The Foundations of Chemistry

There are many additional examples of acidic salts. Sodium hydrogen carbonate,
NaHCO 3 , commonly called sodium bicarbonate, is classified as an acidic salt. It is,
however, the acidic salt of an extremely weak acid—carbonic acid, H 2 CO 3 —and solu-
tions of sodium bicarbonate are slightly basic, as are solutions of salts of other extremely
weak acids.
Polyhydroxy bases (bases that contain more than one OH per formula unit) react with
stoichiometric amounts of acids to form normal salts.

Al(OH) 3 (s)
3HCl(aq)88n AlCl 3 (aq) 
3H 2 O(
)

1 mole 3 moles aluminum chloride,

a normal salt

The reaction of polyhydroxy bases with less than stoichiometric amounts of acids forms
basic salts,that is, salts that contain unreacted OH groups. For example, the reaction of
aluminum hydroxide with hydrochloric acid can produce two different basic salts:

Al(OH) 3 (s)
HCl(aq) 88n Al(OH) 2 Cl(s) 
H 2 O(
)

1 mole 1 mole aluminum dihydroxide chloride,

a basic salt

Al(OH) 3 (s)
2HCl(aq)88n Al(OH)Cl 2 (s) 
2H 2 O(
)

1 mole 2 moles aluminum hydroxide dichloride,

a basic salt

Aqueous solutions of basic salts are not necessarily basic, but they can neutralize acids,
such as,

Al(OH) 2 Cl
2HCl88nAlCl 3 
2H 2 O

Most basic salts are rather insoluble in water.

THE LEWIS THEORY

In 1923, Professor G. N. Lewis (1875–1946) presented the most comprehensive of the
classic acid–base theories. The Lewis definitions follow.

An acidis any species that can accept a share in an electron pair. A baseis any

species that can make available, or “donate,” a share in an electron pair.

10-10

These basic aluminum salts are called

“aluminum chlorohydrate.” They are

components of some deodorants.

10-10 The Lewis Theory 387

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 17.11, Lewis Acids and Bases,

and Screen 17.12, Cationic Lewis Acids.

This is the same Lewis who made

many contributions to our

understanding of chemical bonding.

Sodium hydrogen carbonate, baking soda, is the

most familiar example of an acidic salt. It can

neutralize strong bases, but its aqueous solutions are

slightly basic, as the blue color of the indicator

bromthymol blue shows.