Chapter 4: Concept 4.4
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Objectives
Describe the structure of a water molecule.
List and describe water's unique properties.
Distinguish between an acid and a base.
Explain how Earth's conditions are fit for life.
Key Terms
polar molecule
hydrogen bond
cohesion
adhesion
thermal energy
temperature
solution
solvent
solute
aqueous solution
acid
base
pH scale
buffer
All living things are dependent on water. Inside your body, your cells are
surrounded by a fluid that is mostly water, and your cells themselves are 70
to 95 percent water. The abundance of water is a major reason Earth can
support life. Water is so common that it is easy to overlook its
extraordinary properties, which are linked to the structure and interactions
of its molecules.
The Structure of Water
A water molecule at first may seem pretty simple. Its two hydrogen atoms
are each joined to an oxygen atom by a single covalent bond (Figure 4-12).
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Figure 4-12
Hydrogen bonds form readily among polar
water molecules.
However, the key to water's unusual properties is that the electrons of each
covalent bond are not shared equally between oxygen and hydrogen atoms.
Oxygen pulls electrons much more strongly than does hydrogen. Part of the
reason is that the oxygen nucleus has eight protons, and therefore has a
stronger positive charge than the hydrogen nucleus, which has one proton.
This unequal pull results in the shared electrons spending more of their
time in the "neighborhood" of the oxygen atom. Note the "V" shape of the
water molecule, with the oxygen atom at the base of the "V" opposite the
two hydrogen atoms. The unequal sharing of electrons causes the oxygen
end of the molecule to have a slight negative charge, while the end with the
two hydrogen atoms is slightly positive. A molecule in which opposite ends
have opposite electric charges is called a polar molecule. Water is a
compound consisting of polar molecules.
Water molecules are attracted to one another in a specific way. The slightly
negative oxygen end of one molecule attracts the slightly positive hydrogen
ends of adjacent water molecules, causing the molecules to become
arranged as you see in Figure 4-12. This type of weak attraction between
the hydrogen atom of one molecule and a slightly negative atom within
another molecule is a type of chemical bond called a hydrogen bond.
Because the atoms within the water molecules have not transferred an
electron (and thus a full unit of charge) to another atom, the attraction in a
hydrogen bond is not as strong as that in an ionic bond.
Water's Life-Supporting Properties
The polar nature of water and the effects of hydrogen bonding explain most
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of water's unique properties. These properties include cohesion and
adhesion, temperature moderation, the lower density of ice compared to
liquid water, and water's ability to dissolve other substances.
Cohesion and Adhesion Each hydrogen bond between molecules of
liquid water lasts for only a few trillionths of a second. Yet, at any instant
most of the molecules are involved in hydrogen bonding with other
molecules because new hydrogen bonds form as fast as old ones break.
This tendency of molecules of the same kind to stick to one another is
called cohesion. Cohesion is much stronger for water than for most other
liquids. Water molecules are also attracted to certain other molecules. The
type of attraction that occurs between unlike molecules is called adhesion.
Both cohesion and adhesion are important in the living world. One of the
most important effects of these forces is keeping large molecules organized
and arranged in a way that enables them to function properly in cells. You
will read more about this role of water in Chapters 5 and 6.
As another example, trees depend on cohesion and adhesion to help
transport water from their roots to their leaves (Figure 4-13). The
evaporation of water from leaves pulls water upward from the roots
through narrow tubes in the trunk of the tree. Adhesion between water
molecules and the walls of the tubes helps resist the downward pull of
gravity on the water. And because of cohesion between water molecules,
the pulling force caused by evaporation from the leaves is relayed through
the tubes all the way down to the roots. As a result, water moves against
the force of gravity even to the top of a very tall tree. You've witnessed
another example of cohesion if you've ever seen an insect "skating" across
the surface of a pond. Cohesion pulls the molecules at the surface tightly
together, forming a filmlike boundary that can support the insect. This
effect is known as surface tension.
