Kind: captions
Language: en
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 and while you're
thinking about that let me tell you this
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at the end of the show okay
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
[Music]
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
[Music]
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