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00:00Everyone knows if you shake up a carbonated drink, it explodes.
00:09But why is this?
00:11Well, here I have an identical bottle with a pressure gauge fitted to it.
00:16And I want you to make a prediction right here.
00:18If I shake up this bottle, will the pressure increase, decrease, or remain the same?
00:24And while you're thinking about that, let me tell you this video is sponsored by Squarespace,
00:28the all-in-one platform for building your online presence.
00:32More about Squarespace at the end of the show.
00:35Okay, I hope you've made your prediction and registered it in the poll up here.
00:38This bottle has been sitting stationary for a few days at room temperature.
00:43You can see the pressure reading is about three atmospheres, 330 kilopascals.
00:46And I'm going to shake it up and see what happens.
00:48Ready?
00:49In three, two, one.
00:55And the pressure is still the same.
00:58And you might suspect that, well, maybe this bottle is all out of gas.
01:02Maybe it wouldn't explode on me.
01:03So just to make sure.
01:07Yep, it would go.
01:09So it's not an increase in pressure that causes a bottle to explode like that.
01:16So why is it?
01:17We shouldn't feel bad if you predicted that the pressure would increase because, in fact,
01:21that explanation was published in New Scientist in 1986, leading many other scientists to come
01:28forward saying that is not the real explanation.
01:30So we will find out what it is after we explore the second perplexing physics problem.
01:38Consider this.
01:40If you put identical ice cubes in a cup of fresh water and a cup of salt water, which ice cube
01:46will melt first?
01:48Again, you can register your prediction by answering the poll here.
01:52Now, as you're thinking about that, I want to show you the setup.
01:55Okay, so here I have regular fresh water.
01:58I'm just going to fill up each cup.
02:00Then I'm going to add about a tablespoon of salt into this cup on the right.
02:06Now, if you know a bit about chemistry, you may recognize that adding sodium chloride to
02:10water actually takes energy.
02:13And so it lowers the temperature of this solution by a little bit.
02:16So I've got a thermometer just to check.
02:20And I'm going to let this solution sit here for a while so that it comes back up to room
02:26temperature.
02:27Okay, have you made your prediction?
02:29Let's put these ice cubes in.
02:31In three, two, one.
02:35And they're off.
02:39Watching ice cubes melt.
02:41Isn't this entertaining, YouTube?
02:42It has only been about a minute, but already I can tell that the ice cube in the fresh water
02:50is melting faster than the ice cube in the salt water.
02:54How does that make any sense?
02:56I mean, we put salt on the roads to melt ice faster.
03:00So why isn't this ice cube melting as fast as the one in fresh water?
03:04Well, that is what I'm going to explain.
03:06But first, let's go to the third perplexing physics problem.
03:10Okay, here I have a metal ring and a closed loop of chain.
03:17And I'm going to do this all in one take so you know that I'm not playing any tricks.
03:21So what I'm going to do is dangle the chain and then hold the ring over it like so.
03:28And then I'm going to drop the ring.
03:30And exactly what you expect happens.
03:34The ring just falls off this chain.
03:36And of course, how could anything else possibly happen?
03:39Because, well, it's a closed loop of chain and a closed ring.
03:43But, if you think about it really hard, you can get the ring to stick on the chain.
03:54Have a look at that.
03:57So how does this work?
03:59I think we're going to have to go to some slow-mo footage to really see what's going on.
04:04Now, I'll let you in on the secret.
04:31When you want the ring to stick on the chain, the key is to let it go on one side before the other side.
04:40So I'm going to let it go with my thumb first, and it'll just sort of slide off my finger.
04:47And by doing that, the ring will stick on the chain.
04:50It introduces just a little bit of rotation so that the ring rotates about 90 degrees
04:56and slides down the chain like this.
05:01As it does, these pieces slide up the sides.
05:06And when you get to the bottom, it's almost like this piece at the bottom gets sucked into the middle of the ring.
05:13And then at the last minute, it gets pulled around,
05:15and it snaps on.
05:18And it's locked on like that.
05:20So that's how you can get a ring locked onto a closed loop of chain.
05:44So back to problem number two.
05:48Why is the ice cube in fresh water melting faster than the one in salt water?
05:52Well, I think we can get a better view of this
05:55if I add a little bit of food coloring right on top of the ice cube.
06:01Because the water coming off that ice cube is cold,
06:05it's more dense than the surrounding fresh water.
06:08And so it descends in the glass,
06:12and that brings more warm fresh water up to meet the ice cube, melting it faster.
06:18Whereas on the other hand, in the salt water,
06:22as the ice cube melts,
06:26that fresh water is actually less dense than the salt water around it.
06:32And so it stays that cold water that just melted off the ice cube
06:38stays around the ice cube,
06:40in effect, insulating it from the warmer salt water around it.
06:45Okay, that seems like a very plausible explanation
06:48and maybe a convincing demonstration.
06:50But in the edit, me from the future,
06:54I decided that, you know,
06:56maybe this wasn't the best way to explain this
06:59because, well, you're just dropping food coloring in there.
