200 NON-METALS AND THEIR COMBINATIONS. 



the copper ion. The molecules of copper salts in solution are not blue, as can 

 be shown by the following experiment: If to a solution of copper chloride 

 concentrated hydrochloric acid be added, there will be a point at which the 

 blue changes to a yellowish-green color. The effect of the acid is to reverse 

 the ionization of the copper chloride so far that the color of the remaining 

 ions of copper is overbalanced by the color of the molecules of the salt. 



Again, permanganic acid in dilute solution is deeply colored, due to th e 

 MnO/ ion (hydrogen ion is colorless). Likewise all its salts in dilute solu- 

 tions are deeply colored, because they all form the common ion, MnO/. For 

 equivalent concentrations, the tint of the color is the same in all the solutions. 



Analytical reactions or tests. Since acids, bases, and salts ionize, and 

 chemical actions concern primarily the ions, it is not difficult to see that the 

 tests made in solutions (wet way) for identifying substances are tests for ions. 

 Thus, when we test for carbonic acid by lime-water we are testing, not for the 

 acid H 2 CO 3 , but for the ion CO/'. Nevertheless we infer the nature of the 

 molecules from a study of the reactions of the ions. Since the tests apply to 

 ions, we have an explanation of the fact that tests for a given ion can be used 

 in the case of all substances which give that ion in solution, irrespective of the 

 nature of the other ions. Thus, the silver nitrate test for " chlorine " succeeds 

 for all chlorides that are soluble. Likewise, the barium chloride test for all 

 soluble sulphates. It is evident also that two kinds of tests must be made in 

 the case of each substance, namely, one kind for the positive ion and the other 

 kind for the negative ion. 



Kinds of ions formed by acids. Monobasic acids, as HC1, HNO 3 , etc., 

 can form only one hydrogen ion from each molecule. Dibasic acids, like 

 H 2 SO 4 , form one or two hydrogen ions, according to concentration. In rather 

 concentrated solution of sulphuric acid the ionization is principally thus: 

 H 2 SO 4 : H- + HSO/. The ion HSO/ is an acid, but much less active than 

 sulphuric. When the acid is highly diluted, further ionization takes place to 

 a great degree, thus, HSO/ ^ H* -f SO/'. All dibasic acids dissociate in 

 two stages, like sulphuric. Tribasic acids show a similar behavior. 



Activity or "strength" of acids. A proper comparison of acids can 

 only be made under like conditions of temperature, concentration, etc. For 

 this purpose like concentration means solutions containing in a given volume 

 chemically equivalent weights of the respective acids. These are such weights 

 in grammes as contain the same weight of replaceable hydrogen. For ex- 

 ample, two molecular weights of HC1 are equivalent to one moleculer weight 

 of H 2 SO 4 . All conditions being equal, the activity of acids is proportional to 

 the degree of dissociation ; that is, to the concentration of the hydrogen ions. 

 Those acids that ionize most are the "strongest," and vice versa. 



Bases. Bases are substances which give negative hydroxyl (OH) ions in 

 solution, associated with a positive ion, which is usually metallic, but may be 

 a group of atoms not containing a metal, as NH 4 . The properties common to 

 bases in general when dissolved are due to the hydroxyl ion, for example, 

 action on litmus, soapy taste, neutralization of acids. Most of the bases are 

 so sparingly soluble that not enough (OH) ions are present to affect litmus. 

 Zinc and iron hydroxides are examples of such. In other cases, just enough 



