15.1 chemistry

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15.1 Chemistry

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Water and Its Properties

Water covers about three quarters of Earth’s surface. All life forms that are known to exist are made mostly of water. You will learn about the properties of water and what makes this unique substance essential to life on Earth.

15.1

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Water and Its Properties >

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15.1 Water in the Liquid State

Water in the Liquid State

How can you account for the high surface tension and low vapor pressure of water?

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Water and Its Properties > Water in the Liquid State

You could not live without water, nor could all the plants and animals on Earth.

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Water and Its Properties > Water in the Liquid State

A water molecule is polar.

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Water and Its Properties > Water in the Liquid State

Animation 19 See how hydrogen bonding results in the unique properties of water.

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Water and Its Properties > Water in the Liquid State

Polar molecules are attracted to one another by dipole interactions. The negative end of one molecule attracts the positive end of another molecule.

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Water and Its Properties > Water in the Liquid State

The intermolecular attraction among water molecules results in the formation of hydrogen bonds.

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Water and Its Properties > Water in the Liquid State

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

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Water and Its Properties > 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.

• All liquids have a surface tension, but water’s surface tension is higher than most.

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Water and Its Properties > Water in the Liquid State

Surface tension makes it possible for this water strider to walk on water.

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Water and Its Properties > Water in the Liquid State

A surfactant is any substance that interferes with the hydrogen bonding between water molecules and thereby reduces surface tension.

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Water and Its Properties > Water in the Liquid State

Vapor Pressure

Hydrogen bonding between water molecules also explains water’s unusually low vapor pressure. Because hydrogen bonds hold water molecules to one another, the tendency of these molecules to escape is low, and evaporation is slow.

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Water and Its Properties > Water in the Liquid State

Animation 20 Discover how some insects can walk on water.

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Water and Its Properties >

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Water in the Solid State

Water in the Solid State

How would you describe the structure of ice?

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Water and Its Properties > 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 starts to decrease.

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Water and Its Properties > Water in the Solid State15.1

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Water and Its Properties > Water in the Solid State

Hydrogen bonds hold the water molecules in place in the solid phase.

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Water and Its Properties > Water in the Solid State

The structure of ice is a regular open framework of water molecules arranged like a honeycomb.

When ice melts, the framework collapses, and the water molecules pack closer together, making liquid water more dense than ice.

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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

15.1 Section Quiz.

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1. Many of the unusual properties of water are the result of

a. hydrogen bonding.

b. nonpolar molecules.

c. low molar mass.

d. dispersion forces.

15.1 Section Quiz.

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2. A surfactant causes water to spread out over a surface. This spreading occurs because the surfactant

a. attaches to the surface.

b. interferes with hydrogen bonding.

c. lowers the vapor pressure.

d. lowers the density of water.

15.1 Section Quiz.

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3. Ice is less dense than liquid water because in ice the water molecules

a. expand because of weakened covalent bonds.

b. have a regular open honeycomb framework.

c. expand because of weakened hydrogen bonds.

d. have a more disorderly arrangement with lower density.

15.1 Section Quiz.

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