Science fact confession time

[peterb]

Makes me think of GCSE science classes where water boiling at 100c was taken as a constant, and pretty much taught as such. I get that some things might be too much for most GCSE students to grasp, but would it have been too much to at least mention that the boiling point can change but that for most of us it'll be at 100c? The students who couldn't grasp it can just fall back on the 100c idea whereas those who could would be able to think it through and actually learn a little something else.

Which it does at standard pressure - the all important caveat.

Boiling is the point at which the vapour pressure exceeds the ambient pressure and any additional heat input just converts the water to steam, without raising the temperature of the water further. This is how pressure cookers work. By heating the water under pressure, you get higher cooking temperatures because you raise the boiling point of the water.

A dome pressure cooker with a 15 pound weight will cook at approximately 120C.

[cptwhite_uk]

Gerrard is right, it's to do with atmospheric pressure. As sea level the pressure is greater (more pressure squeezing the H20 molecules together making it hard for them to change state from liquid to gas. As you increase altitude the pressure reduces thus less energy (heat) is required to jump to state from a liquid to a gas, therefore water boils at lower temperature at a higher altitude.

99% sure that's the main reason.

[Domestic_Ginger]

Bpt water on Everest is apparently 69°C. I think if water boiled at 37°C you may have other issues to contend with!

[Bagnaj97]

Bpt water on Everest is apparently 69°C. I think if water boiled at 37°C you may have other issues to contend with!

So after all that climbing you can't have a decent cuppa on the summit. Bah!

[Domestic_Ginger]

Instant?

They quite like black tea with salt and butter in the mountainous regions.

[Ttaskmaster]

Being someone more likely to need a cuppa while halfway up Everest, I was aware of the answer but not the reason. Some sciencey mumbo jumbo is useless to me when I have greater need to just know how to make the stove work.

[GreenPiggy]

I don't know how you didn't get taught that - I thought all Physics teachers *loved* telling the cup of tea on Everest story.

[Ttaskmaster]

I don't know how you didn't get taught that - I thought all Physics teachers *loved* telling the cup of tea on Everest story.

Ours wasn't really a physics teacher... just a lovely pair of boobs with multiple degrees in sports and mathematics, and a need for a third subject to specialise in (a requirement for all our teachers, being a fancy school). I guess she was trying for Physicist...
Besides, we were kids - None of that was interesting at the time!

Also, I understand that the boiling point may be lower due to the air pressure, but the lower temperature up there means it's also harder to get the water up to boiling as the ambient air keeps it cooler?

[TeePee]

I knew it. Actually I commented on it here: http://forums.hexus.net/kitchen-cook...-coffee-2.html

Real life application!

[Butcher]

Also, I understand that the boiling point may be lower due to the air pressure, but the lower temperature up there means it's also harder to get the water up to boiling as the ambient air keeps it cooler?

Somewhat, though an insulated cooker will alleviate that, and in practical terms it just means burning more gas.
One other potential problem in low temperatures is that butane gas canisters have lower pressure at lower temperatures so you get lower gas flow and hence a weaker flame. Adding propane to the mixture helps as it is more volatile.

[scaryjim]

Definitely GCSE science for me: it was one of the most staggering demonstrations of group ignorance I can remember. A question on the mock paper went to great pains to explain that water boiled at a lower temperature at the peak of Everest, giving the reason why. The first part of the actual question stated that distilled water boiled at 101C on the bank of the dead sea, and asked why. About 40% of the GCSE science students in the selective grammar school I went to answered that question with "because it's got lots of salt in it".

I wish I had made that up. I really do...

[jim]

I knew it because years ago I googled freeze-dried fruit to see how it was done.

The answer, IIRC, is that it's frozen, then put in a pressure chamber which changes the pressure until there's no liquid stage at all, and the solid water turns straight into a gas (i.e. it sublimates).

[Spreadie]

We covered this in physics at school - it was over 30 years ago, but I remember using a vacuum pump to reduce the pressure and watching water boil at room temperature.

[me-yeah]

its 102°C if it has salt in the water for hard boiled eggs

[mikerr]

We covered this in physics at school - it was over 30 years ago, but I remember using a vacuum pump to reduce the pressure and watching water boil at room temperature.

Yes, you can do that with a syringe - draw some water into it, put a cap over the end and quickly pull.
you'll create a low pressure area and see the water boil.

[peterb]

You can do it with a Cona coffee filter.



Wait until the coffee starts being drawn back into the lower flask, then hold he flask under cold tap. Pressure drops as the water vapour condenses, and the coffee starts boiling!