Chapter 2
Chemistry Comes Alive
39
2
Other acids found or produced in the body include acetic acid
(HC
2
H
3
O
2
, commonly abbreviated as HAc), which is the acidic
portion of vinegar; and carbonic acid (H
2
CO
3
). Te molecular
formula for an acid is easy to recognize because the hydrogen is
written first.
Bases
Bases
have a bitter taste, feel slippery, and are
proton acceptors
that is, they take up hydrogen ions (H
1
) in detectable amounts.
Common inorganic bases include the
hydroxides
(hi-drok
9
sīds),
such as magnesium hydroxide (milk of magnesia) and sodium
hydroxide (lye). Like acids, hydroxides dissociate when
dissolved in water, but in this case
hydroxyl ions (OH
2
)
(hi-drok
9
sil) and cations are liberated. For example, ionization
of sodium hydroxide (NaOH) produces a hydroxyl ion and a
sodium ion, and the hydroxyl ion then binds to (accepts) a
proton present in the solution. Tis reaction produces water
and simultaneously reduces the acidity (hydrogen ion concen-
tration) of the solution:
NaOH
S
Na
1
1
OH
2
cation hydroxyl
ion
and then
OH
2
1
H
1
S
H
2
O
water
Te cerebrospinal fluid surrounding the brain exemplifies
water’s cushioning role.
Salts
A
salt
is an ionic compound containing cations other than
H
1
and anions other than the hydroxyl ion (OH
2
). As already
noted, when salts are dissolved in water, they dissociate into
their component ions (Figure 2.12). For example, sodium sul-
fate (Na
2
SO
4
) dissociates into two Na
1
ions and one SO
4
2
2
ion.
It dissociates easily because the ions are already formed. All that
remains is for water to overcome the attraction between the op-
positely charged ions.
All ions are
electrolytes
(e-lek
9
tro-līts), substances that conduct
an electrical current in solution. (Note that groups of atoms that bear
an overall charge, such as sulfate, are called
polyatomic ions
.)
Salts commonly found in the body include NaCl, CaCO
3
(calcium carbonate), and KCl (potassium chloride). However,
the most plentiful salts are the calcium phosphates that make
bones and teeth hard. In their ionized form, salts play vital roles
in body function. For instance, the electrolyte properties of so-
dium and potassium ions are essential for nerve impulse trans-
mission and muscle contraction. Ionic iron forms part of the
hemoglobin molecules that transport oxygen within red blood
cells, and zinc and copper ions are important to the activity
of some enzymes. Other important functions of the elements
found in body salts are summarized in ±able 2.1 on p. 26.
Homeostatic Imbalance
2.1
Maintaining proper ionic balance in our body fluids is one of
the most crucial homeostatic roles of the kidneys. When this
balance is severely disturbed, virtually nothing in the body
works. All the physiological activities listed above and thou-
sands of others are disrupted and grind to a stop.
Acids and Bases
Like salts, acids and bases are electrolytes. Tey ionize and dis-
sociate in water and can then conduct an electrical current.
Acids
Acids
have a sour taste, can react with (dissolve) many metals,
and “burn” a hole in your rug. But for our purposes the most
useful definition of an acid is a substance that releases
hydrogen
ions
(H
1
) in detectable amounts. Because a hydrogen ion is just
a hydrogen nucleus, or “naked” proton, acids are also defined as
proton donors
.
When acids dissolve in water, they release hydrogen ions
(protons) and anions. It is the concentration of protons that de-
termines the acidity of a solution. Te anions have little or no
effect on acidity. For example, hydrochloric acid (HCl), an acid
produced by stomach cells that aids digestion, dissociates into a
proton and a chloride ion:
HCl
S
H
1
1
Cl
2
proton
anion
O
Water molecule
Ions in
solution
Na
+
Cl
Cl
Na
+
H
H
Salt
crystal
δ
δ
+
δ
+
Figure 2.12
Dissociation of salt in water.
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