I don't know how you tell if the action is actual boiling
or deairing. I would assume it would de-air before it would
Probably right, but whether it is spectacular or not would
depend upon the amount of air dissolved in the water.
As to why it de-airs at 72 but not at 68, I am not sure.
Could it be that it is boiling, but the boiling gives the
assurance that all the de-airing has already taken place?
Not necessarily. If by “boiling” in the above context you
perhaps mean bubbling, you should also bear in mind that the
appearance of bubbling would also depend upon the amount of solid
material present which would act as sites for the first tiny
bubbles to form upon. (termed ‘nucleation sites’) F’rinstance, if
you take some very pure, micro-filtered distilled water and heat
it with a scrupulously clean vessel and thermometer, you could
well see the temperature rise above 100C - and then large bubbles
of steam form with a series of small thumps. This, in science is
known as ‘bumping’ and some liquids are quite prone to this
phenomenon. Chemistry students are taught to add a few chips of
brick - called ‘bumping stones’ - making nucleation sites to
avoid this and a possible sudden boiling over of the heated
liquid. This is very common when distilling liquids under
reduced pressure. The same bubbling/bumping could occur at the
de-airing stage well before boiling temperatures are reached -
for any given external pressure. ‘Bumping’ is of course, caused
by superheating the water.
Cold water dissolves much more air - or any other gas - than
warm water, by the way - which is why, when you use one of those
home soda pop machines for making carbonated drinks, cold water
produces far more ‘fizz’ than warm would. It is also why you
might boil water to de-air it - but you’d have to keep it
stoppered to stop air dissolving as the water cooled.
Is there such a thing as the vapor pressure of dissolved
air in water?
Let’s imagine a bottle of a liquid - yes, even mercury - with
all the air or gas removed from above the surface, then sealed.
The thin gas above the liquid would be at the natural vapour
pressure of that liquid for any given temperature. Thus, the
vapour pressure of mercury would be very low, whilst the vapour
pressure of - say - acetone, would be relatively high. Remember
the glass tube filled with mercury and inverted in a bowl of
mercury to make a barometer? Well; the space above the mercury
wouldn’t be a true vacuum, but would contain lots of atoms of
mercury vapour all jostling and banging against each other -
giving a slight but real pressure. If you put a little acetone
in the tube as well as the mercury you’d see a difference in
mercury level when you inverted it - the vapour pressure of
acetone! And I’d better stop here, before I get caught up with
Brownian motion and all sorts of other erotica!!
/ / John Burgess,
/ //\ @John_Burgess2
/ / \ \
/ (___) \