106 THE THEORY OF IONIZATIONWhen solutions of two ionized substances are mixed, the oppor-
tunity is thereby furnished for the formation of two new substances.
What will actually take place depends on the properties of these
two substances, as well as upon the properties of the original
substances. The various types of metathetical reactions are
classified on this basis.
Precipitation. When a solution of silver nitrate is added to a
solution of sodium chloride a voluminous, curdy, white precipitate
instantly appears and analysis of the precipitate shows it to be
silver chloride.
The mixing together of the solutions brings together the four
ions, Ag+, NO 3 ", Na+, and Cl", from which four different pairs, or
complete substances, are possible, AgNO 3 , NaCl, NaN0 3 , AgCl.
Reference to a solubility table shows that the first three of these
are very soluble, and reference to the table of ionization values
shows that each of the three is highly ionized; on the other hand,
AgCl is very insoluble. The latter salt, therefore, precipitates
until there are left in solution only the extremely few of its ions
corresponding to its solubility.
Ag+ N(V
Cl" Na+
1
AgCl IFor writing the equations of ionic reactions the set of rules
given on page 104 has been devised. Silver nitrate and sodium
chloride are each about 86 per cent ionized, according to the table.
In the equation these substances both appear as completely
ionized, the 14 per cent un-ionized portion of each which exists
before mixing the solutions being disregarded (Rule 4). As a
matter of fact, as soon as AgCl is precipitated and Ag+ and Cl"
ions are thus removed, the original 14 per cent of un-ionized
AgNO 3 and NaCl are no longer in equilibrium with the ions; fur-
ther ionization ensues, and the salts enter as completely into re-
action as if they had been 100 per cent ionized at the start.
Sodium nitrate, according to the table, is 86 per cent ionized.
Hence 14 per cent of the total amount of this salt must have passed
into the un-ionized condition. It will be noticed that this fact is
neglected in the equation. For all practical purposes the major
part of the Na+ and NO 3 " ions are left in exactly the same con-