you may not have seen Brady's last video
in which he walked through the Himalayas
to Everest base camp and as he walked
his Sherpa who was accompanying him
carried this kettle to boil water to
make tea and Brady did a really lovely
physics experiment or you could say
physical chemistry experiment every time
they stopped as they got higher and
higher in the mountains he measured the
temperature of the water using this
thermometer and he found that the
boiling point of water dropped until at
the highest point he measured a
temperature of only 79 degrees
centigrade Celsius if you prefer and you
remember that at sea level water boils
at 100 degrees so in this video first of
all I'm going to explain to you why the
boiling point of water changes as you go
up in altitude and then we're going to
be a bit critical about the experiment
great in principle could have been
better in detail and then we're going to
look and see what happens with water
when it boils and a really low pressure
and also what happens the boiling point
is you heat it up under high pressure
here we have liquid water and in the
liquid water and the reason why it's
liquid and not a solid is because the
molecules are moving around in the water
and some of the molecules they're near
the surface can come out and so what
this does is to create above the water
some gaseous water water vapor their
higher the temperature the faster the
molecules are moving around and so more
that come out so as you heat water out
you get more and more gaseous water
above it
if you have a closed vessel the water
molecules will go from the gas back into
the liquid and you get an equilibrium
between them and the pressure of the
water vapor is called the vapor pressure
as we heat the water the vapor pressure
increases but as you all know when you
get to a certain temperature the water
starts boiling and steam comes out and
boiling occurs when the vapor pressure
of the water is equal to the pressure of
the air the atmospheric pressure
pressing down because once the vapor
pressure is higher the water can just
escape so what happens before the
boiling point is that the pressure of
the air pushing down effectively stops
the water vapor escaping the air becomes
saturated with water vapor but anymore
will form droplets and go back in again
but as soon as the vapor pressure
exceeds the water just pushes the
atmosphere out of the way it takes
energy to push the atmosphere out of the
way so you have to keep heating the
water but it's temperature doesn't
change just more and more disappears as
you go up into mountains the pressure of
the atmosphere gets less you can
understand this quite easily because you
have the planet Earth which is
surrounded by a layer of air and as you
go towards space eventually there is no
air at all and as you go up the air gets
thinner and thinner and the weight of
the air pressing down gets less and less
and less and so as you go up walking
towards base camp if you're not very fit
you will get shorter and shorter of
breath because there's less air for you
to breathe
and unless you're super fit if you
wanted to climb Everest you would use a
cylinder of oxygen because there's just
not enough air to stop you blanking out
because the air is getting thinner it's
pressure of the gas is less so it is
pressing down less on the water so you
don't have to heat as much till it boils
in effect what Brady is doing is using
the boiling point of water as a way of
showing that the atmospheric pressure is
getting less there's no argument about
the effect up in Everest base camp it's
obvious if you understand the physics or
physical chemistry that the water will
boil at a lower temperature now the
problem is exactly how much lower the
boiling point will be I've got the
Katmandu thermometer and our lab
thermocouple and at present what's not
reading the 10 event the temperature in
the kettle is at 12 12 degrees C and so
on
the clever couple was reading 14 the
first problem is that Brady's
thermometer is slightly dodgy and
particularly worrying is if you watch
his video they measure the temperature
of a frozen lake which should read
naught degrees centigrade and they get a
temperature of minus 4 isn't exactly
minus 4 now this could be because the
thermometer was not very good or it
could be that the water in the lake
wasn't very clean
it's now reading 110 and the
thermocouple was reading 100 point one
so there's a difference
currently oh now it's dropping right
down there's a difference of nine
degrees but there let's look at the
kettle it's a bit dirty inside so when
they boil the water it may not have been
absolutely pure which could met the
boiling point was a bit higher than it
should have been also not very good for
drinking but it seems to be no K for
drinking because Brady is here back
safely but the second thing is that once
it's boiling they have to stick in the
thermometer and read it and when it was
cold and windy they may not have always
stepped that the monitoring at the same
depth so the readings may not have been
completely consistent but having said
all of that if I'd been doing the
experiment and it had been cold and
windy I probably wouldn't have done it
any better if you put water under a
vacuum that is you start pumping out the
air above the water nothing much will
happen until the vapor pressure of the
water is equal to the pressure inside
your vessel and then it'll start boiling
at room temperature and in fact if
you're lucky because when water boils it
takes heat out of the water you can
actually start freezing the water solid
just by pumping on it
some of my students are looking at
reactions in water at higher
temperatures above the boiling wall
this is rather like doing reactions in a
pressure cooker which people use for
cooking food very quickly but we're
going to much higher pressures and our
equipment will go up to pressures of
several hundred atmospheres which allows
us to take water right up to temperature
of 374 degrees centigrade without
boiling so you might ask why 374 and how
I can be so certain that it's precisely
374 and the reason is that as you heat
the water up the liquid water expands
and the water vapor because it's at
higher pressure is denser so the liquid
becomes less dense the vapor becomes
denser and at 374 degrees the density of
the gas and the liquid become the same
and so above that temperature all you
can have is highly compressed steam and
you cannot have liquid water this is
called the critical point and above that
temperature water is so called
supercritical
I think it is three hundred and seventy
four point one or two but I can't
remember but I have a car number plate
on which the first letter is W for water
and the number is three seven four and
the reason was that the I bought the car
secondhand and when I saw it I thought
critical temperature water
I must buy that car is that your car yes
and I've had this car now for thirteen
years
it's getting very old but I'm sort of
superstitious I don't want to sell it
perhaps my water research when flourish