24 GENERAL BIOCHEMISTRY 



pressure increases more rapidly than temperature, yielding the ob- 

 served curve. Figure 2-1 accounts for the curvature. In a typical situa- 

 tion, a temperature rise shifts the entire curve for the distribution of 

 velocities. Since this plot is itself a curve, a small temperature change 

 greatly affects the fraction of molecules having velocities in excess of 

 the critical escape value. Thus the vapor pressure increases as this 

 fraction, and a curve like that of Figure 2-2 results. 



When the temperature is increased until the vapor pressure just 

 equals the external pressure, bubbles of vapor form in and escape from 

 the body of the liquid. This temperature is called the boiling point. 

 Clearly the boiling point depends upon the external pressure, which 

 must be matched by the vapor pressure. In the example of Figure 2-2, 

 the vapor pressure of water equals the normal atmospheric pressure at 

 100°C., and water boils at this temperature under standard sea level 

 pressure conditions. If the pressure is reduced by ascending to a high 

 altitude or by means of a vacuum pump, the boiling point is cor- 

 respondingly reduced. 



Liquids are often boiled and the vapor then brought into contact 

 with a surface cooled below the boiling point. The vapor molecules 

 are thus cooled, slowed down, and condensed to form liquid again. 

 This process is called distillation and is widely employed in separating 

 materials having different boiling points. Artificial reduction of the 

 external pressure, followed by boiling and subsequent vapor condensa- 

 tion, is called vacuum distillation. This process is widely employed in 

 sej^arations at low temperature when the boiling temperatures at atmos- 

 pheric pressure are high enough to cause decomposition of desired 

 materials. For example, the distillation of water at 100°C. in the manu- 

 facture of condensed milk leads to undesirable changes in flavor and 

 nutritional components in the product. When the distillation occurs 

 at low pressure and therefore at low temperature, a much more accept- 

 able condensed milk is obtained. 



When two liquids are mixed, each exhibits a vapor pressure depend- 

 ing upon the natures of the individual liquids, their relative quan- 

 tities, the temperature, and the attractive forces between the molecules 

 of the two liquids. This last factor is important and produces striking 

 effects. When the unlike molecules attract each other very strongly, 

 their escaping tendencies are lower than expected. When these attrac- 

 tive forces are unusually low, the vapor pressures are higher than 

 anticipated. Such liquid systems boil when the combined vapor pres- 

 sures equal the external pressure. Thus the anomalous intermolecular 

 forces lead first to anomalotis vapor pressures and finally to anomalous 

 boiling points. 



