For a pure substance, is it single-phase at all points other than the flash point, where gas and liquid coexist? !
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For a pure substance, is it single-phase at all points other than the flash point, where gas and liquid coexist? ! I don’t quite understand. Take H2O for example: at 20 degrees and 1 atm, there must be a saturated vapor pressure, meaning a gas-liquid phase exists. So why is it a single phase? ! Please give some guidance. @zhxbkofkyo @arpcd
Theoretically speaking, that is indeed the case. However, if you consider this issue from the perspective of a system or framework, then at any time and for any substance, there exists a saturated vapor pressure; the gas phase in contact with it must contain a certain amount of that component.
This post was last edited by zhxbkofkyo on 2018-12-28 at 13:27. The situation you imagine is not that of a pure substance; other inert components must be taken into account to obtain the result you envision. For pure H2O, at 20°C and 1 atm, it is in the liquid phase. You can imagine using a rigid, airtight container to take water from an ambient water system, filling the container completely with water – at this point, there is only water inside, no gas phase. At 20°C and 1 atm, it’s in the same stable state as the surrounding environment; so where would a gas phase come from? From another perspective, it can also be said that the so-called saturated vapor pressure necessarily corresponds to a saturated state – a saturated state, indeed. The process you imagine is non-steady-state, not steady-state; the final state of that process is one in which all the water has evaporated, with no liquid phase left, only a gas phase
Yes, you’re right. Still confused by life’s phenomena. Thank you!