Saturated Vapor PressureThe process of evaporation in a closed container will proceed until there are as many molecules returning to the liquid as there are escaping. At this point the vapor is said to be saturated, and the pressure of that vapor (usually expressed in mmHg) is called the saturated vapor pressure.
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EvaporationOrdinary evaporation is a surface phenomenon - some molecules have enough kinetic energy to escape. If the container is closed, an equilibrium is reached where an equal number of molecules return to the surface. The pressure of this equilibrium is called the saturation vapor pressure.
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Evaporation vs BoilingOrdinary evaporation is a surface phenomenon - since the vapor pressure is low and since the pressure inside the liquid is equal to atmospheric pressure plus the liquid pressure, bubbles of water vapor cannot form. But at the boiling point, the saturated vapor pressure is equal to atmospheric pressure, bubbles form, and the vaporization becomes a volume phenomena. |
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Boiling Point
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Boiling Point VariationThe standard boiling point for water at 100°C is for standard atmospheric pressure, 760 mmHg. It is the experience of high altitude hikers that it takes longer to cook food at altitude because the boiling point of water is lower. On the other hand, food cooks more quickly in a pressure cooker because the boiling point is elevated. Raising or lowering the pressure by about 28 mmHg will change the boiling point by 1°C. Although the vapor pressure variation with temperature is a non-linear one, the boiling point variation can be approximated near 100°C by an empirical fit of the available data. This can provide the following estimate of the boiling point: Any of the data values above may be changed. The empirical calculation is valid only for a few degrees above and below the normal boiling point.Notes: This calculation assumes an air temperature of 20°C and an average air molecule mass of 29 amu. The atmospheric pressure drops off a little faster with height when the air is colder, but the change is not large over the range of reasonable daytime temperatures. If you put in pressures above atmospheric pressure, you get reasonable boiling point values so long as you are within a few degrees of normal, but the negative altitude values you get are not valid.
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Boiling Point Variation Near 100 CValues were taken from the saturated vapor pressure table for water near 100 degrees Celsius. An empirical fit to these data values was made, and the formula obtained is shown on the diagram. It could be considered to be reasonably valid only for a few degrees above and below 100 °C since the curve is very non-linear.
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