WEBVTT
Kind: captions
Language: en

00:00:00.089 --> 00:00:02.810
I want to show you something really cool.

00:00:02.810 --> 00:00:05.050
Eh, well, actually it’s hot.

00:00:05.050 --> 00:00:07.568
But.. eh well it’s not yet.

00:00:07.568 --> 00:00:08.491
Here.

00:00:09.034 --> 00:00:10.034
This thing.

00:00:10.260 --> 00:00:15.980
It’s a little pouch filled with a mysterious
blue liquid and a little metal disc.

00:00:15.980 --> 00:00:18.664
When I squeeze that disc…

00:00:18.664 --> 00:00:21.193
something weird starts happening.

00:00:21.193 --> 00:00:24.611
There’s like a front of light blue crystal
material,

00:00:24.611 --> 00:00:26.160
almost like ice,

00:00:26.160 --> 00:00:28.263
emanating from that disc.

00:00:28.512 --> 00:00:29.793
And now…

00:00:30.019 --> 00:00:31.827
this thing is hot!

00:00:32.664 --> 00:00:33.981
What‽

00:00:33.981 --> 00:00:37.161
Yes, this is one of my favorite objects I’ve
ever encountered.

00:00:37.161 --> 00:00:39.664
This is a reusable hand warmer.

00:00:39.664 --> 00:00:42.817
Reusable? Indeed. I’ll show you how in a bit.

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

00:00:47.884 --> 00:00:50.901
because honestly they’re not that great
at their job

00:00:50.901 --> 00:00:55.870
and the reusability aspect of them is both tedious and energy-intensive.

00:00:55.870 --> 00:01:00.110
But the physical process by which these work
is quite fascinating,

00:01:00.110 --> 00:01:03.902
and they do have at least one advantage compared to other options.

00:01:03.902 --> 00:01:07.527
Speaking of other options, here’s another option!

00:01:07.527 --> 00:01:10.181
This is the traditional not-reusable one.

00:01:10.181 --> 00:01:14.001
These products have been around for a long
while and are quite inexpensive.

00:01:14.001 --> 00:01:18.522
And usually have a pun-adjacent name like Hothands or
Lil’ Hotties.

00:01:18.522 --> 00:01:20.072
I approve!

00:01:20.072 --> 00:01:25.546
To use them, you simply tear open the plastic
packaging which exposes the warmer to air,

00:01:25.546 --> 00:01:28.410
and after a few minutes it starts producing heat.

00:01:28.410 --> 00:01:29.710
How do they work?

00:01:29.710 --> 00:01:32.398
Well, I’m afraid I’m a little rusty…

00:01:32.398 --> 00:01:34.123
Oh wait, right, it’s rust!

00:01:34.123 --> 00:01:35.235
How ironic.

00:01:35.235 --> 00:01:38.690
These contain a mixture of iron and other
support chemicals

00:01:38.690 --> 00:01:43.119
which help the iron quickly oxidize when exposed to oxygen in the air.

00:01:43.119 --> 00:01:46.765
This redox reaction is exothermic so it gives
off heat.

00:01:46.765 --> 00:01:50.255
When you use these, you literally hold onto
a little pouch of powders,

00:01:50.255 --> 00:01:53.802
one of which - the iron - quickly rusts, creating heat.

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.

00:01:59.329 --> 00:02:03.486
These are really quite interesting on their
own, but you can’t see the reaction occurring

00:02:03.486 --> 00:02:07.668
because it happens in this opaque pouch which
it clearly tells you

00:02:07.668 --> 00:02:10.077
not to open, puncture, or tear.

00:02:11.049 --> 00:02:15.362
Fortunately I have scissors and a propensity
to ignore safety instructions.

00:02:16.063 --> 00:02:18.932
Um, don’t do this at home,

00:02:18.932 --> 00:02:20.655
always follow instructions 

00:02:20.655 --> 00:02:22.822
I’m doing it so you don’t have to...

00:02:22.822 --> 00:02:24.269
but here we go!

