everyone knows if you shake up a carbonated drink it explodes but why is this

well here I have an identical bottle with a pressure gauge fitted to it and I want you to make a prediction right here if I shake up this bottle will the pressure increase decrease or remain the same

I hope you've made your prediction and registered it in the poll up here this bottle has been sitting stationary for a few days at room temperature you can see the pressure reading is about three atmospheres 330 kiloPascals

and I'm going to shake it up and see what happens ready in three two one and the pressure is still the same

you might suspect that well maybe this bottle is all out of gas maybe it wouldn't explode on me so just to make sure yep it would go so it's not an increase in pressure that causes a bottle to explode like that

so why is it we shouldn't feel bad if you predicted that the pressure would increase because in fact that explanation was published in New Scientist in 1986 leaving many other scientists to come forward saying that is not the real explanation so we will find out what it is

after we explore the second perplexing physics problem consider this if you put identical ice cubes in a cup of fresh water and a cup of saltwater which ice cube will melt first again you can register your prediction by answering the poll here now as you're thinking about that I want to show you the setup

okay so here I have regular fresh water I'm just gonna fill up each cup then I'm gonna add about a tablespoon of salt into this cup on the right if you know a bit about chemistry you may recognize that adding sodium chloride water actually takes energy and so it lowers the temperature of this solution by a little bit

so I've got a thermometer just to check and I'm gonna let this solution sit here for a while so that it comes back up to room temperature okay have you made your prediction let's put these ice cubes in in three two one

and they're off watching Ice Cube's melt isn't this entertaining YouTube it has only been about a minute but already I can tell that the ice cube in the fresh water is melting faster than the ice cube in the salt water how does that make any sense

and we put salt on the roads to melt ice faster so why isn't this ice cube melting as fast as the one in fresh water well that is what I'm gonna explain but first let's go to the third perplexing physics problem

okay here I have a metal ring and a closed loop of chain and I'm gonna do this all in one take so you know that I'm not playing any tricks so what I'm gonna do is dangle the chain and then hold the ring over it like so and then I'm gonna drop the ring and exactly what you expect happens the ring just falls off this chain

and of course how could anything else possibly happen because well it's a closed loop of chain and a closed ring but if you think about it really hard you can get the ring to stick on the chain

have a look at that so how does this work well I think we're gonna have to go to some slow-mo footage to really see what's going on

now I'll let you in on the secret when you want the ring to stick on the chain the key is to let it go on one side before the other side so I'm going to let let it go with my thumb first and it'll just sort of slide off my finger and by doing that the ring will stick on the chain it introduces just a little bit of rotation so that the ring rotates about 90 degrees and slides down the chain like this

as it does these pieces slide up the sides and when you get to the bottom it's almost like this piece at the bottom gets sucked into the middle of the ring and then at the last minute gets pulled around and it snaps on and it's locked on like that

so that's how you can get a ring locked on to a closed loop of chain

so back to problem number two why is the ice cube in fresh water melting faster than the one in salt water well I think we can get a better view of this if I add a little bit of food coloring right on top of the ice cube because the water coming off that ice cube is cold it's more dense than the surrounding fresh water and so it descends in the glass

and that brings more warm fresh water up to meet the ice cube melting it faster whereas on the other hand in the salt water as the ice cube melts that fresh water is actually less dense than the salt water around it and so it stays that cold water that just melted off the ice cube stays around the ice cube in effect insulating it from the warmer salt water around it

okay that seems like a very plausible explanation and maybe a convincing demonstration but in the Edit me from the future I decided that you know maybe this wasn't the best way to explain this because well you're just dropping food coloring in there and maybe food coloring would just float on the surface of salt water anyway and sink in fresh water so not a good demonstration

so a better demonstration I thought might be if we use colored ice cubes to begin with okay I know there's a lot of food coloring in there and that makes things kind of hard to see but I think you can clearly see the currents of cold water streaming down at the bottom of the cup in the fresh water side and not in the salt water side

so I think this does clearly show what I was saying that cool water that comes off the ice cube doesn't go down deep into the cup over here you can see that's what's happened

so why do shaken carbonated drinks explode well first let's explain why the pressure doesn't increase in the headspace when you shake it up this is because of equilibrium

you know when you pick up a bottle of soda in the grocery store it's been sitting there for a few days so the dissolved gas the dissolved co2 in the liquid is at equilibrium with the gas up here in the head space and that equilibrium only depends on the temperature and the pressure of gas in the headspace so no amount of shaking is going to change the pressure up here

for most soda bottles these days that pressure is about 3 atmospheres now you can actually hear those 3 atmospheres of pressure get released when I open the bottle but of course that's not messy because it's just gas coming at the top there's no liquid

but now the liquid is no longer in equilibrium I mean it used to be under 3 atmospheres of pressure and now it's just under 1 atmosphere ambient pressure and so because of that there is more dissolved co2 in this liquid then would be at equilibrium at this pressure and so the co2 starts to come out of solution and well those are the bubbles that you taste that's why this drink is fizzy non-equilibrium beverage

and if you leave it open those bubbles will keep coming out until the whole drink goes flat now I'm gonna put the pressure gauge on top of this bottle so we can actually see the co2 coming out of solution and increasing the pressure right here

and if I left this bottle alone for long enough the pressure would eventually come back to equilibrium 3 atmospheres but as you can see it is a very slow process and that's because it's actually quite hard for dissolved gas like co2 to spontaneously come out of solution

one way that I can accelerate this process is by introducing nucleation sites into the liquid and one example of a nucleation site is a tiny gas bubble so if I shake up the bottle what am actually doing is introducing little nucleation sites tiny air bubbles into the liquid

and that makes it easier for the co2 to come out of solution and so we'll see this pressure increase much more rapidly you ready I'm going to shake it up in three two one and there you see the pressure has quite quickly come back to about three atmospheres 320 kiloPascals

so if you shake up a closed carbonated drink that's been at equilibrium well you are not increasing the pressure in the bottle but you are introducing tiny air bubbles into the liquid which act as nucleation sites some of them clinging to the walls of the bottle so when you go to open it those bubbles do two things first they expand due to the decrease in pressure and that pushes up the liquid above them and second they act as nucleation sites allowing the dissolved co2 to come out of solution much more rapidly and so that's what leads to the carbonated drink explosion

but this is just a way to disarm a shaken carbonated beverage and that is by flicking the walls of the bottle that gets rid of those bubbles that are clinging to the sides and allows you to open the bottle without incident ah it worked

now is there a way to introduce nucleation sites into of carbonated drink without shaking it yes that's exactly what you're doing when you put a Mentos in a carbonated drink the rough surface of the Mentos acts as a nucleation site which allows the co2 to come out of solution much faster creating the soda fountain

me again so when I showed this video to Diana the physics girl she asked whether paper straws have more nucleation sites than plastic ones and to be honest I'm not sure about the research around this but there are some other YouTube videos as showing how drinks overflow when you put a paper straw in

and also my preliminary analysis with this paper straw show that it does indeed create more bubbles than a plastic straw so if you needed another reason to hate paper straws well there you go they make your carbonated drink more fizzy as it comes up the straw

this has been three perplexing physics problems if you have any other perplexing science problems put them in the comments below

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