Page 79 - Fundamentals of anatomy physiology

66 Unit 1 Levels of Organization

2 3. High Heat Capacity. Heat capacity is the quantity of heat layer of water between two opposing surfaces will greatly
required to raise the temperature of a unit mass of a sub- reduce friction between them. (That is why driving on wet
stance 1°C. Water has an unusually high heat capacity. roads can be tricky. Your tires may start sliding on a layer
Why? The reason is that water molecules in the liquid state of water rather than maintaining contact with the road.)
are attracted to one another through hydrogen bonding. Within joints such as the knee, an aqueous solution pre-
Important consequences of this attraction include the vents friction between the opposing surfaces. Similarly, a
following: small amount of fluid in the body cavities prevents fric-
tion between internal organs, such as the heart or lungs,
The temperature of water must be quite high before and the body wall. p. 45
all the hydrogen bonds are broken between individual
water molecules and they have enough energy to break The Properties of Aqueous Solutions
free and become water vapor, a gas. Therefore, water
remains a liquid over a broad range of environmental Water’s chemical structure makes it an unusually effective sol-
temperatures, and the freezing and boiling points of vent. The covalent bonds in a water molecule are oriented so
water are far apart. that the hydrogen atoms are fairly close together. As a result,
the water molecule has positive and negative ends, or poles
Water carries a great deal of heat away with it when it (Figure 2–9a). For this reason, a water molecule is called a
changes from a liquid to a gas. This feature explains the polar molecule.
cooling effect of perspiration on the skin.
Many inorganic compounds are held together partly or
An unusually large amount of heat energy is required to completely by ionic bonds. In water, these compounds un-
change the temperature of 1 g of water by 1°C. As a re- dergo dissociation (di-so. -se. -A■ -shun), or ionization (ı.-on-i-
sult, a large mass of water changes temperature slowly. ZA■ -shun). In this process, ionic bonds are broken as the indi-
This property is called thermal inertia. Thermal inertia vidual ions interact with the positive or negative ends of polar
helps stabilize body temperature because water accounts water molecules (Figure 2–9b). The result is a mixture of cat-
for up to two-thirds of the weight of the human body. ions and anions surrounded by water molecules. The water
molecules around each ion form a hydration sphere.
4. Lubrication. Water is an effective lubricant because there
is little friction between water molecules. So, even a thin

Figure 2–9 Water Molecules Surround Solutes in Aqueous Solutions.

Hydration Glucose
spheres molecule

Negative CIϪ
pole

2–

O +

H Na+ c Glucose in solution. Hydration
Positive spheres also form around an
b Sodium chloride in solution. Ionic organic molecule containing polar
+ pole compounds, such as sodium chloride, covalent bonds. If the molecule
dissociate in water as the polar water binds water strongly, as does
a Water molecule. In a water molecules break the ionic bonds in the glucose, it will be carried into
molecule, oxygen forms polar large crystal structure. Each ion in solution—in other words, it will
covalent bonds with two hydrogen solution is surrounded by water dissolve. Note that the molecule
atoms. Because both hydrogen molecules, creating hydration spheres. does not dissociate, as occurs for
atoms are at one end of the ionic compounds.
molecule, it has an uneven
distribution of charges, creating
positive and negative poles.