14 PROPERTIES OF SOLUTIONS 



A salt may be defined as a compound which has been produced by the 

 union of an anion or anions of an acid with the cation or cations of a base. 

 Salts are produced when an acid and a base are brought together in a solution, 

 as a result of a chemical union between the hydrogen ion(s) of the acid and 

 the hydroxyl ion(s) of the base, forming water. This reaction is called 

 neutralization. 



Following are several examples : 



HCl + NaOH T± NaCl + H2O 

 H2SO4 + 2 KOH <=± K2SO4 + 1 H2O 

 1 HCl + Ca(0H)2 ^ CaCl2 + 1 H2O 



Since water is practically undissociated, neutralization reactions go rapidly to 

 completion. The reverse reaction, indicated in the above equations by the 

 arrows pointing to the left, is called hydrolysis. Under certain conditions 

 hydrolytic reactions may proceed at a rapid rate, although in the examples 

 presented above the speed of the reaction towards the left (hydrolj'sis) is 

 negligible compared with the speed of the reaction towards the right (neutrali- 

 zation). 



Normal Solutions. — The concentrations of acids and bases are most com- 

 monly expressed in terms of normal solutions. A normal solution of an 

 electrolyte contains in a dissolved state per liter of solution at 20° C. a weight 

 of the compound in grams equal to its molar weight divided by its hydrogen 

 equivalent. The hydrogen equivalent of a compound is defined as the number 

 of replaceable hydrogen atoms in one molecule, or the number of atoms of 

 hydrogen with which one molecule could react. Thus a normal solution of 

 an acid contains 1.008 g. of replaceable hydrogen per liter; a normal solution 

 of a base 17.008 g. of replaceable hydroxyl per liter. By this system the con- 

 centration of any acid, base, or salt can be designated as O.i N, 0.5 N, 2 N, 

 etc., as the case may be. 



The normality of an acid solution is a measure of its total acidity, i. e. 

 of its concentration in terms of replaceable hydrogen ions. Similarly the 

 normality of a base solution is a measure of its total basicity. Since 1.008 g. 

 of replaceable hydrogen represents the same number of ions as 17.008 g. of 

 replaceable hydroxyl (why?) it is evident that equal volumes of acid and base 

 solutions of equal normality will exactly neutralize each other. 



Hydrogen Ion Concentration. — The effects of acids upon chemical re- 

 actions and upon physico-chemical conditions generally in both inorganic sys- 

 tems and in living organisms are due principally to the hydrogen ions which 

 they produce when in solution. Some of the most fundamental of physiological 



