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The Very Weird Reason Hot Water Freezes Faster
The Very Weird Reason Hot Water Freezes Faster
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0:00
- Thank you to Displate for supporting PBS.
0:03
One day as part of a cooking class,
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a boy named Erasto Mpemba was given the assignment
0:08
of making some ice cream.
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He boiled some milk, added some sugar.
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That's probably about enough.
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Then he poured that mixture into a container.
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Of course, he had to wait for it to cool
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before he could put it in the freezer.
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Who has time for that? I want ice cream.
0:30
On that day in 1963, Mpemba was in a hurry too,
0:34
so he put his hot ice cream mixture in the freezer while it
0:37
was still hot.
0:40
Now, his classmates left theirs out on the counter
0:43
to cool first before putting them in the freezer,
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but later, to everyone's surprise,
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even though everyone else's mixture had started at a lower
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temperature, Mpemba's ice cream mix froze first,
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faster than everyone else's.
1:00
All of this happened at a secondary school in
1:03
Tanzania when
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Mpemba was just 13 years old.
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When he found his oddly expeditiously frozen
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treat waiting
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for him the next day, he could have shrugged it off
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as some sort of fluke,
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a weird unexplained event, but he didn't.
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Instead, he asked why.
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That question turned out to be a lot harder
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to answer than anyone expected.
1:26
60 years after the ice cream incident,
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physicists are still arguing about this phenomenon,
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now appropriately called the Mpemba Effect.
1:35
Maybe you've heard about this trick before.
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But what you probably don't know is
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how this strange observation,
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watching hot water freeze faster than cooler water,
1:45
has begun to shake up very different corners of physics.
1:48
From how we heat and cool machines
1:50
to how we design materials,
1:52
even quantum computing.
1:54
Today, we're going to dig into this deceivingly simple
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phenomenon to learn why taking the longer route can
2:01
sometimes be faster.
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After I finish my ice cream.
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Hey, smart people, Joe here.
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It turns out that 13-year-old Erasto Mpemba wasn't the
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first person to notice this.
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Scholars have been pointing out this weird effect
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for thousands of years.
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The first known reference came from Aristotle.
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Doesn't it always? It was all the way back in 350 BCE.
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He wrote this book called Meteorologica,
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in which he mentions how in certain places it was common
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practice to heat up water before freezing it.
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He points, for example, to ice fishermen in Pontus,
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in what's now Turkey.
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They would pour hot water on their rods
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because it freezes quicker
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and then use that ice to hold them in place.
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Over the centuries, the effect was noted again
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and again by scholars.
2:52
Here's one from Roger Bacon in the 13th century,
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or René Descartes in the 17th century.
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Lots of pretty smart people have been fascinated by this.
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And historians have since found numerous accounts of people
3:05
around the world making the same claim.
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Hot water sometimes freezes faster than cooler water.
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But all this seems
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to go against our intuition about how heat works.
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So people have been pretty skeptical of claims like these.
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Some even outright dismissing them as flawed experiments.
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When Mpemba asked his physics teacher why his hot ice cream
3:27
froze first, he was told, "That is Mpemba's physics,
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not the universal physics." Skepticism
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is a good thing, of course.
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It's a necessary ingredient in science,
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but luckily that didn't stop Mpemba.
3:40
Years later, a physicist named Denis Osborne visited his
3:44
school and Mpemba asked him the same question.
3:47
Of course, his classmates laughed at him,
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but Osborne didn't.
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He went back to his lab and ran the experiment himself
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and got the same seemingly impossible
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result that Mpemba got.
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And they published it together with Mpemba as first author.
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My name is Erasto B
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Mpemba, and I'm going to tell you about my discovery,
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which was due to misusing a refrigerator."
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Let's take a second to appreciate
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how weird this actually is.
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When you freeze a cup of water,
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it cools continuously from room
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temperature down to freezing.
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If you start hotter, you've just got farther to travel
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before getting to freezing.
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So it should always take longer, right?
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Well, this was captured in math by Isaac Newton
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and his law of cooling from 1701.
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It describes how things change temperature continuously.
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Compared to a room temperature glass, a hotter glass
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of water will drop in temperature faster at first,
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but then it'll just trail behind the other glass
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as they keep on cooling.
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I mean, that makes sense, right?
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Picture two curiously handsome runners
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racing to a finish line.
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One starts closer to the line,
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the other one starts way back here.
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Obviously the one that starts closer
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to the finish line wins, right?
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Well, that's not always the case.
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Sometimes the runner
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who starts farther back crosses the finish line first,
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but only if he runs the race in a very certain way.