00:02:24.269 --> 00:02:27.097
What’s in here is a very deep black powder.

00:02:27.097 --> 00:02:31.946
Activated carbon, aka activated charcoal is
responsible for the color.

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

00:02:36.433 --> 00:02:38.878
to help evenly distribute the heat produced.

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

00:02:43.120 --> 00:02:46.250
than it does in the warmer with its semi-permeable
membrane.

00:02:46.250 --> 00:02:49.282
You can actually see some steam coming from it -

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.

00:02:54.279 --> 00:02:58.368
It can be any number of things, but one common
material is vermiculite,

00:02:58.368 --> 00:03:01.083
a silicon-based mineral that contains water.

00:03:01.083 --> 00:03:04.953
You can actually see some water droplets forming
on the bottom of this container,

00:03:04.953 --> 00:03:08.806
and they are coming from this powder and not the ambient air.

00:03:08.806 --> 00:03:13.777
We know that because this is getting hot,
thus ambient moisture cannot condense on it.

00:03:13.777 --> 00:03:17.824
Because these pouches are filled with nothing
but commonly-available substances

00:03:17.824 --> 00:03:20.209
they’re quite inexpensive.

00:03:20.209 --> 00:03:23.096
And, the materials in here are all fairly inert,

00:03:23.096 --> 00:03:27.126
meaning that no special consideration needs to be given to their disposal.

00:03:27.126 --> 00:03:30.792
It’s basically just a packet of rocks and
rust when it’s used up,

00:03:30.792 --> 00:03:33.315
making it Mostly Harmless.

00:03:33.315 --> 00:03:36.701
Although the chemical makeup of these does
vary from brand to brand

00:03:36.701 --> 00:03:39.265
and even between offerings within the same brand.

00:03:39.265 --> 00:03:44.222
The reaction is very simple, but can be tuned
to happen more quickly for faster heat,

00:03:44.222 --> 00:03:48.602
or more slowly for longer-lasting heat, depending
on various factors.

00:03:48.602 --> 00:03:53.938
So the chemical composition of these is usually
quite similar but by no means identical.

00:03:53.938 --> 00:03:56.242
But of course, they are one and done.

00:03:56.242 --> 00:03:58.583
Once you open it there’s no going back.

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

00:04:03.161 --> 00:04:06.565
between spent and fresh hand warmer
contents.

00:04:06.565 --> 00:04:11.074
The spent one looks a bit browner, thanks
to the iron now being iron oxide -

00:04:11.074 --> 00:04:11.985
Rust.

00:04:11.985 --> 00:04:14.948
Macro shots help reveal the oxide particles.

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.

00:04:19.673 --> 00:04:24.046
These, on the other hand, work through a reaction
that’s entirely reversible.

00:04:24.046 --> 00:04:25.957
And that’s because…

00:04:25.957 --> 00:04:27.976
it’s not a reaction, exactly.

00:04:27.976 --> 00:04:31.982
Instead it’s our friend latent heat popping up again!

00:04:32.022 --> 00:04:34.615
Sometimes it prefers you call it enthalpy.

00:04:34.615 --> 00:04:36.789
Combine it with another fun phenomenon,

00:04:36.789 --> 00:04:39.181
dotiouscalifragilisticsupersaturation,

00:04:39.181 --> 00:04:44.929
and you’ve got a phase-change material capable of delivering heat on demand.

00:04:44.929 --> 00:04:49.823
What’s in this pouch is a solution of sodium
acetate trihydrate and water.

00:04:49.823 --> 00:04:52.929
Sodium acetate trihydrate is a crystalline
compound

00:04:52.929 --> 00:04:58.151
with a melting point of 58°C or about 136°F.

00:04:58.151 --> 00:05:02.522
Now believe it or not, I don’t
set my thermostat that high.

00:05:02.522 --> 00:05:07.275
It’s currently about 18°C in this room,
something like 65° F,

00:05:07.275 --> 00:05:09.810
and yet this remains a liquid.

