[TheForge] Vapor/Gas (Was: Building a propane forge)

Thu Feb 19 16:40:01 2004

This sounds like a simple confusion.

When you vaporize a liquid to a gas, the gas occupies more volume than
the corresponding liquid.  This ratio can be calculated to an excellent
first approximation, and is mainly dependent upon the molecular weight
of the molecule in question, and the boiling point.

However, boiling point is pressure dependent, so the ratio is not an
absolute.  We would typically think in terms of 1 "atmosphere" pressure
= 14.7 psia= 760 mm Hg.

None of this has anything to due with a gas expanding to fill available
space.  The only way there will be more space is either to reduce
pressure or to mix in another gas, like air.

So the ratio referred to above is the volume of an "ideal" balloon (a
polyethylene bag woudl be a better model of such a thing than a toy
balloon) that is first filled with liquid then heated to the boiling
point of the liquid, all under a pressure of 1 atmosphere.  Now rupture
the bag of gas and the molecules will float away - expanding to fill
available space.  Alternatively, put the bag into a tank and start
pulling air from the tank, and the bag will get bigger as the gas
expands under the reduced pressure.

>>> [email protected] 2/19/2004 3:12:40 PM >>>

On the other hand steam only expands to 1000 times its original volume
as 
water, and propane only expands 270 times its original volume as a
liquid.
A Gas by definition will expand to fill what ever space is available. 
Something is still not right here. Once again we come back to why
didn't they just 
say gas? My head is aching.
Mikey

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