12 ELEMENTS OF BIOLOGY 



For example, zinc chloride, ZnCb, and the gas hydrogen are formed 

 by the reaction between hydrochloric acid, FlCl, and metallic zinc, 

 Zn. The reaction is written: 



i HCl + Zn ;=± ZnCLo + H2 



Zinc (valence 2) combines with two chlorine atoms and hydrogen 

 with but one; therefore two molecules of HCl are necessary in the 

 formation of each molecule of ZnCb. Theoretically, all chemical re- 

 actions are reversible; hence in the equation the equals sign is 

 modified to show this reversibility. Many organic molecules are 

 transformed into difiFerent substances by the addition of a molecule 

 of water. Thus in the human digestive system cane sugar (sucrose) 

 is converted into dextrose and fructose. 



Sucrose Dextrose Fructose 



+ HoO ^-7^,-7^ + 



C12H22O11 " CeHiiOe C6H12O6 



A reaction in which water is involved in this fashion is known as 



HYDROLYSIS. 



Acids and Bases. Inorganic molecules that contain one or 

 more atoms of hydrogen which ionize in water, and some organic 

 molecules that behave in the same fashion are termed acids. Acids 

 are highly reactive, their reactivity being due to the presence of 

 ionized hydrogen and proportional to the extent to which they 

 ionize, in other words, to the number of hydrogen ions. Both 

 inorganic and organic molecules that contain OH radicals which 

 ionize in water are termed bases or alkalis. The OH radical is 

 also highly reactive and whether a substance containing OH radi- 

 cals behaves as a weak or a strong alkali depends on its degree of 

 ionization. Ionized hydrogen atoms, it will be remembered, are 

 positively charged while ionized OH radicals are negatively charged. 

 Thus when both are present in a solution in equal numbers they 

 neutralize each other; the solution is then described as neutral and 

 reacts neither as an alkali nor as an acid. Water is such a solution, 