00:05:09.810 --> 00:05:14.329
It’s a fairly viscous liquid, but liquid
nonetheless.

00:05:14.329 --> 00:05:18.946
That’s because this solution is both supercooled
and supersaturated.

00:05:18.946 --> 00:05:24.276
It contains more dissolved sodium acetate
than is ordinarily possible at this temperature,

00:05:24.276 --> 00:05:29.105
and more importantly the solution is below
the freezing point of sodium acetate.

00:05:29.105 --> 00:05:31.914
This supercooling can happen to many substances-

00:05:31.914 --> 00:05:37.179
you’ve probably seen one of those viral 
videos where a water bottle is frozen but…

00:05:37.179 --> 00:05:42.196
not, and then jostling it causes it to nearly instantly freeze.

00:05:42.196 --> 00:05:47.262
In that case, the water remained a liquid
below its freezing point until it was disturbed.

00:05:47.262 --> 00:05:49.014
It was supercooled.

00:05:49.014 --> 00:05:51.950
That’s more or less what’s going on right here,

00:05:51.950 --> 00:05:57.592
except rather than water, the substance in question is the sodium acetate dissolved in it.

00:05:57.592 --> 00:06:01.166
Sodium acetate is useful in this situation
for two reasons:

00:06:01.166 --> 00:06:03.744
one it has a rather high melting point,

00:06:03.744 --> 00:06:07.358
and two in solution it’s remarkably easy to supercool.

00:06:07.420 --> 00:06:10.074
Once it’s all melted and dissolved in a
solution,

00:06:10.074 --> 00:06:13.840
that solution remains liquid well below its freezing point,

00:06:13.840 --> 00:06:15.874
and that’s what’s happening here.

00:06:15.874 --> 00:06:18.554
This solution is what’s known as metastable.

00:06:18.554 --> 00:06:23.590
It’s in a sort of in-between state; 
not really frozen, obviously

00:06:23.590 --> 00:06:26.698
but not really melted, either.

00:06:26.698 --> 00:06:31.266
The sodium acetate is colder than its melting
point yet still molten.

00:06:31.266 --> 00:06:32.969
And here’s the important thing:

00:06:32.969 --> 00:06:36.580
when it does freeze, it will release heat energy.

00:06:36.580 --> 00:06:39.531
That’s our friend latent heat showing up
again.

00:06:39.531 --> 00:06:42.046
Just as there’s a latent heat of vaporization,

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,

00:06:46.099 --> 00:06:48.738
there’s also a latent heat of fusion,

00:06:48.738 --> 00:06:51.786
the energy needed to get a solid substance to melt

00:06:51.786 --> 00:06:54.201
(fusion is another word for melting, see).

00:06:54.201 --> 00:06:58.662
Materials in the liquid phase contain more
energy than they do in the solid phase,

00:06:58.662 --> 00:07:00.806
and since this is currently a liquid,

00:07:00.806 --> 00:07:04.561
yet it’s colder than the freezing point of sodium acetate,

00:07:04.561 --> 00:07:09.455
it’s got more energy in it than it really
should, in a manner of speaking.

00:07:09.455 --> 00:07:13.843
It managed to cool below its freezing point
without actually freezing,

00:07:13.843 --> 00:07:17.111
which means it hasn’t released its latent heat of fusion.

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.

00:07:22.276 --> 00:07:25.977
Pressing on or bending the metal disc, though,
will fix that.

00:07:25.977 --> 00:07:30.184
Remember it’s metastable, or stable but
barely.

00:07:30.184 --> 00:07:33.743
When the tiny cuts in the metal disc scrape
past each other,

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.

00:07:39.824 --> 00:07:43.676
This creates a nucleation site, sometimes
this is called seeding,

00:07:43.676 --> 00:07:46.049
and this breaks the metastable state.

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.

00:07:52.360 --> 00:07:56.930
This is the sodium acetate precipitating out
of the solution and solidifying.

