --> 00:16:10,852
but hopefully you see the beauty of this.

277
00:16:10,852 --> 00:16:13,846
Ya know where else we find exploitation of
latent heat?

278
00:16:13,846 --> 00:16:17,272
Why, refrigeration and air conditioning, of course!

279
00:16:17,272 --> 00:16:21,908
The same process of absorbing heat in one
place and expelling it in another

280
00:16:21,908 --> 00:16:25,845
is responsible for how we cool our homes and keep our food preserved.

281
00:16:27,451 --> 00:16:31,090
Yeah, OK, it’s nothing like boiling a thing
and storing heat energy

282
00:16:31,090 --> 00:16:33,771
in its intermolecular bonds to be used for later

283
00:16:33,771 --> 00:16:35,983
but the principle of latent heat,

284
00:16:35,983 --> 00:16:39,603
that is the energy absorbed and released as a substance changes phases,

285
00:16:39,603 --> 00:16:42,288
is exactly what makes these devices work.

286
00:16:42,288 --> 00:16:43,382
And in the next video

287
00:16:43,382 --> 00:16:45,940
(weather permitting - it might be the video after the next)

288
00:16:45,940 --> 00:16:51,935
we’ll talk about heat pumps - using that same principle
to make a space warmer.

289
00:16:52,030 --> 00:16:56,700
We have heat pumps everywhere - air conditioners
and refrigerators are heat pumps!

290
00:16:56,700 --> 00:16:59,193
But only recently, particularly in the US,

291
00:16:59,193 --> 00:17:03,660
have we been learning to use that same concept in reverse to provide space heating.

292
00:17:03,660 --> 00:17:07,619
Learning to move heat energy
 rather than produce it

293
00:17:07,619 --> 00:17:11,378
has allowed us to warm our homes more efficiently than ever before,

294
00:17:11,378 --> 00:17:13,014
and soon we’ll learn how

295
00:17:13,014 --> 00:17:17,963
as well as and why it is undoubtedly the heating technology of the future.

296
00:17:18,981 --> 00:17:21,493
♫ enthalpically smooth jazz ♫

297
00:17:22,511 --> 00:17:25,315
When I squeeze that disc…

298
00:17:25,315 --> 00:17:27,046
nothing happens.

299
00:17:27,046 --> 00:17:29,767
When I squeeze that disc…

300
00:17:29,767 --> 00:17:32,899
again, nothing happens!
 [frustrated laughter]

301
00:17:32,899 --> 00:17:35,806
Sodium acetate is useful in this situations…

302
00:17:35,806 --> 00:17:36,980
weooow

303
00:17:36,980 --> 00:17:39,152
...cause a crystal of soble…

304
00:17:39,152 --> 00:17:40,218
[weird noise]

305
00:17:40,218 --> 00:17:43,796
Or stable, but barely 
[weird throat clear]

306
00:17:43,796 --> 00:17:48,622
...precipitating out of the solution and solidifying
in moments. The entire -

307
00:17:48,622 --> 00:17:49,972
No!
[laughs]

308
00:17:49,972 --> 00:17:51,170
Gah-ha!

309
00:17:51,170 --> 00:17:55,256
And some liquid solution remains hangs around.

310
00:17:55,256 --> 00:17:56,076
Dang!

311
00:17:56,076 --> 00:17:58,628
You’ll get noticeable heat output for them...

312
00:17:58,945 --> 00:18:00,229
Out.. out of them.

313
00:18:00,229 --> 00:18:02,041
♫ unprovoked humming ♫

314
00:18:02,041 --> 00:18:04,448
Incredibly small niche and require…

315
00:18:04,448 --> 00:18:05,817
as…

316
00:18:05,817 --> 00:18:06,878
Oops!

317
00:18:07,873 --> 00:18:12,307
Well that sure was a different kind of freezing, wasn't it!

318
00:18:12,307 --> 00:18:14,616
Freezing but getting hot?

319
00:18:14,616 --> 00:18:16,946
How strange and imaginative!

320
00:18:16,946 --> 00:18:20,573
Sounds like something out of Alice in Wonderland 
but it's not!

321
00:18:20,980 --> 00:18:24,137
LATENT HEAT STRIKES AGAIN