Chapter 2
Chemistry Comes Alive
29
2
Water is the body’s chief solvent. Most solutions in the body
are
true solutions
containing gases, liquids, or solids dissolved in
water. True solutions are usually transparent. Examples are sa-
line solution [table salt (NaCl) and water], a mixture of glucose
and water, and mineral water. ±e solutes of true solutions are
minute, usually in the form of individual atoms and molecules.
Consequently, they are not visible to the naked eye, do not settle
out, and do not scatter light. In other words, if a beam of light is
passed through a true solution, you will not see the path of light.
Concentration of Solutions
We describe true solutions in
terms of their
concentration
, which may be indicated in various
ways. Solutions used in a college laboratory or a hospital are
oFen described in terms of the
percent
(parts per 100 parts) of
the solute in the total solution. ±is designation always refers
to the solute percentage, and unless otherwise noted, water is
assumed to be the solvent.
Milligrams per deciliter
(
mg/dl
) is another common concen-
tration measurement. (A deciliter is 100 milliliters or 0.1 liter.)
Still another way to express the concentration of a solution
is in terms of its
molarity
(mo-lar
9
ĭ-te), or moles per liter, indi-
cated by
M
. ±is method is more complicated but much more
useful. To understand molarity, you must understand what a
mole is. A
mole
of any element or compound is equal to its
atomic weight or
molecular weight
(sum of the atomic weights)
weighed out in grams. ±is concept is easier than it seems, as
illustrated by the following example.
characteristics of the compound. ±is concept is important be-
cause the properties of compounds are usually very different
from those of the atoms they contain. Water, for example, is very
different from the elements hydrogen and oxygen. Indeed, it is
next to impossible to tell what atoms are in a compound without
analyzing it chemically.
Mixtures
Mixtures
are substances composed of two or more components
physically intermixed
. Most matter in nature exists in the form of
mixtures, but there are only three basic types:
solutions
,
colloids
,
and
suspensions
(Figure 2.4)
.
Solutions
Solutions
are homogeneous mixtures of components that may
be gases, liquids, or solids.
Homogeneous
means that the mixture
has exactly the same composition or makeup throughout—a
sample taken from any part of the mixture has the same com-
position (in terms of the atoms or molecules it contains) as a
sample taken from any other part of the mixture. Examples in-
clude the air we breathe (a mixture of gases) and seawater (a mix-
ture of salts, which are solids, and water). ±e substance present
in the greatest amount is called the
solvent
(or dissolving me-
dium). Solvents are usually liquids. Substances present in smaller
amounts are called
solutes
.
Solution
Solute
particles
Solute
particles
Solute
particles
Plasma
Unsettled
Settled
Settled red
blood cells
Example
Example
Example
Solute particles are very tiny,
do not settle out or scatter light.
Colloid
Solute particles are larger than
in a solution and scatter light;
do not settle out.
Suspension
Solute particles are very large,
settle out, and may scatter light.
Mineral water
Jello
Blood
Figure 2.4
The three basic types of mixtures.
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