00:07:56.930 --> 00:08:00.182
In moments, the entire thing is frozen and,

00:08:00.182 --> 00:08:01.621
counterintuitively,

00:08:01.621 --> 00:08:02.942
hot.

00:08:02.942 --> 00:08:06.817
As the sodium acetate falls out of the solution
and crystals form,

00:08:06.817 --> 00:08:08.621
in other words as it freezes,

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.

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,

00:08:19.508 --> 00:08:21.489
but that’s precisely what happens.

00:08:21.489 --> 00:08:26.160
It's just… the freezing point of sodium
acetate isn’t cold like ice.

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" -

00:08:31.399 --> 00:08:36.141
that was this exact substance, though
not in a pouch like you see here.

00:08:36.141 --> 00:08:38.927
And it generates heat extremely quickly.

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.

00:08:43.549 --> 00:08:47.234
It doesn’t quite reach the freezing point
of sodium acetate trihydrate,

00:08:47.234 --> 00:08:48.758
but it’s pretty close.

00:08:48.758 --> 00:08:51.418
Just look at my hands compared to the hand
warmer -

00:08:51.418 --> 00:08:53.563
they’re quite a bit colder.

00:08:53.563 --> 00:08:55.848
Luckily, it doesn’t all freeze immediately.

00:08:55.848 --> 00:08:59.454
After the crystals have spread out, you can
knead this thing

00:08:59.454 --> 00:09:02.566
and you’ll discover that it’s pretty soft.

00:09:02.566 --> 00:09:06.797
It feels almost like lightly packed snow,
but warm.

00:09:06.797 --> 00:09:10.764
The sensible heat energy released as the initial
crystallization occurred

00:09:10.764 --> 00:09:13.447
brings the entire volume up to the freezing point

00:09:13.447 --> 00:09:16.698
(which again is anything but what we would call cold)

00:09:16.698 --> 00:09:19.278
and some liquid solution hangs around.

00:09:19.278 --> 00:09:24.209
The latent heat it contains is slowly released
as more acetate falls out of solution

00:09:24.209 --> 00:09:25.886
and continues freezing.

00:09:25.886 --> 00:09:28.946
Over time these things get stiffer and stiffer,

00:09:28.946 --> 00:09:33.452
and by the time they’re all used up it’s quite firm and inflexible.

00:09:33.452 --> 00:09:37.913
Every single time I’ve used one of these
it’s a fascinating experience.

00:09:37.913 --> 00:09:44.007
The idea that something can just spontaneously
release energy by freezing

00:09:44.007 --> 00:09:45.279
is wild!

00:09:45.279 --> 00:09:47.605
And it’s a delightful sight to behold.

00:09:47.605 --> 00:09:52.965
Something about seeing the crystals propagate
like this is just amazing.

00:09:52.965 --> 00:09:57.983
The most recent set that I got came with a couple of
much larger heating pads,

00:09:57.983 --> 00:10:00.295
and it’s mesmerizing to see this occur.

00:10:00.295 --> 00:10:01.678
Just look at that!

00:10:02.718 --> 00:10:04.639
Seriously, look at that!

00:10:05.770 --> 00:10:07.085
Wowzers!

00:10:07.696 --> 00:10:11.017
One thing I’ve learned is that these aren’t
all created equal.

00:10:11.080 --> 00:10:13.529
I was trying to get an interdimensional portal going

00:10:13.529 --> 00:10:16.014
so I got these ones, too.

00:10:16.014 --> 00:10:20.879
So far no luck with the portal but the orange ones
are just… better.

00:10:20.879 --> 00:10:24.220
They produce heat for longer, and I suspect
the reason they do

00:10:24.220 --> 00:10:28.912
is that there’s more sodium acetate in this solution than in the blue ones.

00:10:28.912 --> 00:10:33.960
When these are spent, they’re incredibly
solid - practically rock-like.

00:10:33.960 --> 00:10:37.267
The blue ones, though, remain a bit soft
and somewhat flexible,

00:10:37.267 --> 00:10:39.319
even once at room temperature.

