tnhrbtnhb
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Boiling water in a vacuum - Heat of vaporization correction
If I boil water in a vacuum of 20 torr, it requires practically the same amount of heat to boil it as it would at room temperature and pressure,
right? I'm pretty sure, but I thought I'd better double-check before I build my apparatus.
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benzbenz
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I don't agree with you. The amout of heat required for boiling gets smaller when pressure gets bigger. When we heat the same two water on the same
heat source at the same time, the two times are different, the time for big pressure is less than that for small pressure. hope you understand my
meaning for my poor English.
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Magpie
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Quoting from "Steam Tables"
at 760 torr: enthalpy of evap = 970.3 BTU/lb
at 20 torr: enthalpy of evap = 1052.3 BTU/lb
Sorry for the English units, but I think you can see the relative difference, which is about 8.5% higher for the water at 20 torr.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Sauron
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@magpie, you get the impression that these guys were off fishing during p-chem class?
The boiling point changes dramatically with pressure. The heat required to get over the phase change 'hump' changes little, and in fact as you can
see, increases not decreases at reduced pressure. So, the total amount of heat required is the sum of the heat required to get to the b.p. plus the
heat of vaporization. The former goes way down with dropping pressure, the latter slightly increases with reduced pressure.
It's non-intuitive but real. All that pesky hydrogen bonding no doubt.
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tnhrbtnhb
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ah, okay, thanks.
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16MillionEyes
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Boil water in a narrow point bottle. Little eruptions of water occur. Take caution though, the "eruptions" period starts getting longer and longer as
preassure builds up so that the "eruption" itself gets more violent. It's cool to watch but sort of risky.
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