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Old 01-16-2013, 01:12 PM   #20
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Join Date: Oct 2005
Posts: 34,195

Due to evaporation, there is always some water vapor in the upper half and some kind of equilibrium between the vapor and the liquid due to dynamic exchange of molecules between the 2 phases. If a lid was put, true dynamic equilibrium will settle in, and there will be a vapor pressure above the liquid phase. In that case, half empty is also like full.
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