All of the second- and third-row metals are found in the crust
of the Earth in the form of silicates (which we will discuss in
the next section), carbonates, oxides, and nitrates. Sodium carbonate,
is the salt of a strong base, NaOH, and a weak acid, H2CO3
Carbonic acid is weak because the carbonate ion has nearly as strong
an affinity for protons as water does. About half of the carbonic
acid remains in undissociated form in water solution, half exists
as bicarbonate ion, HCO3-,
and very little CO32-
is present (See 2).
When sodium carbonate is dissolved in water, the carbonate ions
thus released take protons away from water molecules and make bicarbonate
and hydroxide ions (See 3).
This is called hydrolysis, or "cleaving with water,"
under the original but erroneous impression that water simply takes
sodium carbonate apart, rather than the carbonate ion pulling the
water molecule apart.
A solution of sodium carbonate, or of any salt of a strong base
and weak acid, will be mildly basic.
or washing soda, is used as a mild source of hydroxide ions in the
home and in industry. It dissolves grease and oils by turning them
into soaps that can be washed away, but is not as corrosive to people
or property as sodium hydroxide is.
Sodium bicarbonate, NaHCO3, is an
even weaker base because only the second hydrolysis (See4)) can
take place and is used internally as an ant-acid (see 5). It is
also used as a raising agent in baking as even with mild acetic
acid it produces CO2 gas. (see 6)
(1) 2NaOH + H2CO3
------> 2Na+ + CO3-
+ H2O ------> HCO3-
+ OH- (almost completely)
+ H2O ------> H2CO3
+ OH- (only halfway)
(5) Na+ + HCO3-
+ H2O ------> Na+
(6) H+ + HCO3-
------> H2O + CO2(gas)