00:10:39.319 --> 00:10:43.011
I suspect the blue ones have more water in
them than the orange ones,

00:10:43.011 --> 00:10:46.815
which keeps them more flexible at the expense of runtime.

00:10:46.815 --> 00:10:49.372
So how are these reusable?

00:10:49.372 --> 00:10:51.150
Well, you may have already guessed.

00:10:51.150 --> 00:10:53.669
You simply need to melt them again.

00:10:53.669 --> 00:10:55.953
And to do that, you boil them.

00:10:55.953 --> 00:10:57.564
Like a weird egg.

00:10:57.564 --> 00:11:01.191
Bring water to a boil, add a used pad, and
keep the water simmering

00:11:01.191 --> 00:11:03.435
until the entire thing has melted.

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,

00:11:08.562 --> 00:11:11.386
the remaining crystals will
prevent it from supersaturating

00:11:11.386 --> 00:11:14.248
and it will simply freeze again as it cools.

00:11:14.248 --> 00:11:16.574
Ya know, like things normally do.

00:11:16.574 --> 00:11:19.162
But, assuming it did melt completely,

00:11:19.162 --> 00:11:23.004
as it cools down it’ll sneakily slip right past its freezing point

00:11:23.004 --> 00:11:25.889
without releasing its latent
heat of fusion.

00:11:25.889 --> 00:11:30.924
It’ll save that for later, providing an
intriguing and effective source of portable heat.

00:11:30.924 --> 00:11:34.817
By the way, in normal circumstances, for instance with
plain water,

00:11:34.817 --> 00:11:40.111
when it cools to the freezing point it will effectively pause getting colder as it freezes.

00:11:40.111 --> 00:11:42.691
No matter how cold the temperature is around
it,

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.

00:11:47.320 --> 00:11:50.777
Only then, after it's released all the latent
heat of fusion,

00:11:50.777 --> 00:11:53.027
can it get below its freezing point.

00:11:53.027 --> 00:11:55.516
That’s how latent heat normally behaves,

00:11:55.516 --> 00:11:59.028
although the rules are bent in all sorts of circumstances.

00:11:59.028 --> 00:12:02.625
The biggest trouble with these is that, well,

00:12:02.625 --> 00:12:04.426
they don’t work all that well.

00:12:04.426 --> 00:12:06.808
Initially they work fantastically!

00:12:06.808 --> 00:12:10.497
It is near-instant heat at its full intensity,

00:12:10.497 --> 00:12:13.728
which simply does not happen with the rusty ones,

00:12:13.728 --> 00:12:16.436
or even an electric heating pad for that matter.

00:12:16.436 --> 00:12:21.570
But their heat output starts falling rapidly
after only about 10 minutes.

00:12:21.570 --> 00:12:25.408
In fact, there’s a significant measurable
difference after only five minutes.

00:12:25.408 --> 00:12:28.834
You’ll get noticeable heat out of them for
maybe a half hour

00:12:28.834 --> 00:12:33.080
but by then it’s already pretty tepid, and also quite stiff.

00:12:33.080 --> 00:12:34.547
[creaky sounds as warmer is flexed]

00:12:34.547 --> 00:12:39.136
Now as I said this does seem to depend somewhat
on the warmer’s acetate concentration,

00:12:39.136 --> 00:12:43.419
but more important than that is its overall mass
and how you use it.

00:12:43.419 --> 00:12:46.181
These tiny ones came with the blue set.

00:12:46.181 --> 00:12:47.034
Shocker.

00:12:47.034 --> 00:12:50.223
And they’re… next to useless.

00:12:50.223 --> 00:12:53.327
You get mere minutes of heat before they’re
used up.

00:12:53.327 --> 00:12:57.783
There’s just too much surface area for the
volume of solution inside.

00:12:57.783 --> 00:13:02.016
The larger orange pads, though, last the longest
of any of these.

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.

00:13:07.906 --> 00:13:10.605
And the other greatest factor is how you use it.

