The Chemical Foundation of Life > Water
Water
• Water’s Polarity
• Water’s States: Gas, Liquid, and Solid
• Water’s High Heat Capacity
• Water’s Heat of Vaporization
• Water’s Solvent Properties
• Water’s Cohesive and Adhesive Properties
• pH, Buffers, Acids, and Bases
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The Chemical Foundation of Life > Water
Water’s States: Gas, Liquid, and Solid
• As water is boiled, kinetic energy causes the hydrogen bonds to break
completely and allows water molecules to escape into the air as gas
(steam or water vapor).
• When water freezes, water molecules form a crystalline structure
maintained by hydrogen bonding.
• Solid water, or ice, is less dense than liquid water.
• Ice is less dense than water because the orientation of hydrogen
bonds causes molecules to push farther apart, which lowers the
Ice Density
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density.
• For other liquids, solidification when the temperature drops includes
the lowering of kinetic energy, which allows molecules to pack more
tightly and makes the solid denser than its liquid form.
• Because ice is less dense than water, it is able to float at the surface
of water.
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The Chemical Foundation of Life > Water
Water’s High Heat Capacity
• Water has the highest heat
capacity of all liquids.
• Oceans cool slower than the
land due to the high heat
capacity of water.
• To change the temperature of 1
gram of water by 1 degree
Celsius, it takes 1.00 calorie.
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The Chemical Foundation of Life > Water
Water’s Polarity
• The difference in electronegativities between
oxygen and hydrogen atoms creates partial
negative and positive charges, respectively, on
the atoms.
• Water molecules attract or are attracted to other
polar molecules.
Nonpolar Molecules
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• Molecules that do not dissolve in water are
known as hydrophobic (water fearing) molecules.
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The Chemical Foundation of Life > Water
Water’s Heat of Vaporization
• The dissociation of liquid water molecules, which
changes the substance to a gas, requires a lot of
energy.
• The boiling point of water is the temperature in which
there is enough energy to break the hydrogen bonds
between water molecules.
Humidity, Evaporation, and Boiling
• Water is converted from its liquid form to its gaseous
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form (steam) when the heat of vaporization is
reached.
• Evaporation of sweat (mostly water) removes heat
from the surface of skin, cooling the body.
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The Chemical Foundation of Life > Water
Water’s Solvent Properties
•Water is considered the universal
solvent because so many other
substances dissolve in it
•This contributes to the fact that water
found on Earth is rarely ‘pure’
Dissociation of NaCl in water
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•Water dissolves many biomolecules,
because they are polar and therefore
hydrophilic (can be mixed or dissolved
with water)
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The Chemical Foundation of Life > Water
Water’s Cohesive and Adhesive Properties
• Cohesion holds hydrogen bonds together to create
surface tension on water. Water molecules are
attracted to other water molecules, because of water’s
polarity
• Since water is attracted to other molecules, adhesive
forces pull the water toward other molecules.
Surface Tension
• Water is transported in plants through both cohesive
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and adhesive forces; these forces pull water and the
dissolved minerals from the roots to the leaves and
other parts of the plant.
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The Chemical Foundation of Life > Water
pH, Acids, and Bases
• A basic solution will have a pH above 7.0, while an
acidic solution will have a pH below 7.0.
pH of Drinking Water
• Although the pH of pure water is 7, drinking water and
natural water exhibits a pH range because it contains
dissolved minerals and gases. Surface waters typically
The pH scale
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range from pH 6.5 to 8.5 while groundwater ranges
from pH 6 to 8.5.
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The Chemical Foundation of Life > Water
Surface Tension
Water molecules that are surrounded by other water
molecules are attracted to the molecules in every
direction. Water molecules that are found at the top
surface of some water, such as the top of a cup or the
surface of a pond, have only a small number of water
molecules in the air above them. These surface water
molecules are attracted downward by the water
molecules below them. The strong attraction below
causes the water to have something like a strong
invisible “skin” at the top surface. The scientific name
given to this “skin” is surface tension.
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The Chemical Foundation of Life
Surface Tension, cohesive and adhesion
Water's cohesive and adhesive properties allow this water strider (Gerris sp.) to stay
afloat.
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The Chemical Foundation of Life
Nonpolar Molecules
Oil and water do not mix. As this macro image of oil and water shows, oil does not dissolve in
water but forms droplets instead. This is due to it being a nonpolar compound.
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The Chemical Foundation of Life
Dissociation of NaCl in water
When table salt (NaCl) is mixed in water, spheres of hydration form around the ions (atoms
with extra electrons or missing electrons. When you are missing an electron or two, you have a
positive charge. When you have an extra electron or two, you have a negative charge).
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The Chemical Foundation of Life
Humidity, Evaporation, and Boiling
(a) Because of the distribution of speeds and kinetic energies, some water molecules can
break away to the vapor phase even at temperatures below the ordinary boiling point. (b) If the
container is sealed, evaporation will continue until there is enough vapor density for the
condensation rate to equal the evaporation rate. This vapor density and the partial pressure it
creates are the saturation values. They increase with temperature and are independent of the
presence of other gases, such as air. They depend only on the vapor pressure of water.
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The Chemical Foundation of Life
Surface Tension
The weight of the needle is pulling the surface downward; at the same time, the surface
tension is pulling it up, suspending it on the surface of the water and keeping it from sinking.
Notice the indentation in the water around the needle.
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The Chemical Foundation of Life
Ice Density
Hydrogen bonding makes ice less dense than liquid water. The (a) lattice structure of ice
makes it less dense than the freely flowing molecules of liquid water, enabling it to (b) float on
water.
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The Chemical Foundation of Life
The pH scale
The pH scale measures the concentration of hydrogen ions (H+) in a solution.
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