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Figure 4-13
Cohesion and adhesion contribute to the rise of water molecules within a
tree's water transport system. The dotted lines in the diagram indicate
hydrogen bonds.
Temperature Moderation If you have ever burned your finger on a
metal pot while waiting for the water in it to boil, you know that water
heats up much more slowly than metal. In fact, because of hydrogen
bonding, water has a better ability to resist temperature change than most
other substances. To understand why, it is first helpful to know a little
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about energy and temperature. Thermal energy is the total amount of
energy associated with the random movement of atoms and molecules in a
sample of matter. Temperature is a measure of the average energy of
random motion of the particles in a substance. When two substances differ
in temperature, thermal energy in the form of heat is transferred from the
warmer substance to the cooler one.
When you heat a substance—such as a metal pan or water—its temperature
rises because its molecules move faster. But in water, some of the thermal
energy that is absorbed goes to break hydrogen bonds. That doesn't happen
in the metal pan, which has no hydrogen bonds. As a result, the water
absorbs the same amount of thermal energy but undergoes less temperature
change than the metal. Conversely, when you cool a substance, the
molecules slow and the temperature drops. But as water cools, it forms
hydrogen bonds. This releases thermal energy in the form of heat, so there
is less of a drop in temperature than in metal.
One result of this property is that it causes oceans and large lakes to
moderate the temperatures of nearby land areas. In other words, coastal
areas generally have less extreme temperatures than inland areas. For
example, a large lake can store a huge amount of thermal energy from the
sun during the day. Then at night, heat given off by the gradually cooling
water moderates the otherwise more rapid cooling of the air and land.
Water also moderates temperature through evaporation, such as when you
sweat. Evaporation occurs when molecules at the surface of a liquid escape
to the air. As water molecules evaporate, the remaining liquid becomes
cooler. The process of evaporation requires thermal energy to break
hydrogen bonds and release water molecules into the air. In sweating, this
energy is absorbed from the skin, cooling the body.
Low Density of Ice Density is the amount of matter in a given volume.
A high-density substance is more tightly "packed" than a low-density
substance. In most substances, the solid state is more dense than the liquid
state. Water is just the opposite—its solid form (ice) is less dense than the
cold liquid form. Once again, hydrogen bonds are the reason. Every water
molecule in ice forms four long-lasting hydrogen bonds with neighboring
water molecules, which keep the molecules spaced in a regular pattern
(Figure 4-15). Because the molecules in liquid water are moving faster than
those in ice, there are fewer and more short-lived hydrogen bonds between
molecules. The liquid water molecules can fit more closely together than
the molecules in ice. Since substances of lesser density float in substances
of greater density, ice floats in liquid water.
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Figure 4-15
Ice floats because its molecules are less
densely packed than those in liquid water.
How is the fact that ice floats important to living things? If ice sank, it
would form on the bottom of a body of water as the water was cooling.
Ponds and lakes would freeze from the bottom up, trapping the fish and
other organisms in a shrinking layer of water without access to the
nutrients from the muddy bottom. Instead, when a deep body of water
cools, the floating ice insulates the liquid water below, allowing life to
persist under the frozen surface.
Water's Ability to Dissolve Other Substances When you stir
table salt into a glass of water, you are forming a solution, a uniform
mixture of two or more substances. The substance that dissolves the other
substance and is present in the greater amount is the solvent (in this case,
water). The substance that is dissolved and is present in a lesser amount is
the solute (in this case, salt). When water is the solvent, the result is called
an aqueous solution (from the Latin word aqua, "water").
Water is the main solvent inside all cells, in blood, and in plant sap. Water
dissolves an enormous variety of solutes necessary for life. Figure 4-16
illustrates how water dissolves ionic compounds such as table salt (sodium
chloride). The positive sodium ions at the surface of a sodium chloride
crystal attract the oxygen ends of the water molecules. The negative
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chloride ions attract the hydrogen ends of the water molecules. As a result,
water molecules surround each ion, breaking the salt crystal apart in the
process.
