John and everyone, could you elucidate on boiling in
vacuum? Is the definition of boiling the change from a liquid
state to vapour? Is temperature change only an incidental due
to energy produced by change in form in this application ?
G’day Kat; the scientific definition of boiling goes something
Boiling is a process whereby a liquid is changed to a vapour
with consequent absorption of heat. Thus, a liquid (at
whatever temperature) which is being heated will absorb heat from
the heat source at a constant rate with no apparent change in
appearance, although the temperature of the liquid will rise.
When the liquid reaches a certain temperature which is it’s
boiling point at any given pressure, bubbles of vapour will be
seen, and the liquid will evaporate but the temperature will not
rise further. So, at normal temperature, 20C and standard
pressure 760mm of mercury pure water =
will boil at 100C, ethanol at 78C, ether at 34C, and so on.
Reduce the pressure and if low enough, water will boil at 20C.
Which is why you don’t get good tea up a tall mountain unless you
use a pressure cooker to boil the water - and as soon as you took
the lid off you’d get boiling water all over the place. A bit
dodgy, eh? (Tried it with your car radiator which is under
Is temperature change only an incidental due to energy
produced by change in form in this application ?
When a liquid changes to a vapour it requires a large input of
heat to do so; “The latent heat of vaporisation”, and it will
give out that same amount of heat as it returns to liquid form
(water to steam at 100C and steam to water.)
When a solid changes to a liquid it absorbs a large quantity of
heat, and when the liquid changes to a solid it gives out that
same heat. “The latent heat of crystallisation” (ice to water and
water to ice at 0C) But the solid/liquid phase change in this
case requires large changes in pressure to become apparent.
If temperature is graphed against time when a liquid is heated
or cooled, the temperature change will be constant and
increase/fall steadily. Until a phase change takes place, when
the temperature will suddenly plateau.
Cor, blimey, gov, wot will my tech union say about me now? But
here endeth the lesson. Cheers
/ / John Burgess,
/ //\ @John_Burgess2
/ / \ \
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