152 EFFECTS OF REACTION OF SUBSTRATE ON FUNGI 



gen-ion concentration, without mastering the underlying theories 

 and principles. 



Meaning of hydrogen-ion concentration. Substances are con- 

 ceived to be aggregates of molecules; a molecule is the smallest 

 unit of mass that possesses the characteristics of the given sub- 

 stance. A molecule is, however, conceived to be an aggregate 

 of atoms, which, if all alike, constitute an element. 



Included in atomic structure are positive electric units or 

 charges (protons) and negative units (electrons). In each ele- 

 ment protons and electrons are arranged according to a definite 

 configuration, which is concerned with the behavior of the atoms. 

 A complete atom may be deprived of one or more positively 

 charged electrons, whereupon the remaining particle is a posi- 

 tively charged ion and is designated a cation. Some atoms, how- 

 ever, are able to acquire extra electrons, thereby becoming nega- 

 tively charged ions which are designated anions. 



The behavior of acids, bases, and salts in water solutions is at- 

 tributed to the activities of their constituent ions. If, for example, 

 an acid (base or salt) is dissolved in water, its molecules become 

 dissociated to a certain amount. The radical of the molecule ac- 

 quires an electron from the acid hydrogen atom, becoming a 

 negatively charged ion and leaving the nucleus of the hydrogen 

 atom positively charged. This hydrogen ion is designated as H + 

 to distinguish it from the complete atom H; the remainder of the 

 molecule may be symbolized by A - . Dissociation of an acid may 

 therefore be formally expressed as follows: [HA] ^± [H + ] + 

 [A~ ]. This reaction is shown to be reversible, but if temperature 

 is kept constant, equilibrium will eventually become established. 



ThCn [H+] X [A-] = „ 



[HA] ai 



meaning that the product of the number of positively charged 

 ions and negatively charged ions, divided by the number of un- 

 dissociated molecules, is a constant for each given acid. It follows 

 from this concept that if the value of K is large, the numerator 

 must be large in proportion to the denominator. Such acids arc 

 strong acids. If, on the other hand, the numerator is small in 

 proportion to the denominator, the acid is a weak acid. The in- 

 tensity of reaction of an acid therefore depends upon the hydro- 

 gen-ion concentration. 