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,

00:13:15.847 --> 00:13:21.082
it will lose its heat rapidly and whatever liquid
acetate remains will freeze faster.

00:13:21.082 --> 00:13:23.546
Therefore it gets used up more quickly.

00:13:23.546 --> 00:13:28.089
If it’s somewhat insulated, though, this
is delayed and it will last longer.

00:13:28.089 --> 00:13:34.084
The trouble there is, especially in the
beginning, these are borderline uncomfortably hot against bare skin.

00:13:34.084 --> 00:13:38.299
And of course, well, eventually you’ll have
a weird rock in your gloves,

00:13:38.299 --> 00:13:40.211
if you could even fit it in there.

00:13:40.211 --> 00:13:43.852
Lastly, there’s the energy needed to melt
them for re-use.

00:13:43.852 --> 00:13:46.054
This is extremely variable, mind you,

00:13:46.054 --> 00:13:50.778
but they need a fairly long time in near-boiling water to be reset.

00:13:50.778 --> 00:13:53.273
It takes a fair bit of energy to heat water,

00:13:53.273 --> 00:13:57.001
especially the quantity needed to reset these large packs.

00:13:57.001 --> 00:14:00.044
And when you get such poor runtime out of
them,

00:14:00.044 --> 00:14:02.223
it seems almost a futile exercise,

00:14:02.223 --> 00:14:07.126
especially when you consider the miniscule fraction of the energy used to reactivate these

00:14:07.126 --> 00:14:11.281
that they are actually able to capture and store for later release.

00:14:11.281 --> 00:14:16.449
Therefore I consider these to be more of a
novelty than a truly useful object.

00:14:16.449 --> 00:14:19.859
That said, I will give them one bit of genuine
praise.

00:14:19.859 --> 00:14:25.387
The heat they generate is distinctly different
from that of these disposable things.

00:14:25.387 --> 00:14:31.256
Being a large, dense, solid hot thing feels more
therapeutic to me,

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.

00:14:37.307 --> 00:14:40.162
Still, if that’s a thing you regularly need…

00:14:40.162 --> 00:14:43.148
I would advise getting an electric heating
pad.

00:14:43.148 --> 00:14:48.322
These are neat, but tedious and boiling them
gets old fast.

00:14:48.322 --> 00:14:50.402
They’re also not cheap.

00:14:50.402 --> 00:14:54.010
This set of 8 hand warmers was $25.

00:14:54.010 --> 00:14:57.232
That can buy a LOT of disposable hand warmers,

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.

00:15:03.949 --> 00:15:07.748
Again - this is just some iron and various
rocks.

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,

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

00:15:18.250 --> 00:15:21.381
than I think makes sense, at least to me.

00:15:21.381 --> 00:15:26.089
But, they are very useful at demonstrating
the concept of latent heat.

00:15:26.089 --> 00:15:31.366
There’s hidden heat energy in here because
substances in a higher phase of matter

00:15:31.366 --> 00:15:35.984
have more energy in them thanks to the difference
in intermolecular forces between the phases.

00:15:35.984 --> 00:15:41.824
It’s the process of adding that extra energy
that gets stuff to melt or vaporize.

00:15:41.824 --> 00:15:44.378
When you come back down the ladder of phases,

00:15:44.378 --> 00:15:48.779
that energy gets released as substances condense or solidify.

00:15:48.779 --> 00:15:53.928
In my eyes, there’s nothing that demonstrates
this quite so effectively as these things.

00:15:53.928 --> 00:15:56.196
Take a liquid at room temperature,

00:15:56.196 --> 00:15:59.297
see it instantly fall from the liquid phase to the solid,

00:15:59.297 --> 00:16:02.146
and feel the heat it releases as it does so.

00:16:02.146 --> 00:16:06.257
It’s a remarkably tangible demonstration of latent
heat,

00:16:06.257 --> 00:16:08.057
and yes I know you can’t feel this,

00:16:08.057 --> 00:16:10.852
but hopefully you see the beauty of this.

