Sciencemadness Discussion Board

Bioling Point of Water at Different pressure

ssdd - 9-9-2007 at 13:32

What equation can I use to find the boiling point of water at different pressures?

-ssdd

Yes I know this is a remedial question, but I can't seem to find the answer.... :(

S.C. Wack - 9-9-2007 at 16:07

http://www.chemsoc.org/exemplarchem/entries/pkirby/exemchem/...

There are equations for correction of boiling points but these have variable constants AFAIK.

BTW when using official barometers in the US (not having a home barometer) for the pressure for bp determination, one must also correct for altitude as these are calibrated for the altitude that they are stationed at. This caused me some perplexity until I figued this out, here at 1400' with stated pressure at 29.9" but bp's strangely low. (i.e. The Denver Post says that the current pressure there is 30.22". I don't think so.)

[Edited on 9-9-2007 by S.C. Wack]

ssdd - 9-9-2007 at 17:08

Thanks

-ssdd

gsd - 9-9-2007 at 17:43

The good old Antoine's Equation for water holds good for considerable range :

Log (Ps) = 7.96681 - (1668.2/(228+T))

Where Ps is Vapour Pressure of water in mmHg
T is temperature in Deg. C.
and Log is to the base 10

gsd

[Edited on by gsd]

16MillionEyes - 9-9-2007 at 19:26

What I think would be more interesting is understand how those guys came up with those equations.

gsd - 10-9-2007 at 06:58

To the best of my understanding, Antoine's Equation is an excercise in curve fitting.

You have a basic equation : Log (Ps) = A - (B/(C+T))

This is an equation with 2 variables and 3 constants.

Now Take known vapour pressure data of any pure conpound.
Plot it on a graph paper. From sufficient number of data points find out value of A B C.

What you have is an impirical formula for vapour pressure as a function of temperature.

Here are few web-pages pertaining to Vapour Pressure and Antoine's Equation

http://www.msstate.edu/dept/Chemistry/scf2/4411/vaporP.htm

http://www.ece.umd.edu/~nsw/ench250/antoine.htm

http://imartinez.etsin.upm.es/dat1/ePv.htm


gsd

Quibbler - 12-9-2007 at 05:59

Just to add a bit of substance to these approximate equations.

Water (or anything) boils when G(liquid)=G(vapour)
[G = Gibbs free energy]
G(liquid) changes very little with presssure, but for the vapour dG=VdP=RT dP/P (ideal gas)

So fiddling around and integrating

ln(P) = -/\H/RT +/\S/R

where T is the boiling point (in kelvin) at P (in atm)

Magpie - 12-9-2007 at 14:52

S.C. Wack says:
Quote:

BTW when using official barometers in the US (not having a home barometer) for the pressure for bp determination, one must also correct for altitude as these are calibrated for the altitude that they are stationed at. This caused me some perplexity until I figued this out, here at 1400' with stated pressure at 29.9" but bp's strangely low. (i.e. The Denver Post says that the current pressure there is 30.22". I don't think so.)



This is an important observation; thanks for pointing it out. I have been mistakenly using airport values.

Researching this I find that when you call your local airport tower to get a barometric pressure what you actually get is a barometric pressure adjusted to a sea level condition. This is for use by pilots flying in that locality. By setting their altimeter to this adjusted value it will indicate the correct altitude above mean sea level.

For example, I live about 400 feet above sea level. The airport gave me 29.84"Hg. A local weather station gave me a true barometric pressure of 29.41"Hg.

[Edited on by Magpie]

S.C. Wack - 12-9-2007 at 15:58

Local weather station can mean a couple of different things...what I was trying to say is that local NWS stations give pressure adjusted for sea level...for the purpose of nice isobars instead of isobars that follow altitude contours more than anything else.

[Edited on 12-9-2007 by S.C. Wack]

Magpie - 12-9-2007 at 18:58

Yes, there is that too. I did make sure that the pressure my "weather station" was giving me was a true barometric pressure.