Figure 4-16
Sodium chloride dissolves as Na+
and Cl- ions become attracted to
water molecules and break away
from the surface of the solid.
Water can also dissolve many nonionic compounds, such as sugars. The
structures of sugar molecules include polar areas where electrons are
shared unevenly between atoms. These areas of slight electric charge
attract the polar ends of water molecules. Water molecules cling to these
charged regions and separate the sugar molecules from one another.
Acids, Bases, and pH
In aqueous solutions, a very small percentage of the water molecules
themselves break apart into ions. The ions formed are positively charged
hydrogen ions (H+) and negatively charged hydroxide ions (OH-). (A
hydroxide ion is a combination of an oxygen atom and a hydrogen atom
that carries a 1- charge.) For the chemical processes of life to work
correctly, the right balance of H+ ions and OH- ions is critical.
Some chemical compounds contribute additional H+ ions to an aqueous
solution while others remove H+ ions from it. A compound that donates H+
ions to a solution is called an acid. An example is hydrochloric acid (HCl),
the acid in your stomach. In an aqueous solution, hydrochloric acid breaks
apart completely into H+ and Cl- ions. A compound that removes H+ ions
from an aqueous solution is called a base. Some bases, such as sodium
hydroxide (NaOH), do this by adding OH- ions, which then combine with
H+ ions and form water molecules.
The pH Scale The pH scale describes how acidic or basic a solution is.
The scale ranges from 0 (most acidic) to 14 (most basic) (Figure 4-17).
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Each pH unit represents a tenfold change in the concentration of H+ ions.
For example, lemon juice at pH 2 has 10 times more H+ ions than an equal
amount of grapefruit juice at pH 3. Pure water and aqueous solutions that
have equal amounts of H+ and OH- ions are said to be neutral. They have a
pH of 7 and are neither acidic nor basic. The pH of the solution inside most
living cells is close to 7.
Figure 4-17
A solution having a pH of 7 is neutral. Many fruits have pH values less than
7, making them acidic. Various household cleaners have pH values greater
than 7, making them basic.
Buffers Because the molecules in cells are very sensitive to
concentrations of H+ and OH- ions, even a slight change in pH can be
harmful to organisms. Many biological fluids contain buffers, substances
that cause a solution to resist changes in pH. A buffer works by accepting
H+ ions when their levels rise and donating H+ ions when their levels fall,
thereby maintaining a fairly constant pH in the solution. An example of the
importance of buffers is their role in regulating the pH of the blood. Human
blood normally has a pH of about 7.4. Certain chemical reactions within
your cells can lead to an increase in the amount of H+ ions. When these
ions move into the blood, buffers take up some of them, preventing the
blood from becoming acidic enough to endanger cell function.
An Environment Fit for Life
You have just explored some of the unique properties of water—an
essential substance of life. The abundance of liquid water is one example
of how conditions on Earth provide a favorable environment for life.
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Another condition is the planet's location in the solar system. Earth is far
enough from the sun that the planet receives a moderate quantity of the
energy radiating from the sun, but not so far away that temperatures are too
cold to sustain life. At the same time, ozone (a gas made of oxygen atoms)
in Earth's upper atmosphere shields the planet's surface from some of the
sun's harmful radiation. Yet another factor is the availability in the soil,
rock, and atmosphere of elements essential to life. As you will read in
Chapter 36, carbon, hydrogen, oxygen, and nitrogen are recycled through
living and nonliving parts of the environment and so are constantly
available to living organisms. In the next chapter, you will explore how
these essential elements are arranged into the molecules of life.
Concept Check 4.4
1. Explain how the structure of water molecules results in attractions
among them.
2. Give an example of how cohesion among water molecules is important
to living things.
3. Describe the information the pH scale provides.
4. Name three conditions on Earth that make the planet suitable for life.
5. Explain one way in which water can moderate temperature.
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