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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

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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

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Water in the Liquid State

What factor causes the high surface tension, low vapor pressure, and high boiling point of water?

Water in the Liquid State

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Water in the Liquid State

Water, H₂O, is a simple molecule consisting of three atoms.

The oxygen atom forms a covalent bond with each of the hydrogen atoms.

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Water in the Liquid State

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 than hydrogen, so the oxygen atom attracts the electron pair of the covalent O—H bond to a greater extent than the hydrogen atom.

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Water in the Liquid State

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 than hydrogen, so the oxygen atom attracts the electron pair of the covalent O—H bond to a greater extent than the hydrogen atom.
Thus, the O—H bond is highly polar.

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Water in the Liquid State

The oxygen atom acquires a partial negative charge (δ^–).

Molecule has net polarity

Polar bonds

δ^–

δ⁺

δ^–

δ⁺

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Water in the Liquid State

The oxygen atom acquires a partial negative charge (δ^–).

The less electronegative hydrogen atoms acquire partial positive charges (δ⁺).

Molecule has net polarity

Polar bonds

δ^–

δ⁺

δ^–

δ⁺

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Water in the Liquid State

How do the polarities of the two O—H bonds affect the polarity of the molecule?

Molecule has net polarity

Polar bonds

δ^–

δ⁺

δ^–

δ⁺

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Water in the Liquid State

The molecule has a bent shape.

How do the polarities of the two O—H bonds affect the polarity of the molecule?

Molecule has net polarity

Polar bonds

δ^–

δ⁺

δ^–

δ⁺

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Water in the Liquid State

The molecule has a bent shape.
The two O—H bond polarities do not cancel.

How do the polarities of the two O—H bonds affect the polarity of the molecule?

Molecule has net polarity

Polar bonds

δ^–

δ⁺

δ^–

δ⁺

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Water in the Liquid State

The molecule has a bent shape.
The two O—H bond polarities do not cancel.
The water molecule as a whole is polar.

How do the polarities of the two O—H bonds affect the polarity of the molecule?

Molecule has net polarity

Polar bonds

δ^–

δ⁺

δ^–

δ⁺

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Water in the Liquid State

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.

δ⁺

δ^–

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Water in the Liquid State

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.

Liquid water

Hydrogen bond

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Water in the Liquid State

Many unique and important properties of water—including its high surface tension, low vapor pressure, and high boiling point—result from hydrogen bonding.

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Water in the Liquid State

Have you ever noticed that water forms nearly spherical droplets on a leaf?

Surface Tension

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Water in the Liquid State

The water molecules within the body of the liquid form hydrogen bonds with the other molecules that surround them on all sides.

The attractive forces on each of these molecules are balanced.

Surface Tension

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Water in the Liquid State

The water molecules within the body of the liquid form hydrogen bonds with the other molecules that surround them on all sides.

The attractive forces on each of these molecules are balanced.
Water molecules at the surface of the liquid experience an unbalanced attraction.
As a result, water molecules at the surface tend to be drawn inward.

Surface Tension

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Water in the Liquid State

Surface Tension

The inward force, or pull, that tends to minimize the surface area of a liquid is called surface tension.

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Water in the Liquid State

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.

Surface Tension

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Water in the Liquid State

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.

Surface Tension

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Water in the Liquid State

Vapor Pressure

Hydrogen bonding between water molecules also explains water’s unusually low vapor pressure.

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Water in the Liquid State

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.

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Water in the Liquid State

Boiling Point

Molecular compounds of low molecular mass are usually gases or liquids with low boiling points at normal atmospheric pressure.

Ammonia (NH₃) has a molar mass of 17.0 g/mol and boils at about –33˚C.

Water has a molar mass of 18.0 g/mol, but it has a boiling point of 100˚C.

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Water in the Liquid State

Boiling Point

The difference between the boiling points of ammonia and water is due to hydrogen bonding, which is more extensive in water than in ammonia.

It takes much more heat to disrupt the attractions between water molecules than those between ammonia molecules.

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Some insects are able to walk across water. How do the properties of water explain their ability?

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Some insects are able to walk across water. How do the properties of water explain their ability?

The surface tension of water is relatively high. As long as the forces holding the surface water molecules together are stronger than the forces exerted down on the water by the insect, the insect will not sink.

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Water in the Solid State

How can you describe the structure of ice?

Water in the Solid State

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Water in the Solid State

Ice cubes float in your glass of iced tea because solid water has a lower density than liquid water.

This situation is not usual for liquids.

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Water in the Solid State

As water begins to cool, it behaves initially like a typical liquid.

It contracts slightly, and its density gradually increases.
When the temperature of the water falls below 4˚C, the density of water actually starts to decrease.

Density of Liquid Water and Ice

Temperature (˚C)

Density (g/cm³)

100 (liquid water)

0.9584

50

0.9881

25

0.9971

10

0.9997

4

1.0000

0 (liquid water)

0.9998

0 (ice)

0.9168

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Water in the Solid State

Below 4˚C, water no longer behaves like a typical liquid.

Ice, which forms at 0˚C, has about a 10% lower density than water at 0˚C.
Ice is one of only a few solids that floats in its own liquid.

Density of Liquid Water and Ice

Temperature (˚C)

Density (g/cm³)

100 (liquid water)

0.9584

50

0.9881

25

0.9971

10

0.9997

4

1.0000

0 (liquid water)

0.9998

0 (ice)

0.9168

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Water in the Solid State

Why is ice less dense than liquid water?

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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?

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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.

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What properties of water that result from hydrogen bonding make it essential to life on Earth?

CHEMISTRY & YOU

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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

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In ice, how many hydrogen bonds can be formed between one hydrogen atom of a water molecule and the oxygen in surrounding water molecules?

A. 0

B. 1

C. 2

D. 3

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In ice, how many hydrogen bonds can be formed between one hydrogen atom of a water molecule and the oxygen in surrounding water molecules?

A. 0

B. 1

C. 2

D. 3

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Key Concepts

Many unique and important properties of water—including its high surface tension, low vapor pressure, and high boiling point—result from hydrogen bonding.

The structure of ice is a regular open framework of water molecules in a hexagonal arrangement.

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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

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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

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