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15.1 Water and Its Properties >
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Chapter 15 Water and Aqueous Systems 15.1 Water and Its Properties 15.2 Homogeneous Aqueous Systems 15.3 Heterogeneous Aqueous Systems
15.1 Water and Its Properties >
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What properties of water make it essential to life on Earth?
Water covers about three quarters of Earth’s surface. All known life forms are made mostly of water.
CHEMISTRY & YOU
15.1 Water and Its Properties >
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Water, H2O, is a simple molecule consisting of three atoms.
• The oxygen atom forms a covalent bond with each of the hydrogen atoms.
• Oxygen has a greater electronegativity (wants electrons more)than hydrogen, so the oxygen atom attracts the electron pair of the covalent O—H bond to a greater extent than the hydrogen atom.(share unevenly)
Water in the Liquid State
15.1 Water and Its Properties >
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Water, H2O, is a simple molecule consisting of three atoms.
• The oxygen atom forms a covalent bond with each of the hydrogen atoms.
• Thus, the O—H bond is highly polar. (Opposite sides like a magnet or the earth)
Water in the Liquid State
15.1 Water and Its Properties >
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• The oxygen atom acquires a partial negative charge (δ–).
Water in the Liquid State
Molecule has net polarity Polar bonds
δ– δ–
δ+ δ+
δ– δ–
δ+ δ+
15.1 Water and Its Properties >
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• The oxygen atom acquires a partial negative charge (δ–).
• The less electronegative hydrogen atoms acquire partial positive charges (δ+).
Water in the Liquid State
Molecule has net polarity Polar bonds
δ– δ–
δ+ δ+
δ– δ–
δ+ δ+
15.1 Water and Its Properties >
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How do the polarities of the two O—H bonds affect the polarity of the molecule?
Water in the Liquid State
Molecule has net polarity Polar bonds
δ– δ–
δ+ δ+
δ– δ–
δ+ δ+
15.1 Water and Its Properties >
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• The molecule has a bent shape.
How do the polarities of the two O—H bonds affect the polarity of the molecule?
Water in the Liquid State
Molecule has net polarity Polar bonds
δ– δ–
δ+ δ+
δ– δ–
δ+ δ+
15.1 Water and Its Properties >
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• The molecule has a bent shape.
• The water molecule as a whole is polar.
How do the polarities of the two O—H bonds affect the polarity of the molecule?
Water in the Liquid State
Molecule has net polarity Polar bonds
δ– δ–
δ+ δ+
δ– δ–
δ+ δ+
15.1 Water and Its Properties >
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In general, polar molecules are attracted to one another by dipole interactions.
• The negative end of one molecule attracts the positive end of another molecule.
Water in the Liquid State
δ+
δ+
δ–
15.1 Water and Its Properties >
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However, in water, this attraction results in hydrogen bonding.
• Hydrogen bonds are attractive forces that arise when a hydrogen atom is covalently bonded to a very electronegative atom and also weakly bonded to an unshared electron pair of another electronegative atom.
Water in the Liquid State
Liquid water
Hydrogen bond
15.1 Water and Its Properties >
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Many unique and important properties of water—including its high surface tension, low vapor pressure, and high boiling point—result from hydrogen bonding.
Water in the Liquid State
15.1 Water and Its Properties >
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Have you ever noticed that water forms nearly spherical droplets on a leaf?
Water in the Liquid State
Surface Tension
15.1 Water and Its Properties >
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The inward force, or pull, that tends to minimize the surface area of a liquid is called surface tension. • All liquids have a surface tension, but
water’s surface tension is higher than most.
• The surface tension of water tends to hold a drop of liquid in a spherical shape.
Water in the Liquid State
Surface Tension
15.1 Water and Its Properties >
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It is possible to decrease the surface tension of water by adding a surfactant. • A surfactant is any substance that
interferes with the hydrogen bonding between water molecules and thereby reduces surface tension.
• Soaps and detergents are surfactants. • http://science360.gov/obj/tkn-video/81074969-11e0-4a2e-b674-8fc8886fd9c3
Water in the Liquid State
Surface Tension
15.1 Water and Its Properties >
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Vapor Pressure Hydrogen bonding between water molecules also explains water’s unusually low vapor pressure. • An extensive network of hydrogen bonds holds
the molecules in liquid water to one another.
• These hydrogen bonds must be broken before water changes from the liquid to the vapor state, so the tendency of these molecules to escape is low and evaporation is slow.
Water in the Liquid State
15.1 Water and Its Properties >
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Water in the Solid State
Why is ice less dense than liquid water?
15.1 Water and Its Properties >
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Water in the Solid State
The structure of ice is a regular open framework of water molecules in a hexagonal arrangement.
The hexagonal symmetry of a snowflake reflects the structure of the ice crystal.
Why is ice less dense than liquid water?
15.1 Water and Its Properties >
19
http://phet.colorado.edu/en/simulations/category/chemistry
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15.1 Water and Its Properties >
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Water in the Solid State
The unique properties of ice are a result of hydrogen bonding.
• Extensive hydrogen bonding in ice holds the water molecules farther apart in a more ordered arrangement than in liquid water.
Hydrogen bond
Ice
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Water in the Solid State
The fact that ice floats has important consequences for all organisms. • The liquid water at the bottom of an
otherwise frozen body of water is warmer than 0˚C, so fish and other aquatic life are better able to survive.
• If ice were denser than liquid water, bodies of water would tend to freeze solid during the winter months, destroying many types of organisms.
15.1 Water and Its Properties >
22
Which is most likely liquid water?
A B C
15.1 Water and Its Properties >
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What properties of water that result from hydrogen bonding make it essential to life on Earth? • The low vapor pressure of water keeps the
liquid water in all of Earth’s lakes and oceans from evaporating rapidly.
• If water did not have such a high boiling point, it would be a vapor at the usual temperatures found on Earth.
• The fact that ice floats allows fish and other aquatic life to survive the winter months.
CHEMISTRY & YOU
15.1 Water and Its Properties >
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Glossary Terms
• surface tension: an inward force that tends to minimize the surface area of a liquid; it causes the surface to behave as if it were a thin skin
• surfactant: any substance that interferes with the hydrogen bonding between water molecules and thereby reduces surface tension; soaps and detergents are surfactants
15.1 Water and Its Properties >
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• Water molecules are held together through hydrogen bonds.
• The hydrogen bonding interactions between water molecules account for the unique properties of water, including its high surface tension, low vapor pressure, and high boiling point.
• Hydrogen bonding also accounts for the fact that ice is less dense than liquid water.
BIG IDEA
Bonding and Interactions
