15.1 Water and Its Properties > Chapter 15 Water and Aqueous Systems Water and Its Properties Homogeneous Aqueous Systems

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1 Chapter 15 Water and Aqueous Systems 15.1 Water and Its Properties 15.2 Homogeneous Aqueous Systems 15.3 Heterogeneous Aqueous Systems 1 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

2 CHEMISTRY & YOU 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. 2 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

3 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. 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) 3 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

4 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. Thus, the O H bond is highly polar. (Opposite sides like a magnet or the earth) 4 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

5 The oxygen atom acquires a partial negative charge (δ ). Polar bonds δ δ δ + δ + Molecule has net polarity δ δ δ + δ + 5 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

6 The oxygen atom acquires a partial negative charge (δ ). δ δ δ + δ + The less electronegative hydrogen atoms acquire partial positive charges (δ + ). Polar bonds Molecule has net polarity δ δ δ + δ + 6 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

7 How do the polarities of the two O H bonds affect the polarity of the molecule? δ δ δ + δ + Polar bonds Molecule has net polarity δ δ δ + δ + 7 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

8 How do the polarities of the two O H bonds affect the polarity of the molecule? The molecule has a bent shape. δ δ δ + δ + Polar bonds Molecule has net polarity δ δ δ + δ + 8 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

9 How do the polarities of the two O H bonds affect the polarity of the molecule? The molecule has a bent shape. The water molecule as a whole is polar. Polar bonds δ δ δ + δ + Molecule has net polarity δ δ δ + δ + 9 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

10 In general, polar molecules are attracted to one another by dipole interactions. δ δ + The negative end of one molecule attracts the positive end of another molecule. δ + 10 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

11 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 11 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

12 Many unique and important properties of water including its high surface tension, low vapor pressure, and high boiling point result from hydrogen bonding. 12 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

13 Surface Tension Have you ever noticed that water forms nearly spherical droplets on a leaf? 13 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

14 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. The surface tension of water tends to hold a drop of liquid in a spherical shape. 14 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

15 Surface Tension 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 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

16 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. 16 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

17 Water in the Solid State Why is ice less dense than liquid water? 17 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

18 Water in the Solid State Why is ice less dense than liquid water? 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. 18 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

19 tegory/chemistry 19 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

20 Water in the Solid State The unique properties of ice are a result of hydrogen bonding. Hydrogen bond Ice Extensive hydrogen bonding in ice holds the water molecules farther apart in a more ordered arrangement than in liquid water. 20 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

21 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. 21 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

22 Which is most likely liquid water? A B C 22

23 CHEMISTRY & YOU 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. 23 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

24 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 24 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

25 BIG IDEA Bonding and Interactions 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. 25 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.