Chemicals, including those that take part in metabolism (the
cell’s energy reactions), are of two general types.
ik) compounds have carbon and hydrogen. All other
ik). Many organic mole-
cules have long chains or ring structures that can form because
of a carbon atom’s ability to form four covalent bonds.
Inorganic substances usually dissociate in water, forming
ions; thus, they are
Many organic compounds
dissolve in water, but most dissolve in organic liquids such
Many bases are present in body ﬂ
uids, but because of the
way bases react in water, the concentration of hydroxide
ions is a good estimate of the total base concentration.
The concentrations of hydrogen ions and hydroxide ions
are always in balance such that if one increases, the other
decreases, and vice versa. Solutions with more hydrogen
ions than hydroxide ions are
That is, acidic solutions
have pH values less than 7.0
. Solutions with fewer
hydrogen ions than hydroxide ions are
have pH values greater than 7.0.
summarizes the relationship between hydro-
gen ion concentration and pH. Chapter 21 (p. 819) discusses
the regulation of hydrogen ion concentrations in the internal
Many ﬂ uids in the human body function within a nar-
row pH range. Illness results when pH changes. The normal
pH of blood, for example, is 7.35 to 7.45. Blood pH of 7.5
to 7.8, called
sis), makes one feel agi-
tated and dizzy. This can be caused by breathing rapidly at
high altitudes, taking too many antacids, high fever, anxiety,
or mild to moderate vomiting that rids the body of stomach
sis), in which blood pH falls to 7.0 to
7.3, makes one feel disoriented and fatigued, and breathing
may become difF
cult. This condition can result from severe
vomiting that empties the alkaline small intestinal contents,
diabetes, brain damage, impaired breathing, and lung and
are chemicals that resist pH change. They com-
bine with hydrogen ions when these ions are in excess, or
they donate hydrogen ions when these ions are depleted.
Buffers are discussed in chapter 21 (pp. 820–823).
Describe three types of chemical reactions.
Compare the characteristics of an acid, a base, and a salt.
What does the pH scale measure?
What is a buF
Hydrogen Ion Concentrations
Grams of H
Neutral—neither acidic nor basic
The pH scale re±
ects the hydrogen ion (H
) concentration. As the concentration of H
increases, a solution becomes more acidic and
the pH value decreases. As the concentration of ions that bond with H
(such as hydroxide ions) increases, a solution becomes more basic (alkaline)
and the pH value increases. The pH values of some common substances are shown.