How Do Disposable Hand Warmers Work?

These tiny packets are able to give off a lot of heat for a few hours thanks to the same process that causes rust to form -- just sped way up. Learn more in this episode of BrainStuff, based on this article: https://science.howstuffworks.com/disposable-hand-warmers.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff, a production of iHeart Radio. Hey
brain Stuff, Lauren Vogelbam Here. When the weather is so
cold that even your best gloves and socks can't keep
your digits toasty, those little packets of magic that we
call disposable handwarmers can bring big relief. Handwarmers are good

(00:23):
to have an emergency's and they're perfect to tuck inside
your bittens during winter activities like skiing, ice skating, or
just on your daily commute. But how exactly do these
little packets work. They are not filled with tiny demons
who warm your hands with hell fire, nor is it
some gamma radiating goo that will turn your paws into

(00:43):
hulk smash style superhands. The science behind those handwarmers is
actually pretty simple. It turns out they use the same
process that transforms iron into rest, just a lot faster.
There are five main ingredients inside a disposable handwarmer iron powder, water, salt,

(01:03):
activated charcoal, and some kind of water absorbent material. But
of course they don't get hot until you open the
air tight plastic wrappers that the packets come in. That's
because the final ingredient that makes them work is oxygen
to become hot. Oxygen from the air flows into the
packet and reacts with the iron powder, water and salt

(01:27):
that's already there, which oxidizes the particles of iron, that is,
it turns them into iron oxides, also known as rust.
Oxidation is sometimes used as another term for rust, because
rust is perhaps the most well known version of oxidation,
but lots of different substances can oxidize anyway. What's happening

(01:47):
in the packet is that the iron powder is giving
away electrons to the oxygen that comes in. Water has
to be present in order to break up molecules of
oxygen so that the reaction can happen. The water absorbent
material in the packet hangs onto the water molecules for
safe keeping until you open the packet. Meanwhile, the salt

(02:08):
helps the oxidation along because it accelerates the flow of electrons.
You've probably observed this if you've noticed that exposed metal
on a car will rest more quickly if the car
is driven on salt treated roads in the winter, or
if you've noticed a lot of rust on chains or
fences that are in or near seawater the versus ones

(02:28):
that aren't. But okay, the process of rusting is exothermic,
meaning it gives off heat, But when your car's fender rusts,
it doesn't feel hot, and that's because that process happens
very slowly, even when it is accelerated by salt. The
reaction is greatly sped up in disposable handwarmers, and that's

(02:52):
where the construction of the packet and the other ingredients
come in. It may be simple science driving this reaction,
but control holling it for the precise effect of creating
a certain amount of warm for a certain period of
time is complicated. The material of the packet has to
let in just the right amount of oxygen. They're made

(03:13):
with microporous materials, meaning they have tiny holes to let
oxygen in at the gaseous equivalent of a slow drip.
Warmers for hands have very tiny holes, whereas the warmers
designed for feet have slightly larger ones. That's because there's
less air inside your boots and shoes, so those warmers
need more access to oxygen in order to activate. Then

(03:35):
you've got that activated charcoal, which serves to distribute the
heat that's created evenly throughout the packet so that you
don't wind up with any extra hot clumps. Manufacturers can
tweak how fast the reaction happens and how long it
continues producing heat by changing the size of the iron
powder particles. Smaller particles with more surface area will heat

(03:56):
up faster, and by increasing the amount of iron in
the PA could Overall, they're designed to last anywhere from
a few hours up to a full day. So it's
not demons and it's not scary chemicals. It's not even magic.
But the heat in that handwarmer is toasty. Those little
packets can produce temperatures between a hundred and a hundred

(04:17):
and eighty degrees fahrenheit. That's eighty two degrees celsius, which
should keep your digits nice and cozy. And it's totally
safe throw used disposable handwarmers in the garbage. No has
Matt team required. Today's episode is based on the article
how do disposable handwarmers work? On how stuffworks dot com

(04:40):
written by Kristen Hall Geisler. Brain Stuff is production of
i Heeart Radio in partnership with how stuffworks dot com,
and it's produced by Tyler Klang. Four more podcasts my
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