00:16:10.852 --> 00:16:13.846
Ya know where else we find exploitation of
latent heat?

00:16:13.846 --> 00:16:17.272
Why, refrigeration and air conditioning, of course!

00:16:17.272 --> 00:16:21.908
The same process of absorbing heat in one
place and expelling it in another

00:16:21.908 --> 00:16:25.845
is responsible for how we cool our homes and keep our food preserved.

00:16:27.451 --> 00:16:31.090
Yeah, OK, it’s nothing like boiling a thing
and storing heat energy

00:16:31.090 --> 00:16:33.771
in its intermolecular bonds to be used for later

00:16:33.771 --> 00:16:35.983
but the principle of latent heat,

00:16:35.983 --> 00:16:39.603
that is the energy absorbed and released as a substance changes phases,

00:16:39.603 --> 00:16:42.288
is exactly what makes these devices work.

00:16:42.288 --> 00:16:43.382
And in the next video

00:16:43.382 --> 00:16:45.940
(weather permitting - it might be the video after the next)

00:16:45.940 --> 00:16:51.935
we’ll talk about heat pumps - using that same principle
to make a space warmer.

00:16:52.030 --> 00:16:56.700
We have heat pumps everywhere - air conditioners
and refrigerators are heat pumps!

00:16:56.700 --> 00:16:59.193
But only recently, particularly in the US,

00:16:59.193 --> 00:17:03.660
have we been learning to use that same concept in reverse to provide space heating.

00:17:03.660 --> 00:17:07.619
Learning to move heat energy
 rather than produce it

00:17:07.619 --> 00:17:11.378
has allowed us to warm our homes more efficiently than ever before,

00:17:11.378 --> 00:17:13.014
and soon we’ll learn how

00:17:13.014 --> 00:17:17.963
as well as and why it is undoubtedly the heating technology of the future.

00:17:18.981 --> 00:17:21.493
♫ enthalpically smooth jazz ♫

00:17:22.511 --> 00:17:25.315
When I squeeze that disc…

00:17:25.315 --> 00:17:27.046
nothing happens.

00:17:27.046 --> 00:17:29.767
When I squeeze that disc…

00:17:29.767 --> 00:17:32.899
again, nothing happens!
 [frustrated laughter]

00:17:32.899 --> 00:17:35.806
Sodium acetate is useful in this situations…

00:17:35.806 --> 00:17:36.980
weooow

00:17:36.980 --> 00:17:39.152
...cause a crystal of soble…

00:17:39.152 --> 00:17:40.218
[weird noise]

00:17:40.218 --> 00:17:43.796
Or stable, but barely 
[weird throat clear]

00:17:43.796 --> 00:17:48.622
...precipitating out of the solution and solidifying
in moments. The entire -

00:17:48.622 --> 00:17:49.972
No!
[laughs]

00:17:49.972 --> 00:17:51.170
Gah-ha!

00:17:51.170 --> 00:17:55.256
And some liquid solution remains hangs around.

00:17:55.256 --> 00:17:56.076
Dang!

00:17:56.076 --> 00:17:58.628
You’ll get noticeable heat output for them...

00:17:58.945 --> 00:18:00.229
Out.. out of them.

00:18:00.229 --> 00:18:02.041
♫ unprovoked humming ♫

00:18:02.041 --> 00:18:04.448
Incredibly small niche and require…

00:18:04.448 --> 00:18:05.817
as…

00:18:05.817 --> 00:18:06.878
Oops!

00:18:07.873 --> 00:18:12.307
Well that sure was a different kind of freezing, wasn't it!

00:18:12.307 --> 00:18:14.616
Freezing but getting hot?

00:18:14.616 --> 00:18:16.946
How strange and imaginative!

00:18:16.946 --> 00:18:20.573
Sounds like something out of Alice in Wonderland 
but it's not!

00:18:20.980 --> 00:18:24.137
LATENT HEAT STRIKES AGAIN