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Chapter 2 Chemistry, Matter, and Life 33
proton acceptors (hydroxide ions), and the pH is 7. Pure water
is a neutral solution with a pH of 7. Solutions that are acidic
have a pH of less than 7, and the lower is the pH, the stronger
is the acid. Basic or alkaline solutions have a pH greater than
7, and the higher the pH, the stronger it is. Because the scale
is logarithmic, each unit on the pH scale represents a 10-fold
difference in the hydrogen ion concentration. For example, a
solution with a pH of 6 has 10 times the hydrogen ion concentration
as a solution with a pH of 7, and a solution with a pH of
5 has 10 times more than a solution with a pH of 6. A solution
with a pH of 5 has 100 times the hydrogen ion concentration of
a pH 7 solution. Figure 2-14 compares the pH values for some
familiar acids and bases.
Quick Applications
The pH of human milk ranges from 7.0 to 7.4. The
initial breast secretion, colostrum, has a pH of 7.45.
This drops to a low of 7.0 for milk during the second
week of lactation. The pH remains near this level for
about 3 months, then increases gradually to 7.4 at
about 10 months.
In this way the hydrogen ions from the strong acid are incorporated
into the weak acid and will have less effect on the pH.
If a base such as NaOH is added to the buffer system, it will be
neutralized by the acid to form water and a salt:
NaOH + H 2
CO 3
→ NaHCO 3
+ H 2
O
Base Buffer acid Buffer salt Water
When either an acid or a base is added to a buffered solution,
something neutral (water or salt) and a component of the buffer
(weak acid or its salt) are formed. The pH does not change.
Buffers are very important physiologically. Even a slight deviation
from the normal pH range can cause pronounced changes
in the rate of cellular chemical reactions and thus threaten survival.
Buffers provide one of the homeostatic control mechanisms
that maintain normal pH.
Quick Applications
In the human body, acid-base balance is regulated by
chemical buffer systems, the lungs, and the kidneys.
Neutralization Reactions
Neutralization is the reaction between an acid and a
base. The hydrogen ion (H + ) from the acid reacts with the
hydroxide ion (OH − ) from the base to form water. The acid
removes or neutralizes the effect of the base and vice versa.
The other product is called a salt. Salts are ionic compounds
produced from neutralization reactions and consist of cations
other than H + and anions other than OH − . They are
formed from the positive ion of the base and the negative
ion of the acid:
HCl + NaOH → NaCl + H 2
O
Acid Base Salt Water
Neutralization reactions help maintain the proper pH of the
blood. They also have many practical applications. One such
application involves antacid medications. Antacids are basic
and are often used to neutralize excess acid in the stomach.
Buffers
If you add a few drops of a strong acid to some distilled water
(pH = 7), the pH of the solution will decrease because hydrogen
ions have been added. If a few drops of acid are placed in a
buffer solution, the pH will change only slightly. A buffer is a
solution that resists change in pH when either an acid or a base
is added.
A buffer solution contains a weak acid and a salt of that same
acid, the salt functions as a weak base. One of the most common
buffering systems and one that is important in human
physiology involves carbonic acid (H 2
CO 3
) and its salt, sodium
bicarbonate (NaHCO 3
). When hydrogen ions from a strong
acid are added, they react with the salt to form a weaker acid
and a neutral salt:
HCl + NaHCO 3
→ H 2
CO 3
+ NaCl
Strong acid Buffer salt Weak acid Neutral salt
Quick Check
2.12 Black coffee is an acid. Is its pH greater or less than 7.0?
2.13 Many of the foods we eat are acids, yet the pH of body
fluids remains relatively constant. Why?
Organic Compounds
The term organic chemistry is inherited from the day when the
science of chemistry was comparatively primitive. Compounds
formed by living processes were termed organic. In the nineteenth
century, it was discovered that organic compounds can
be created artificially in the laboratory from substances that do
not arise from life processes. Since then, the field of organic
chemistry has grown until today more than 1 million different
organic compounds have been identified.
Organic compounds are important because they are associated
with all living matter in both plants and animals. Nearly all
compounds related to living substances contain carbon; thus,
organic compounds are carbon compounds. Carbohydrates,
fats, proteins, hormones, vitamins, enzymes, and many drugs
are organic compounds. Wool, silk, linen, cotton, nylon, and
rayon all contain organic compounds. Add to this the perfumes,
dyes, flavorings, soaps, gasoline, and oils and you see
that the study of organic compounds is extensive.
The most important groups of organic compounds in the
body are the carbohydrates, proteins, lipids, nucleic acids, and
adenosine triphosphate.
Carbohydrates
Carbohydrate (kar-boh-HYE-drayt) molecules are composed
of carbon, hydrogen, and oxygen. They are the product of photosynthesis,
a process by which plants convert solar energy into
chemical energy. Carbohydrate molecules range in size from
small to very large. They are an important energy source in the