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That's the Mpemba Effect.
5:15
It's actually not hard to show
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that hot water can freeze really fast.
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On a super cold day, throw some boiling water into the air
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and it'll instantly freeze into these beautiful
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icy rainbow shapes.
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We try that with lukewarm water, just get cold rain.
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But it has turned out to be incredibly challenging
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to demonstrate the Mpemba Effect rigorously
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and repeatedly in the lab.
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In the decades after Mpemba
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and Osborne published their paper, physicists had attempted
5:44
to replicate the effect with, honestly, pretty mixed results.
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I mean, some studies seemed to replicate the findings,
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but others directly contradicted them.
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Now, there are two big reasons for this controversy.
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One reason is that these experiments are really tough
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to control perfectly.
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Whenever you compare when two liquids freeze, what tips
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that balance to one or the other can be so many things.
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The shape of the container, dissolved gases, how smooth
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or rough the surface is
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or if something disrupts a super cool liquid,
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because even tiny ice crystals can start a chain
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reaction of freezing.
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There's an absurd number of variables to control.
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The second reason is that water itself is very weird.
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It's actually most dense at four degrees Celsius,
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just above freezing.
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So as it cools, the coldest layer sinks
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and warmer water rises to the surface
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where it evaporates faster.
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Once ice does actually start forming, those crystals float
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and insulate the water underneath,
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slowing the heat loss back down.
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Water is just surprisingly strange stuff.
6:48
And for that matter, what does freezing even mean?
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Is it when the first ice crystal forms?
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Or when the average temperature hits zero?
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When it's fully solid?
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Different labs have used different definitions
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and gotten different answers.
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This is tricky stuff.
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So maybe the Mpemba Effect happens just
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because water is uniquely weird, right?
7:08
Well, not quite.
7:09
Over the last few decades, that exact same signature effect,
7:13
starting hotter and getting to frozen faster,
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keeps showing up in materials that they have nothing to do
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with water, which means that whatever's going on
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is bigger than just one strange liquid.
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Take these molecules, for example. Polylactic acid,
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or PLA.
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It's what many compostable cups are made of.
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And if you got a 3D printer,
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you've almost certainly used this stuff.
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But to form it, you have to get the molecules
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to crystallize.
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They lock into an ordered structure,
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which is sort of like freezing.
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And guess how you can get them to freeze faster?
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By heating them up first.
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But it gets even weirder than that.
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The effect doesn't just apply to temperature either.
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You can find a magnetic Mpemba Effect too.
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If you look at the magnetic field of certain materials,
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you'll find that objects
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with a stronger magnetic field can be demagnetized faster.
8:04
Again, winning the race
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by starting farther from the finish line.
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It seems like the Mpemba Effect isn't some one-off quirk.
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It's a real feature of nature.
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Now, the thing that they all have in common,
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the more extreme it is where you start,
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the faster you can find your way back to equilibrium.
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A state where everything is settled
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and stable, done changing.
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You see, nature loves equilibrium.
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Leave out a cup of ice in a warm room,
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and eventually it melts.
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It settles at the same temperature as the room.
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A leaf falls onto the ground and eventually it'll decompose
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and become indistinguishable with the soil.
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Everything, given enough time,
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ultimately seeks out sameness.
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Why does the world crave equilibrium?
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Well, it's simple statistics, really.
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Basically, there's just many more ways for things
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to be randomly mixed up than to stay clumped together.
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The lukewarm water is vastly more probable than an ice cube.
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Mixed up soil is more probable than an intact leaf.
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So what does this have to do
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with Mpemba's ice cream experiment?
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Well, inside of a freezer, the equilibrium state is,
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well, frozen solid.
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Both the hot and cold cup of water are racing to
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that finish line.
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The question is, how does the hot cup get there first if the
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cold cup has a head start?
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Does it somehow want it more?
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It turns out, well, sort of, yes.
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See, scientists have worked out a mathematical
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explanation for this shortcut.
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They showed that the fastest path to equilibrium
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isn't always the most direct one,
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especially when systems start with more energy.
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Imagine firing a ball into a maze.
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The harder that you shoot it, the more that it can ricochet
9:52
around and maybe find a path to the target.
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A system with more energy and more things inside, jittering
9:58
and moving in all sort of quantum and atomic-y ways.
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They can explore more pathways to equilibrium,
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potentially finding faster shortcuts along the way.
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Picture the road to equilibrium as a downhill journey.
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For a ball that starts here, it'll roll sort
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of sluggishly to the finish.
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But for a ball that rolls from here at a higher energy
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starting point, it has more energy to carry it
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to the end faster.
