Human Physiology, 14th edition (2016)

Chemical Composition of the Body 29

Figure 2.7 Hydrogen bonds between water
molecules. The oxygen atoms of water molecules are weakly
joined together by the attraction of the negatively charged
oxygen for the positively charged hydrogen. These weak bonds
are called hydrogen bonds.

O

H

+ +

• – Water molecule

H

H

O

O

H

+

Hydrogen bonds

• +H

H

H

O H

+

H

H+

• – +

ionize, so the concentrations of H^1 and OH^2 are each equal

to only 10^2 7 molar (the term molar is a unit of concentra-

tion, described in chapter 6; for hydrogen, 1 molar equals

1 gram per liter). A solution with 10^2 7 molar hydrogen ion,

which is produced by the ionization of water molecules in

which the H^1 and OH^2 concentrations are equal, is said to

be neutral.

A solution that has a higher H^1 concentration than that

of water is called acidic; one with a lower H^1 concentration

is called basic, or alkaline. An acid is defined as a molecule

that can release protons (H^1 ) into a solution; it is a “proton

donor.” A base can be a molecule such as ammonia (NH 3 ) that

can combine with H^1 (to form NH 4 1 , ammonium ion). More

commonly, it is a molecule such as NaOH that can ionize to

produce a negatively charged ion (hydroxide, OH^2 ), which, in

turn, can combine with H^1 (to form H 2 O, water). A base thus

removes H^1 from solution; it is a “proton acceptor,” thereby

lowering the H^1 concentration of the solution. Examples of

common acids and bases are shown in table 2.2.

pH

The H^1 concentration of a solution is usually indicated in pH

units on a pH scale that runs from 0 to 14. The pH value is

equal to the logarithm of 1 over the H^1 concentration:

p H 5 log^1

[H^1 ]

where [H^1 ] 5 molar H^1 concentration. This can also be

expressed as pH 52 log[H^1 ].

Pure water has a H^1 concentration of 10^2 7 molar at 25 8 C,

and thus has a pH of 7 (neutral). Because of the logarithmic

relationship, a solution with 10 times the hydrogen ion con-

centration (10^2 6 M) has a pH of 6, whereas a solution with

one-tenth the H^1 concentration (10^2 8 M) has a pH of 8. The

pH value is easier to write than the molar H^1 concentration,

but it is admittedly confusing because it is inversely related

to the H^1 concentration. That is, a solution with a higher H^1

concentration has a lower pH value, and one with a lower H^1

concentration has a higher pH value. A strong acid with a high

H^1 concentration of 10^2 2 molar, for example, has a pH of 2,

whereas a solution with only 10^2 10 molar H^1 has a pH of 10.

Acidic solutions, therefore, have a pH of less than 7 (that of

dashed or dotted lines ( fig. 2.7 ) to distinguish them from strong
covalent bonds, which are shown with solid lines.
Although each hydrogen bond is relatively weak, the sum
of their attractive forces is largely responsible for the folding
and bending of long organic molecules such as proteins and
for the holding together of the two strands of a DNA mol-
ecule (described in section 2.4). Hydrogen bonds can also
be formed between adjacent water molecules ( fig. 2.7 ). The
hydrogen bonding between water molecules is responsible
for many of the biologically important properties of water,
including its surface tension and its ability to be pulled as a
column through narrow channels in a process called capillary
action.

Acids, Bases, and the pH Scale

The bonds in water molecules joining hydrogen and oxygen
atoms together are, as previously discussed, polar covalent
bonds. Although these bonds are strong, a small proportion of
them break as the electron from the hydrogen atom is com-
pletely transferred to oxygen. When this occurs, the water mol-
ecule ionizes to form a hydroxide ion (OH^2 ) and a hydrogen
ion (H^1 ), which is simply a free proton (see fig. 2.4 ). A proton
released in this way does not remain free for long, however,
because it is attracted to the electrons of oxygen atoms in water
molecules. This forms a hydronium ion, shown by the formula
H 3 O^1. For the sake of clarity in the following discussion, how-
ever, H^1 will be used to represent the ion resulting from the
ionization of water.
Ionization of water molecules produces equal amounts
of OH^2 and H^1. Only a small proportion of water molecules

Acid Symbol Base Symbol

Hydrochloric acid HCl Sodium hydroxide NaOH

Phosphoric acid H 3 PO 4 Potassium hydroxide KOH

Nitric acid HNO 3 Calcium hydroxide Ca(OH) 2

Sulfuric acid H 2 SO 4 Ammonium hydroxide NH 4 OH

Carbonic acid H 2 CO 3

Table 2.2 | Common Acids and Bases