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.006 

 VAPOR PRESSURE - ATMOSPHERES 



.008 



Figure 10. — Phase diagram for pure water in the vicinity of the freezing point. The inter- 

 section of the three shaded areas, near 0.006 atmospheres and 32° C, is the triple point, the 

 only point at which equilibrium between water, ice, and water vapor can exist. Note that 

 although the freezing point of water is exactly 32° F. at 1.0 atmosphere pressure, decreasing 

 the pressure of the system to 0.006 atmosphere raises the freezing point to 32.013°, since 

 the melting point of ice decreases 0.013'^ for each atmosphere increase of pressure. The solid 

 line between VAPOR and LIQUID represents the vapor pressure of water; that between LIQUID 

 and SOLID the freezing point of water; that between VAPOR and SOLID the vapor pressure 

 of ice. The dashed curve below the triple point represents the vapor pressure of super-cooled 

 water, which may exist in the absence of suitable nuclei to initiate crystal formation. 

 The phase diagram for sea water has the LIQUID-SOLID boundary shifted downward 3° or 

 4° to correspond with the freezing point of sea water, but there is very little shift in the vapor 



pressure curves. 



superficial layer of the ice surface is raised above the freezing point, 

 but evaporation may occur at any temperature. Figure 10, illustrat- 

 ing the equilibrium relationships between the solid, liquid, and vapor 

 states for pure water, shows that vapor pressure, the tendency for 

 evaporation, increases rapidly w^ith increasing temperature, but that 

 there is no appreciable change in this tendency in passing from the 

 solid to the liquid state. Under still air conditions, the layer of air 



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