108 THE THEORY OF IONIZATION- Cd(NO 3 ) 2 + Na 2 S.
- CaCl 2 + Na 2 SO 4.
- CaSO 4 (solid) + Na 2 CO 3.
Neutralization. Acids and bases have the ability to mutually
neutralize the distinctive properties of each other.
An acid tastes sour, it turns blue litmus red, and it imparts dis-
tinctive colors to a number of other organic substances which may
be used in the same way as litmus; it reacts with active metals,
hydrogen being evolved and salts of the metal being left; it
reacts with calcium carbonate with an effervescence due to the
escape of carbon dioxide. These properties of an acid are all lost
when the acid has reacted with an equivalent quantity of a base.
A base tastes alkaline, that is, like lime water; it turns red
litmus blue, as well as imparting distinctive colors to the other
organic substances which may be used in a similar manner; it
causes a slippery feeling if a drop of its solution is rubbed between
the finger tips. These properties of a base are all lost when the
base has reacted with an equivalent quantity of an acid.
When the neutralization has been very carefully carried out, so
that exactly equivalent quantities of acid and base have been used,
the resulting solution shows none of the characteristic properties
or either acid or base. It still conducts electricity strongly, show-
ing that it contains ions; if it is evaporated a solid salt is left.
Since we know that all acids yield hydrogen ions when dissolved,
although the negative ions may be of most divergent kinds, it is
obvious that the distinctive properties of acid solutions must be the
properties of hydrogen ions. Likewise it is obvious that the dis-
tinctive properties of solutions of bases must be the properties of
hydroxyl ions.
Since we may see in the table of ionization values that pure
water contains but 0.000,000,1 mole of H+ ions and 0.000,000,1
mole of OH~ ions per liter, we may know in advance that when an
acid and a base are mixed the H+ and OH" ions cannot remain in
the presence of each other, but must unite according to the reaction
H+ + OH" -» H 2 O
until only a number corresponding to the exceedingly small con-
centration just stated is left.
A salt may be defined as a compound consisting of the positive
radical of a base and the negative radical of an acid. Hence the