Ch15 Sec1.ppt

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15.1 Water and Its Properties >

Chapter 15

Water and Aqueous Systems

15.1 Water and Its Properties

15.2 Homogeneous 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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15.1 Water and Its Properties > Water in the Liquid StateWater in the Liquid State

Water in the Liquid State

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Water, H

₂

O, is a simple molecule

consisting of three atoms.

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

Water in the Liquid State

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Water, H

₂

O, is a simple molecule

consisting of three 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, H

₂

O, is a simple molecule

consisting of three 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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• The oxygen

atom acquires a partial negative charge (δ–).

Molecule has net polarity Polar bonds

δ– δ–

δ+

δ– δ–

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• The oxygen

atom acquires a partial negative charge (δ–).

• The less

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

δ– δ–

δ+

δ– δ–

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How do the polarities of the two O—H

bonds affect the polarity of the molecule?

δ– _δ–

δ+

δ– _δ–

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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?

δ– _δ–

δ+

δ– _δ–

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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?

δ– _δ–

δ+

δ– _δ–

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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?

δ– _δ–

δ+

δ– _δ–

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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.

δ+

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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.

Liquid water

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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.

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Have you ever noticed that water forms

nearly spherical droplets on a leaf?

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• 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.

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• 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.

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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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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.

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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.

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Vapor Pressure

Hydrogen bonding between water

molecules also explains water’s unusually

low vapor pressure.

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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.

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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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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?

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

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15.1 Water and Its Properties > Water in the Solid StateWater in the Solid State

Water in the Solid State

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Ice cubes float in your glass of iced tea

because solid water has a lower density

than liquid water.

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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

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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

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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.

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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

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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

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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

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

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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

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15.1 Water and Its Properties > Key ConceptsKey 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

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15.1 Water and Its Properties > Glossary TermsGlossary 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

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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

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