Water participates in chemical reactions. Because
water can dissolve many different substances, it is an
ideal medium for chemical reactions. Water also actively
during digestion break down large nutrient molecules
into smaller molecules through the addition of water
molecules. This type of reaction is called hydrolysis
-lysis =
to loosen or break apart). As you
will see later in this chapter, the double sugar sucrose
gets broken down into two single sugars, glucose and
fructose, by the addition of a water molecule. Water
also participates in synthesis reactions. In one type
of synthesis reaction, called dehydration synthesis
(de-hi-DRA-shun), a water molecule is removed from
the reactants. For example, sucrose is formed from
glucose and galactose by the removal of a water molecule.
Water absorbs and releases heat very slowly. In
comparison to most other substances, water can absorb
or release a relatively large amount of heat with only a
slight change in its own temperature. The large amount
of water in the human body thus moderates the effect
of changes in the environmental temperature, thereby
helping maintain the homeostasis of body temperature.
Water requires a large amount of heat to change
from a liquid to a gas. When the water in sweat
evaporates from the skin surface, it takes with it large
quantities of heat and with it provides an excellent
cooling mechanism.
Water serves as a lubricant. Water is a major part of
saliva, mucus, and other lubricating fluids. Lubrication
is especially necessary in the thoracic and abdominal
cavities, where internal organs touch and
slide over one another.
Many inorganic compounds can be classified
as acids, bases, or salts. An acid is a substance
that breaks apart or
(dis-SO-se-ats') into one or
hydrogen ions
(H+) when it dissolves in water (Fig-
ure 2.8a) ; an alternative definition of an acid is a sub-
stance that releases one or more H+ when dissolved in
water. In contrast, a base usually dissociates into one
or more
hydroxide ions
(OH-) when it dissolves in water
(Figure 2.8b); an alternative definition of a base is a
substance that absorbs one or more H+ when dissolved
in water. A salt, when dissolved
in water, dissociates into cations
and anions, neither of which is
H+ or OH- (Figure
2.8c). Ac-
ids and bases react with one an-
other to form salts. For example,
the reaction of hydrochloric acid
(HCl) and the base potassium hy-
droxide (KOH) produces the salt
potassium chloride (KCl), along
with water (H2O). This exchange
reaction is written as follows:
HCl + KOH ^ KCl + H2O
To ensure homeostasis, body fluids must contain al-
most balanced quantities of acids and bases. The more
H+ dissolved in a solution, the more acidic it is; converse-
ly, the more OH-, the more basic (alkaline) it is. The
chemical reactions that take place in the body are very
sensitive to even small changes in the acidity or alkalin-
ity of body fluids. Any departure from the narrow limits
of normal H+ and OH- concentrations greatly disrupts
body functions. Acidity or alkalinity is expressed on the pH
scale, which ranges from 0 to 14 (Figure 2.9).
This scale is based on the concentration of hy-
drogen ions in a solution. The midpoint of the
pH scale, where the concentrations of H+ and
OH- are equal, is 7. A solution with a pH of 7,
inorganic com-
A substance
that does not contain
A positively
charged ion that is at-
tracted to a negatively
charged electrode
called a cathode.
A negatively
charged ion that is at-
tracted to a positively
charged electrode
called an anode.
Acids, bases, and salts • Figure 2.8
Acids dissociate into
Bases dissociate into
hydrogen ions HH+).
hydroxide ions (O H-).
Salts dissociate into anions and
cations that are neither H+ nor O H - .
Introductory Chemistry
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