07:02Maybe food coloring would just float on the surface of salt water anyway
07:05and sink in fresh water.
07:07So, not a good demonstration.
07:09So a better demonstration, I thought, might be
07:11if we use colored ice cubes to begin with.
07:19Okay, I know there's a lot of food coloring in there
07:21and that makes things kind of hard to see.
07:23But I think you can clearly see the currents of cold water
07:27streaming down to the bottom of the cup
07:29in the fresh water side and not in the salt water side.
07:32So I think this does clearly show what I was saying.
07:36The cool water that comes off the ice cube
07:39doesn't go down deep into the cup.
07:44Over here you can see that's what's happened.
07:47So why do shaken carbonated drinks explode?
07:53First, let's explain why the pressure doesn't increase
07:56in the headspace when you shake it up.
07:58This is because of equilibrium.
08:00You know, when you pick up a bottle of soda
08:03in the grocery store, it's been sitting there for a few days.
08:06So the dissolved gas, the dissolved CO2 in the liquid
08:09is at equilibrium with the gas up here in the headspace.
08:13And that equilibrium only depends on the temperature
08:16and the pressure of gas in the headspace.
08:18So no amount of shaking is going to change the pressure up here.
08:22For most soda bottles these days,
08:24that pressure is about three atmospheres.
08:27Now you can actually hear those three atmospheres
08:30of pressure get released
08:32when I open the bottle.
08:35But of course that's not messy
08:36because it's just gas coming out the top.
08:38There's no liquid.
08:39But now the liquid is no longer in equilibrium.
08:44I mean, it used to be under three atmospheres of pressure
08:46and now it's just under one atmosphere, ambient pressure.
08:51And so because of that,
08:52there is more dissolved CO2 in this liquid
08:55than would be at equilibrium at this pressure.
08:58And so the CO2 starts to come out of solution
09:01and well, those are the bubbles that you taste.
09:04That's why this drink is fizzy.
09:06Non-equilibrium beverage.
09:07And if you leave it open, those bubbles will keep coming out
09:10until the whole drink goes flat.
09:12Now I'm going to put the pressure gauge on top of this bottle
09:16so we can actually see the CO2 coming out of solution
09:21and increasing the pressure right here.
09:25And if I left this bottle alone for long enough,
09:28the pressure would eventually come back to equilibrium,
09:30three atmospheres.
09:31But as you can see, it is a very slow process.
09:34And that's because it's actually quite hard
09:36for dissolved gas like CO2 to spontaneously come out of solution.
09:42One way that I can accelerate this process
09:44is by introducing nucleation sites into the liquid.
09:48And one example of a nucleation site is a tiny gas bubble.
09:51So if I shake up the bottle,
09:54what I'm actually doing is introducing little nucleation sites,
09:58tiny air bubbles into the liquid.
10:00And that makes it easier for the CO2 to come out of solution.
10:03And so we'll see this pressure increase much more rapidly.
10:07Are you ready?
10:08I'm going to shake it up in three, two, one.
10:11And there you see the pressure has quite quickly come back
10:18to about three atmospheres, 320 kilopascals.
10:22So if you shake up a closed carbonated drink
10:26that's been at equilibrium,
10:27well, you are not increasing the pressure in the bottle,
10:30but you are introducing tiny air bubbles into the liquid,
10:34which act as nucleation sites.
10:36Some of them cling to the walls of the bottle.
10:38So when you go to open it, those bubbles do two things.
10:42First, they expand due to the decrease in pressure,
10:45and that pushes up the liquid above them.
10:47And second, they act as nucleation sites,
10:50allowing the dissolved CO2 to come out of solution
10:52much more rapidly.
10:54And so that's what leads to the carbonated drink explosion.
10:58But this suggests a way to disarm a shaken carbonated beverage,
11:01and that is by flicking the walls of the bottle.
11:04That gets rid of those bubbles that are clinging to the sides
11:08and allows you to open the bottle without incident.
11:13Ha!
11:14It worked.
11:16Now, is there a way to introduce nucleation sites
11:18into a carbonated drink without shaking it?
11:21Yes.
11:22That's exactly what you're doing
11:23when you put a mentos in a carbonated drink.
11:26The rough surface of the mentos acts as a nucleation site,
11:30which allows the CO2 to come out of solution much faster,
11:33creating the soda fountain.
11:35Me again.
11:35So when I showed this video to Diana, the physics girl,
11:38she asked whether paper straws have more nucleation sites
11:42than plastic ones.
11:43And to be honest,
11:44I'm not sure about the research around this,
11:46but there are some other YouTube videos
11:49showing how drinks overflow when you put a paper straw in.
11:52And also my little preliminary analysis
11:56with this paper straw
11:57show that it does indeed create more bubbles
12:00than a plastic straw.
12:03So if you needed another reason to hate paper straws,
12:06well, there you go.
12:09They make your carbonated drink more fizzy
12:11as it comes up the straw.
12:13This has been three perplexing physics problems.
12:15If you have any other perplexing science problems,
12:18put them in the comments below.
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