242 THE REALITIES OF MODERN SCIENCE 



the liquid would grow at the expense of the other. 

 Now this is exactly what does happen if the temperature 

 is altered. Thus you notice that as the temperature 

 falls below that of the triple point the pressure of the 

 supercooled liquid, shown by the dotted extension of 

 the curve B, is higher than that of the solid. Molecules 

 leaving the surface of the water as vapor then serve to 

 supersaturate the vapor above the solid and thus con- 

 dense on it. In other words some of the water freezes. 

 Conversely it may be seen that the curve A is steeper 

 at than is curve 1 B, so that if the temperature rises 

 above the freezing point the solid loses molecules to 



its vapor more rapidly 

 than it gains from it 

 and there is thus an ex- 

 cess of molecules which 

 condense as a liquid. 

 Melting thus occurs. 



If a substance is dis- 

 solved in the liquid, 

 for which we have the 



curves in Fig. 31, its 



vapor pressure is reduced according to equation (1). 

 The vapor pressure curve B' corresponding to a solu- 

 tion is shown dotted. The intersection of B', with A 



1 Curves like A and B represent equilibrium conditions. Thus for 

 any temperature greater than freezing there is one and only one 

 pressure at which the liquid and its vapor may coexist in equilibrium, 

 and that is the pressure corresponding to this temperature on the 

 curve. We may use the term "reaction" to describe these actions 

 and indicate by arrows the direction in which a reaction occurs. 

 Thus for water we have H 2 (liquid)^ H 2 O (vapor). The double 

 arrows are made similar to indicate that this reversible reaction is 

 in equilibrium. 



