2 PHYSICAL CHEMISTRY 



cannot be obtained or interpreted without the aid of the methods 

 of physical chemistry. Quantitative chemistry is based on the 

 molecular and atomic theories, and since molecules and atoms 

 obey physical laws, physical chemistry is necessary in order to 

 understand the reactions that take place between them. Though 

 the problems considered in this book are physiological, the 

 methods of attack are chiefly those of the physical chemist. 



As an illustration of the usefulness of physical chemistry we 

 may consider the grouping of the elements into the periodic sys- 

 tem. Fig. i shows the elements grouped according to their 

 physical characters. It may be seen that Li, Na, K, Rb and Cs 

 have corresponding positions in the curves. They also have sim- 

 ilar effects physiologically. Ca, Sr and Ba form another series, 

 whereas Mg stands somewhat apart physiologically as well as 

 physically, and the same is probably true of Be (near B). F, CI, 

 Br and I form another series (F, CI and Br are indicated by 

 spaces homologous with I). 



It is sometimes supposed that very exact data are not necessary 

 in biochemistry owing to the errors in taking samples and the 

 individual variation of the organisms. We might be interested, 

 for instance, in determining whether dextrose were burned in 

 an animal, but consider the exact rate of oxidation to be of sec- 

 ondary importance. In reality, all qualitative distinctions are 

 based on quantitative differences. When we say that coal does 

 not burn in air at room temperature we mean that the combustion 

 is very slow. Furthermore, the rate of a chemical reaction may 

 determine the possibility of finally detecting the end products. 



All chemical reactions are theoretically reversible. That is to 

 say, not only does the observed reaction take place, but the op- 

 posite reaction is taking place at the same time at a slower rate. 

 The most rapid reactions occur between electrically charged 

 atoms or complexes called ions. Positive ions, called cations, 

 are attracted to and combine with negatively charged ions, anions. 

 If we mix an acid (containing H ions) with a base (containing 

 OH ions) the H* and OH' combine to form H 2 0. But the re- 

 verse reaction also takes place. Water dissociates into H and 

 OH ions, but their number is so small that in all ordinary chem- 

 ical work they are never detected. The reason for this lies in 

 the difference in speed of the two reactions. The combination 



