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From: Chris W on 6 May 2010 01:06


Uncle Al wrote:
> Chris W wrote:
>> suppose I have a sealed container of CO2 at room temperature, where
>> approximately half of the volume of the container has liquid CO2.
>
> Carbon doxide critical temperature is 31.04�C. If you want a
> mensicus, take care it is not an especially warm room, like 88�F or
> higher. Don't shove it up yer butt.

Since CO2 cylinders are often used for paint ball guns and other things,
and you don't hear about them blowing up, I guess those cylinders have
some other substance in there with the CO2 that changes the critical
temperature?


Chris W


>
>> Is
>> the pressure inside this container going to always be equal to the vapor
>> pressure of CO2 at whatever temperature the container and it's contents
>> are currently at?
>
> If CO2 is the only substance in there, the internal pressure (chemical
> activity) has no other sources. What does "vapor pressure" mean above
> the critical temperature where there is only one phase?
>
From: tadchem on 6 May 2010 19:41
On May 6, 1:06 am, Chris W <1qaz...(a)cox.net> wrote:
> Uncle Al wrote:
> > Chris W wrote:
> >> suppose I have a sealed container of CO2 at room temperature, where
> >> approximately half of the volume of the container has liquid CO2.
>
> > Carbon doxide critical temperature is 31.04°C.  If you want a
> > mensicus, take care it is not an especially warm room, like 88°F or
> > higher.  Don't shove it up yer butt.
>
> Since CO2 cylinders are often used for paint ball guns and other things,
> and you don't hear about them blowing up,  I guess those cylinders have
> some other substance in there with the CO2 that changes the critical
> temperature?

You guessed wrong. When two fluid phases of one suubstance are
present, one is liquid and one is vapor, and the pressure equals the
equilibrium vapor pressure of the substance at the ambient
temperature.

In this case the state of the system lies somewhere along the liquid-
vapor line, such as the black line here (to the left of the black dot)
http://upload.wikimedia.org/wikipedia/commons/1/13/Carbon_dioxide_pressure-temperature_phase_diagram.svg

If you draw off some vapor, then the remaining liquid will boil until
the pressure returns to the equilibrium vapor pressure. This is
called a feedback mechanism, and applies to everything from a droplet
to an ocean, of any liquid.

Note that the boiling will require heat, which the liquid will provide
by cooling down as it boils.

Tom Davidson
Richmond, VA
From: Chris W on 6 May 2010 23:17
tadchem wrote:
> On May 6, 1:06 am, Chris W <1qaz...(a)cox.net> wrote:
>> Uncle Al wrote:
>>> Chris W wrote:
>>>> suppose I have a sealed container of CO2 at room temperature, where
>>>> approximately half of the volume of the container has liquid CO2.
>>> Carbon doxide critical temperature is 31.04�C. If you want a
>>> mensicus, take care it is not an especially warm room, like 88�F or
>>> higher. Don't shove it up yer butt.
>> Since CO2 cylinders are often used for paint ball guns and other things,
>> and you don't hear about them blowing up, I guess those cylinders have
>> some other substance in there with the CO2 that changes the critical
>> temperature?
>
> You guessed wrong. When two fluid phases of one suubstance are
> present, one is liquid and one is vapor, and the pressure equals the
> equilibrium vapor pressure of the substance at the ambient
> temperature.
>
> In this case the state of the system lies somewhere along the liquid-
> vapor line, such as the black line here (to the left of the black dot)
> http://upload.wikimedia.org/wikipedia/commons/1/13/Carbon_dioxide_pressure-temperature_phase_diagram.svg
>
> If you draw off some vapor, then the remaining liquid will boil until
> the pressure returns to the equilibrium vapor pressure. This is
> called a feedback mechanism, and applies to everything from a droplet
> to an ocean, of any liquid.
>
> Note that the boiling will require heat, which the liquid will provide
> by cooling down as it boils.
>
> Tom Davidson
> Richmond, VA


So what happens when it gets above 31.04C?

Chris W
From: tadchem on 7 May 2010 18:32
On May 6, 11:17 pm, Chris W <1qaz...(a)cox.net> wrote:
> tadchem wrote:
> > On May 6, 1:06 am, Chris W <1qaz...(a)cox.net> wrote:
> >> Uncle Al wrote:
> >>> Chris W wrote:
> >>>> suppose I have a sealed container of CO2 at room temperature, where
> >>>> approximately half of the volume of the container has liquid CO2.
> >>> Carbon doxide critical temperature is 31.04°C.  If you want a
> >>> mensicus, take care it is not an especially warm room, like 88°F or
> >>> higher.  Don't shove it up yer butt.
> >> Since CO2 cylinders are often used for paint ball guns and other things,
> >> and you don't hear about them blowing up,  I guess those cylinders have
> >> some other substance in there with the CO2 that changes the critical
> >> temperature?
>
> > You guessed wrong.  When two fluid phases of one suubstance are
> > present, one is liquid and one is vapor, and the pressure equals the
> > equilibrium vapor pressure of the substance at the ambient
> > temperature.
>
> > In this case the state of the system lies somewhere along the liquid-
> > vapor line, such as the black line here (to the left of the black dot)
> >http://upload.wikimedia.org/wikipedia/commons/1/13/Carbon_dioxide_pre...
>
> > If you draw off some vapor, then the remaining liquid will boil until
> > the pressure returns to the equilibrium vapor pressure.  This is
> > called a feedback mechanism, and applies to everything from a droplet
> > to an ocean, of any liquid.
>
> > Note that the boiling will require heat, which the liquid will provide
> > by cooling down as it boils.
>
> > Tom Davidson
> > Richmond, VA
>
> So what happens when it gets above 31.04C?
>
> Chris W- Hide quoted text -
>
> - Show quoted text -

Above the critical temperature a vapor in equilibrium with its liquid
becomes a "supercritical fluid" - essentially a dense gas.

http://en.wikipedia.org/wiki/Supercritical_fluid

Supercritical CO2 is routinely used in laboratory and some industrial
processes.

http://en.wikipedia.org/wiki/Supercritical_carbon_dioxide

Tom Davidson
Richmond, VA
From: glen herrmannsfeldt on 17 May 2010 14:55
Chris W <1qazse4(a)cox.net> wrote:
> suppose I have a sealed container of CO2 at room temperature, where
> approximately half of the volume of the container has liquid CO2. Is
> the pressure inside this container going to always be equal to the vapor
> pressure of CO2 at whatever temperature the container and it's contents
> are currently at?

The pressure will be (less than or equal to) the vapor pressure
at the coldest part of the container. The less than is when
there is no liquid left. That includes large containers that
might not all be at one temperature.

Easier to see is the vapor presure of water in air on a cold day.
As above, the coldest part of a house is usually the windows, and
that is where excess water will condense out, such that the vapor
pressure of water in the house is that of the cold windows.

-- glen
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