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Boiling Water and Pressure
Name: Greg
Status: other
Grade: other
Location: NY
Country: USA
Date: March 2009
Question:
Hi; I understand that water boils at different
temperatures at different air pressures. Does the boiling point
also change with water depth? For example, if the air pressure is
746 mm Hg, it looks like boiling point would be about 99.47 degrees
C. If I have a container with 8 inches or so of water in it, as far
as I can tell the water would add about 15 mm Hg pressure at the
bottom. So, does the water at the bottom boil at something like
100.04 C while the water at the top boils at 99.47 C? Or is there
just one "boiling point"? If I have a quick responding (and precise)
thermometer would I expect to see one temperature (in a container of
boiling water)? Or a fluctuating temperature (because of the water
circulation between a gradient of temperatures between the top and
bottom)?
Perhaps a bit more detail will help.
I am probably getting a little too far ahead of myself, but I was basically trying to
understand what really is happening in an open container of water being boiled. (As
I have discovered, when looking closely/precisely at how things work, things are
usually
much more complicated they I would have thought.)
For example, if I stick a fast reacting precision digital thermometer in an open
container of boiling water, is all the water going to be one temperature (the
'boiling point'), or is there going to be
some small temperature variation (say 0.2 C) above and below an average boiling
temperature?
Or, small variations with the boiling point being the absolute upper limit, etc?
The water is losing heat from the exposed top, and through the sides of a container
(being heated from the bottom) and convecting. So, it seemed to me that some small
variability of temperature
(in all that turbulence) might be observed if water from these areas was being cooled
and then
churned back into the main body of water.
Thinking about this, I began to wonder if all the water in a given container actually
has
one single boiling point to begin with. As I understand it (perhaps incorrectly),
the boiling point of water is affected by the
atmospheric pressure, and this is related to the vapor pressure of the water at a
given
temperature working against the atmospheric pressure. When the vapor pressure
exceeds
the atmospheric pressure, the water will boil.
There are formula that I found for predicting the boiling point based on the current
atmospheric pressure.
But, since the water exerts a pressure of its own (increasing with depth), my question
was
what, if any, effect does this have on 'the boiling point'. Does the pressure of the
water
at a given depth add to the atmospheric pressure (working against the vapor pressure)
to increase the boiling point?
Does water at the bottom of a 1 inch deep container boil at a different temperature
than water
at the bottom of a 3 foot deep vat (at the same atmospheric conditions)? (If so, I
thought perhaps I
could look up the predicted boiling point by adding the atmospheric pressure to the
additional water pressure at a given depth).
And, if the pressure of the water is relevant, and the water pressure increases with
the depth, then does this create different 'boiling points' at different depths in an
open container?
And if that was the case, what is the practical result of having these different
boiling points at different depths (with the water being churned by the boiling
action
and presumably changing depth between the different pressure areas)?
As I said, I am probably getting a bit too far ahead of myself, since the later
questions
arise from assumptions I do not know the answer to. Sorry to be confusing.
Replies:
Greg,
This is very astute reasoning.
One of the definitions of boiling point is - the temperature at which the pressure
of the particles entering the gas phase from the liquid phase is equal to the pressure
exerted by the fluid on the liquid phase. Since it is not relevant what is exerting a
pressure on the liquid phase (it could be the atmosphere, a different liquid, the
liquid itself) then the temperature needed for the liquid to form an equilibrium
between the gas and liquid phase should be higher at the bottom of a liquid column.
As far as measuring this temperature difference and accounting for convections and
variations in temperature and so forth - remember that another definition of boiling
point is - the temperature at which the gas phase is in equilibrium with the liquid
phase. This is a steady state condition, there is no net change, the amount of
particles entering the gas phase is the same as the amount entering the liquid
phase, and should not involve kinetic effects or factors that vary over time.
To prove that the column of liquid has an effect on the boiling point of the liquid,
we need only to look at deep sea steam vents, measure the temperature of the gaseous
and liquid water to realize that indeed the water here boils at a much higher
temperature.
We can then interpolate to smaller column heights.
Greg (Roberto Gregorius)
Let us go to nature, in fact to Yellowstone N. P. and Old Faithful geyser, for what
I hope will be an answer to all your questions.
