190 SMITH'S INTERMEDIATE CHEMISTRY 



(b) Transition acids, such as phosphoric acid, hydrofluoric 

 acid. These substances are slightly ionized in 0.1 N solution. 

 Their solutions conduct fairly well, and show moderate activity. 



(c) Weak acids, such as acetic acid, carbonic acid, boric acid. 

 These substances are scarcely ionized at all in 0.1 N solution. 

 Their solutions conduct the current very poorly, and exhibit 

 little activity as acids. 



3. Bases also show very extreme differences in their degrees 

 of ionization. We have two main classes strong bases, such as 

 sodium hydroxide, barium hydroxide; and weak bases, such as 

 ammonium hydroxide. Between these fall certain transition 

 bases, such as silver hydroxide (which is only very slightly soluble, 

 but which is noticeably stronger than ammonium hydroxide) 

 and some organic derivatives of ammonium hydroxide. 



4. Water itself is both an exceedingly weak acid and an ex- 

 ceedingly weak base. It breaks up into the two ions H + and OH~ 

 to a very minute extent indeed. At ordinary temperature the 

 fraction ionized is less than 0.000,000,002. Pure water conse- 

 quently conducts the electric current practically not at all. The 

 ionization of water, however, is a factor of vital importance in 

 the explanation of certain reactions, as we shall see later. 



Ions and Vapor-Pressure, Boiling-Point and Freezing' 

 Point Abnormalities. The cause of the abnormal changes in 

 these three physical properties, exhibited by solutions of electro- 

 lytes, will now be evident. A solution of sodium chloride containing 

 one-gram molecular weight of salt in 1000 g. of water gives almost 

 twice the calculated effect in each case, because almost all of the 

 solute is present as Na + and Cl~, instead of as undissociated NaCl. 

 The number of solute molecules present is almost doubled by 

 ionization. Similarly a dilute solution of ferric chloride shows 

 nearly four limes the normal change in freezing-point lowering, 

 for example, owing to the fact that the majority of Feds mole- 

 cules are broken up into four ions, Fe +++ and 3C1~. 