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If there are intermediate valleys along the way,
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the faster ball might be able to find shortcuts
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to the lowest energy state.
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That's how it reaches equilibrium faster.
10:31
One group of scientists tested the effect
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by building this energy landscape in real life.
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When they drop tiny glass beads into this landscape,
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they could stop at this first energy hump,
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or they could roll through that first one
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to this more stable resting place.
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They found that the beads that started out hotter tended
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to find the lowest resting place faster,
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just like the math predicted.
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And it goes even smaller than that.
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Down at the scale of individual atoms,
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physicists have found a quantum version
10:59
of the Mpemba Effect too.
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Picture a row of trapped ions, kind
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of like tiny compass needles all pointing the same way.
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That's a state of symmetry. Now tilt them out of alignment.
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If you just leave them alone,
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they'll eventually drift back into sync.
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But if you tilt them further out of alignment,
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sometimes they snap back into sync faster than ions
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that were only disturbed a little.
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Physicists confirmed this quantum Mpemba Effect in the lab
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for the first time in 2024.
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And this isn't just a curiosity.
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Researchers have started using this exact effect
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to quickly reset the memory in quantum computers.
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It's sure starting to seem like this is all the same beast
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in different forms.
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From freezing ice cream to wiping out magnets
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to quantum alignments.
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Now across all these different materials
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and different scales, there's one consistent theme.
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The shortest route is not always the fastest one.
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And we still don't fully understand why the Mpemba Effect
11:58
happens in every case,
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but that doesn't stop people from putting it to work.
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Physicists are already exploring Mpemba-inspired tricks
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to make cooling and heating systems more efficient,
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which not a huge surprise.
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But Mpemba's turning up in some much weirder places too,
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like improving the manufacturing of ceramic materials.
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Now some very special ceramics have
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to be compressed really tightly to be formed.
12:21
Instead of using a big squish machine,
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one group used the expansion of freezing ice
12:26
to squeeze the material.
12:28
And sure enough, they found
12:29
that the freezing happened faster if they
12:31
started with hot water.
12:33
Thanks, Mpemba. Who knows where else Mpemba will show up?
12:36
I mean, that's the beauty of discoveries like this.
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You start with this tiny little oddity, something
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that doesn't seem quite right.
12:43
And if you keep poking,
12:45
you might end up exposing a new way of looking at the world.
12:48
In this case, the mystery comes from assuming
12:51
that everything is close to equilibrium.
12:53
Whether or not we realize it, much
12:55
of our intuition about the world is
12:57
built on that assumption.
12:59
I mean, we imagine that the air in a room is exactly the
13:02
same temperature everywhere.
13:04
We assume that flipping a coin a hundred times will land us
13:07
exactly 50 heads or tails,
13:09
or that the balls in a ball pit are randomly
13:12
distributed by color.
13:13
But if you think about it, the things
13:15
that are far from equilibrium are often
13:17
the most interesting.
13:19
Take fire. If you leave it in an empty room,
13:22
it'll eventually peter out and just blend in with the air.
13:25
But if you keep feeding it oxygen
13:27
and fuel, if you keep pushing it out of equilibrium,
13:30
it can light up your room, even drive your engine.
13:34
And you and I
13:35
and all life as we know it, we are all out
13:38
of equilibrium too.
13:39
All the work that we do in the world is only possible
13:43
because we draw energy from the sun
13:45
and from our food to constantly push ourselves away
13:48
from equilibrium.
13:50
When we fall into equilibrium, we die. We decompose.
13:54
Let me just blend in with the soil, just like that leaf.
13:57
Far from equilibrium is
13:58
where all the exciting stuff happens.
14:00
But it's also where our intuition about the world
14:02
kind of goes out the window.
14:03
It's where nature can play all sorts
14:05
of surprising tricks on us, like the origin of life
14:08
or earthquakes or the Mpemba Effect.
14:11
This story is a wonderful reminder of the power
14:14
of questioning the world around us.
14:15
Even things as trivial as a bowl of ice cream.
14:18
You never know what it will turn up. New questions?
14:21
Faulty assumptions, deep insights,
14:23
just hiding in plain sight.
14:25
I think Mpemba and Osborne put it best in their very first
14:29
paper about this effect:
14:31
"No question should be ridiculed.
14:33
Everyday events are seldom as simple as they seem."
14:37
Or in other words, stay curious.
14:40
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We'll see you in the next video.
17:04
Sorry, Carl, I win.
17:06
I don't need, I don't need to eat more ice cream at the end.
17:10
Director: "rolling"
17:11
Joe: "cool drumming noises"