Water in O. F. is heated from the rocks around and below it, yet it erupts only
occasionally. This is because, at depth, the water pressure prevents water from
turning to a gas at its normal boiling temperature. The waters at depth continue
to heat up to temperatures past their normal boiling temperature.
These superheated waters heat water nearer the surface until some of the near surface
water boils. (This surface boiling is seen as a small overflow of water from the
basin just before the big eruption.) The hydrostatic pressure below this boiling
water (which has a lot of gas [steam] mixed in) drops allowing that deeper water to
boil which in turn reduces the pressure on even deeper waters.
This pressure reduction cascades down the water column as more and more of the water
column goes gaseous. (Note that the eruption takes time to build up to maximum?). At
the height of the eruption pretty much the entire water column is taking part and
shoots out the top of the geyser.
So . . . water pressure does affect boiling point and there can be a temperature
gradient in a fluid that is boiling.
Hope this helps.
Bob Avakian
OSU Institute of Technology
Okmulgee, OK
Hi Greg,
These are great questions. There are a few things I suggest for you to
keep in mind in answering this question.
First is convection, which is movement of mass (in this case) water.
When you have different temperatures of water in the same vessel, the
water will begin to mix (by itself). The hotter water will tend to
rise to the top, while the colder water will sink. This is known as
autoconvection ('auto' because it does it itself -- you can cause
'regular' convection with a spoon too). So when you apply heat to the
surface of a container (such as a flame to a pot), you will get some
areas of the water that are warmer than others, and thus you will get
autoconvection. There are many factors that affect how much
autoconvection you get, such as viscosity and thermal conductivity. If
you have a very viscous (or, say, a solid) material and very good
thermal conductivity, the heat would conduct faster than the material
would move -- and thus you would have very little autoconvection.
Conversely, if you have low viscosity and low thermal conductivity,
you would get lots of autoconvection because it's easier for the
material to move around rather than for heat to be conducted.
Next is nucleation. If you look at boiling at the molecular level, you
would see atoms (or molecules) constantly changing configurations
relative to each other. In the case of a phase change, such as boiling
or freezing, you get little clusters of molecules forming one phase or
the other -- in the case of boiling, you get a small 'bubble' of water
vapor forming. This pre-bubble is known as a 'nucleus' -- not to be
confused with the nucleus of an atom. Once a nucleus forms, because
the water is hot, it prefers to be a vapor, and a liquid water
molecule next to the bubble might 'join' the bubble. So you have
nucleii forming and then growing. There are thermodynamic explanations
for how and why this happens -- let me know if you want more detail on
nucleation. In any event, after a bubble nucleates, it grows, and then
gets big enough to float to the surface. There are lots of factors
that influence nucleation, but perhaps the most important one is that
the bubbles need a 'home' to begin. These are known as nucleation
points. Usually, a 'home' that a bubble likes is a small scratch a
little bigger than the nucleus size. In a vessel with lots of
nucleation points, bubbles will form and grow quickly. If there are
few nucleation points (a very smooth container), then bubbles won't
form as quickly. In fact, there are special drinking glasses with
laser-etched bottoms to enhance bubble formation for carbonated
beverages. In the case of boiling, a smooth surface can be very
dangerous. If you heat water (say, in a glass inside a microwave), but
the glass is smooth, it may become super-heated, which means it's
above it's boiling point, but hasn't boiled. Think of a
stretched-rubber-band that's ready to snap. If you then provide a
nucleation point (by adding a tea bag, for example), the water can
explosively and instantaneously come to a boil -- and possibly hurt
the tea-maker.
Last, 'boiling point' is not a very exact term. It reflects an average
temperature at which phase changes occur, but does not represent any
immutable constant or property of matter. Lots of factors affect
boiling point, including pressure and nucleation.
Put these together, and you create a complex situation where boiling
may or may not occur at exactly the "boiling point", as affected by
many parameters. To address your specific question, yes, if you had a
'super-thermometer' that could instantly divine the water temperature
at different locations (without transferring heat and without mixing
and without providing a nucleation point), then yes, it would measure
a range of temperatures throughout the 'boiling' (or near-boiling)
water. The water might be a little colder here and a little warmer
there based on the parameters above (and there are more if you are
interested). I say 'super-thermometer' because the act of measuring
the temperature may also impact the water, especially if it is
superheated.
Hope this helps,
Burr Zimmerman
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