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Vapor pressure -- How does water still boil at 100°C in an open pot?
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[QUOTE="Drakkith, post: 6828496, member: 272035"] How can there be a liquid-vapor equilibrium inside the liquid water if there's no vapor and no surface that liquid-gas or gas-liquid transitions can occur? This doesn't really make any sense. Vapor pressure isn't 'developed', and an equilibrium is something that is reached, not something in which you can develop something else. Instead I'd say that at boiling temperature, the vapor pressure equals the surrounding pressure. Note that this [I]does not[/I] mean that bubbles will immediately form, as the phase transition from liquid to gas itself takes energy. You can indeed have a mass of water at 100 c (or whatever its boiling point happens to be depending on the surrounding pressure) without it boiling. Also, remember that when you boil a pot of water the bubbles form at the BOTTOM of the pot, not the top. This means that the bubbles are forming not at a boundary of liquid and gas, but within the liquid water itself. The vapor pressure of the water has to equal the combined atmospheric pressure plus an additional amount equal to the pressure exerted by the water at whatever depth the bottom of the pot is. No, this is incorrect. Boiling can and does occur in a sealed container if you heat the water up fast enough or if the container is large enough. What needs to happen is that you need to reach the boiling temperature of the water before the maximum allowable pressure of the container is reached. Or, as I explain below, you seal the container after boiling begins. Note that in a pressure cooker (at least the one that I have in my kitchen) the water boils BEFORE it pressurizes. There are two valves that allows for the release of pressure located in the lid. One is a manual valve that you open when the food is done to release the pressure completely. The other valve is smaller and is designed to close when a large enough pressure differential is developed between the inside and outside of the cooker. Being small, it cannot release the water vapor from the inside of the cooker faster than the cooker can generate it by heating the water. The water starts to boil, generating large amounts of water vapor, which pressurizes the inside of the cooker until the valve close completely. The water continues to boil until the rising pressure inside pushes the boiling point high enough to stop boiling. So a pressure cooker itself is an example of boiling water in a closed system. [/QUOTE]
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Vapor pressure -- How does water still boil at 100°C in an open pot?
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