He measured the temperature of only 79 degrees centigrade.

Sale says, 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 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 under 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 that near the surface can come out.

And so what this does is to create above the water, some gaseous water, water vapor.

They're hired the temperature of the fast, the molecules are moving around and so Maur that come out.

So as you heat water out, you get more and more gaseous water bucket.

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 of the water starts boiling and steam comes out on.

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 putting 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 its temperature doesn't change.

Just more and more disappears this way.

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, you eventually there is no air, a tall and as you go up, the air gets thinner and thinner, and the weight of the air pressing down gets less Annelise unless and so as you go up walking towards base camp.

If you're not very fit, you will get sorted and sort 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 the 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 a CZ much till it boils.

In effect, what Brady is doing is using the boiling point of water as a way off, showing that the atmospheric pressure is getting less.

There's no argument about the effect AP 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 Kathmandu thermometer on our lab firm couple, and at present, what's that reading the 10.

The temperature in the cattle is a 12 12 degrees C and the club a couple is reading 14.

The first problem is that Brady's thermometer is slightly dodgy, and particularly worrying is if you watched his video.

They measured the temperature of a frozen lake, which should read no degrees centigrade, and they get the temperature of minus.

Fool is exactly minus.

For 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 no reading 110 on the firm.

A couple is reading 100.1.

There's a difference currently.

Oh, and I was dropping right down.

There's a difference of nine degrees, but now let's look at the kettle.

It's a bit dirty inside, so when they boil the water, it may not have bean absolutely pure, which could make the boiling point was a bit higher than it should have.

Bean also not very good for drinking, but it seems to be no que for drinking because Brady's 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 stuck the thermometer in at the same depth, so the readings may not been completely consistent.

But having said all of that, if I had been doing the experiment and its being cold and windy, I probably wouldn't have done it any better.

If you put water under a vacuum, that is, you stop 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 temp.

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 point.

This is rather like doing reactions in a pressure cooker, which people used for cooking food very quickly.

But we're going to much higher pressures, and our equipment will go up to pressures of several 100 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 could 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 a tire pressure is denser, so the liquid becomes less dense, the vapor becomes denser, and a 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 Onda.

Above that temperature water is so called super critical.

I think it is 374.1 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 374 And the reason waas that I bought the car second hand and when I saw it, I thought critical temperature of water.

I must buy that car.

Yes, on I've had this car now for 13 years is getting very old.

But I'm sort of superstitious.

I don't want to sell it.