15.1 water and its properties > 1 copyright © pearson education, inc., or its affiliates. all...

15.1 Water and Its Properties >15.1 Water and Its Properties >

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

Chapter 15Water and Aqueous Systems

15.1 Water and Its Properties

15.2 Homogeneous Aqueous Systems

15.3 Heterogeneous Aqueous Systems



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 & YOUCHEMISTRY & YOU



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.

Water in the Liquid StateWater 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 polarityPolar bonds

δ– δ–

δ+δ+

δ– δ–

δ+ δ+



• 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 polarityPolar bonds

δ– δ–

δ+δ+

δ– δ–

δ+ δ+



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+

δ–



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



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




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


Surface Tension



• 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



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



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



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.



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.




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.




HEAT CAPACITY

GO TO OTHER STUFF




Water in the Solid StateWater 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.




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 (ice) 0.9168




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)


50 0.9881

25 0.9971

10 0.9997

4 1.0000


0 (ice) 0.9168




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?




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




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.



DISSOLVED GASES NOTES




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.




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.




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



Although it sounds absurd, an ordinary dill pickle from the deli can be a source of light when connected to an electric current!


How can you make a pickle glow?



SolutionsSolutions

Solutions

What types of substances dissolve most readily in water?

• An aqueous solution is water that contains dissolved substances.



SolutionsSolutions

Solvents and Solutes

• In a solution, the dissolving medium is the solvent.

• The dissolved particles in a solution are the solute.



SolutionsSolutions


• A solvent dissolves the solute.

• The solute becomes dispersed in the solvent.

• Solvents and solutes may be gases, liquids, or solids.



SolutionsSolutions


• Solutions are homogeneous mixtures.

• Solute particles can be atoms, ions, or molecules.

• If you filter a solution through filter paper, both the solute and solvent pass through the filter.



SolutionsSolutions

Substances that dissolve most readily in water include ionic compounds and polar covalent compounds.

• Nonpolar covalent compounds, such as methane, and compounds found in oil, grease, and gasoline, do not dissolve in water.

• However, oil and grease will dissolve in gasoline.




SolutionsSolutions

The Solution Process

• A water molecule is polar, with a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms.

• As individual solute ions break away from the crystal, the negatively and positively charged ions become surrounded by solvent molecules and the ionic crystal dissolves.



SolutionsSolutions

The process by which the positive and negative ions of an ionic solid become surrounded by solvent molecules is called solvation.

Solvated ions

Surface of ionic solid

The Solution Process



SolutionsSolutions

The Solution Process• Polar solvents such as

water dissolve ionic compounds and polar compounds.

• Nonpolar solvents such as gasoline dissolve nonpolar compounds.

• This relationship can be summed up in the expression “like dissolves like.”



Which of these compounds should not dissolve in water?

A.HCl

B.C4H10

C.KI

D.NH3

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