Page 97 - Basic physical chemistry for the atmospheric sciences

5

Ac d s and b a s e s
i

o
Very early i n the history f chemistry many substances were desig­
g
nated as acids, bases, and salts. Acids have a sour taste (e. . , citric
acid gives lemon u ice its sour taste); they dissolve certain metals ; and
j
they also dissolve carbonate minerals to produce carbon dioxide.
Bases have a bitter taste (e.g. , sodium carbonate); they feel slippery
when touched; and they react with many dissolved metal salts to form
s
precipitate . However, the most striking characteristic of bases is their
s
ability to neutralize the properties of acid ; when a base reacts with an
acid a salt is produced.
The French chemist Lavoisier thought that all acids contain oxygen
(the word oxygen means "acid former" in Greek). However, it was
subsequently found that many acids contain no oxygen (e.g. , hydro­
chloric acid, HCI), but that they all contain hydrogen.
Acids and bases figure prominently in the equilibrium of aqueous
s
solution , where they significantly enhance the electrical conductivity
of water. In this chapter, we will explore some of the important
properties of acids and bases in aqueous solutions. This will lead us to
a discussion of several theories of acids and bases.

5. 1 Some definitions and concepts
Equation (4.22) is valid for aqueous solutions as well as for pure water.
A solution for which [H + ( aq)] = [ OH - ( aq)] is said to be neutral; thu ,
s
H
pure water is neutral. I f [ + (aq)] > [ OH - ( aq)] the solution is said to
be acidic. If [OH - (aq)] > [H + (aq)] the solution is said to be basic.
Exercise 5 1 . If 0.02 mole of hydrochloric acid is dissolved in 1 L of
.
water, what are the concentratio s of H + (aq) and oH -(aq) ions in the
n
solution at 25°C?

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