Properties of Water
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
• A molecule in which opposite ends have
opposite electric charges is called a polar
molecule. Water is a compound consisting
of polar molecules.
polar molecule: molecule in which
opposite ends have opposite electric
charges
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.
• Water's Life-Supporting Properties
The polar nature of water and the effects of
hydrogen bonding explain most 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
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.
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.
Because of hydrogen bonding, water has a better ability
to resist temperature change than most other substances.
Why?
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.
• 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.
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
Sodium chloride
dissolves as Na+ and
Cl- ions become
attracted to water
molecules and break
away from the surface
of the solid.
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-).
• For the chemical processes of life to work
correctly, the right balance of H+ ions and OH-
ions is critical.
Acids
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.
Base
•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).
•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
•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.
•.
Example
•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