3() PHINCIFLES OF ANIMAL BIOLOGY 



called acids. Other substances which easily accept protons are called 

 bases. They do so through the formation of negative ions consisting of 

 oxygen and hydrogen (0H~), known as hydroxyl ions. Bases in solution 

 have the properties of lye, are said to be alkaline, and are recognized by 

 the blue color they confer on litmus. 



It will be observed that the characteristic positive ions (H+) of acids 

 and the characteristic negative ions (0H~) of bases together contain the 

 components of ordinary water (H2()). Now water is an exceedingly 

 stable compound. It is to be expected, therefore, that when an acid 

 and a base are brought together in a solution the above ions will promptly 

 unite to form water. This they do. But what becomes of the other 

 radicals that belong to the acid and the base? They also combine in the 

 sense that sodium ions (Na+) and chloride ions (Cl~") combine to produce 

 sodium chloride. What they produce depends on what the other 

 radicals of the acid and base were, but in any ease the product is called a 

 salt. A salt is defined as a substance which produces, or is a combination 

 of, positive and negative ions other than H+ and 0H~. 



If the acid used was hydrochloric (HCl) and the base was sodium 

 hydroxide (NaOH), the solution containing the former would contain 

 hydrogen ions (H+) and chloride ions (Cl~), while the latter in solution 

 would consist of sodium ions (Na+) and hydroxyl ions (0H~). When 

 these two solutions are mixed, the hydrogen ions (H+) and hydroxyl 

 ions (0H~) promptly unite to form water. The ions of the other two 

 kinds, Na+ and Cl~, do not actually unite, but they form a solution of 

 sodium chloride. If such a solution is dried up, crystals of sodium chlo- 

 ride having the lattice structure shown in Fig. 25 are formed. The 

 sodium chloride is a salt. 



If sulfuric acid (II2SO4), in which there are hydrogen ions (11+) and 

 sulfate ions (S04=), is mixed with potassium hydroxide (KOH). in which 

 there are potassium ions (K+) and hydroxyl ions (()H~), water is again 

 formed by the H+ and 0H~ ions. This leaves the potassium ions (K+) 

 and sulfate ions (S()4=°) to form potassium sulfate (K2SO4). The potas- 

 sium sulfate is likewise a salt. 



Salts may be obtained in other -ways than by mixing acids and bases. 

 Mixing two salts gives rise to two other diffei-ent salts. Thus, if a 

 solution of soduun chloride is mixed with a solution of potassium sulfate, 

 the combined solution contains two kinds of positive ions (Na+ and K+) 

 and two kinds of negative ions {C\- and SOr). While the ions do not 

 join in solution, it is just as correct to regard the solution as containing 

 potassium chloride (KCl) and sodium sulfate (Na2S04) as the original 

 two. 



Electrolytes. — Ions, because of their charges, are able to carry an 

 electric current when they are free to move. The sodium and chloride 



