Water Boiling Point Question

February 27, 2008 7:09 PM

At what air pressure will water boil around 65 Degrees farenheight?

I'm curious to see how you figure it out too. I keep getting negative numbers :-(

posted by GregX3 to science & nature (13 comments total)

It's been a long time since I did this, but with an online calculator, I'm getting 0.001 inches mercury = ~156 degrees F, which seems to suggest that it's the lower limit. (It doesn't work with 0 or negative numbers.)

Is "it doesn't" a valid answer? (Is it definitely Fahrenheit? 65 Celsius is right near the lower limit...)

Don't trust my answer more than anyone who's taken chemistry in the past decade.
posted by fogster at 7:14 PM on February 27, 2008

Right around 2 kPa.

See Vapor Pressure of Water and Vapor Pressure.

Are you sure you are using the correct units?
posted by slavlin at 7:15 PM on February 27, 2008

About 2 kPa.

I looked it up on a steam table.
posted by mr_roboto at 7:16 PM on February 27, 2008

You're not going to be able to do this from first principles and simple thermodynamic equations: water is highly nonideal. I'm sure there's some theory out there, but engineers use empirical relationships.
posted by mr_roboto at 7:19 PM on February 27, 2008

Are you using Arden Buck?
posted by vacapinta at 7:24 PM on February 27, 2008

Is "it doesn't" a valid answer?

No. The triple point for water is at about 0.01 ºC. Above that, you can have a liquid-to-vapor transition (boiling).
posted by mr_roboto at 7:25 PM on February 27, 2008

4.58 kPa according to the Arden Buck equation. I don't know if this is actually possible to do though.
posted by rancidchickn at 7:36 PM on February 27, 2008

Thanks for the tips. I guess the online calculators don't work for some reason for low pressures?

BTW, what would be the cheapest way to achieve 2 to 3 kPa?
posted by GregX3 at 7:53 PM on February 27, 2008

Why is the Arden Buck result so far off from the Wikipedia steam tables ?
posted by GregX3 at 8:08 PM on February 27, 2008

I threw 18.3degC (65degF) in to Arden Buck as linked by vacapinta and got 21hPa (which is 2kPa) which is what the steam tables in WP give.
posted by markr at 8:24 PM on February 27, 2008

I had to do some looking up to figure out that 2 kilopascals is about 15 torr, the unit I'm used to using. That's about 2% of normal atmospheric pressure at sea level.

You can go below 15 torr using a diaphragm pump.
posted by Class Goat at 8:32 PM on February 27, 2008

Nope, way cheaper than that is a 50' hose and a (winding) staircase.

Fill the hose with water, cap the top of the hose, and carry it up a staircase. To avoid requiring umpteem miles of hose, dangle it through the center of the staircase. Near or after the third story, the inside of the hose cap will reach the partial vapor pressure of water at room temperature, and it will boil away, so that the water won't rise any further.

You can use clear tubing, obviously, if you actually want to see the boiling occur.

(Just took a wonderful class on vacuum technology techniques, and this was suggested. Credit where due. Pity I can't remember the teacher's full name: Luke ?, PhD.)
posted by IAmBroom at 8:44 PM on February 27, 2008 [2 favorites has favorites]

Caveat: the tubing would have to be stiff enough that it wouldn't simply collapse under the external air pressure.
posted by DevilsAdvocate at 5:04 AM on February 28, 2008