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

CHEMISTRY & YOU

How can you make a pickle glow?

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Solutions

What types of substances dissolve most readily in water?

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Solutions

What types of substances dissolve most readily in water?

An aqueous solution is water that contains dissolved substances.

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Solutions

Solvents and Solutes

In a solution, the dissolving medium is the solvent.
The dissolved particles in a solution are the solute.

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Solutions

Solvents and Solutes

A solvent dissolves the solute.
The solute becomes dispersed in the solvent.
Solvents and solutes may be gases, liquids, or solids.

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Solutions

Solvents and Solutes

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.

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Solutions

Solvents and Solutes

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

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Solutions

Solvents and Solutes

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.

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Solutions

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.

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Solutions

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

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Solutions

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.”

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Which of these compounds should not dissolve in water?

A. HCl

B. C₄H₁₀

C. KI

D. NH₃

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Which of these compounds should not dissolve in water?

A. HCl

B. C₄H₁₀

C. KI

D. NH₃

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Electrolytes and Nonelectrolytes

Why are all ionic compounds electrolytes?

Electrolytes and Nonelectrolytes

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Electrolytes and Nonelectrolytes

Why are all ionic compounds electrolytes?

Electrolytes and Nonelectrolytes

An electrolyte is a compound that conducts an electric current when it is in an aqueous solution or in the molten state.

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Electrolytes and Nonelectrolytes

All ionic compounds are electrolytes because they dissociate into ions.

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Electrolytes and Nonelectrolytes

Sodium chloride, a strong electrolyte, is nearly 100% dissociated into ions in water.

In order for the bulb to light, an electric current must flow between the two electrodes that are immersed in the solution.

To (+) electrode

To (–) electrode

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Electrolytes and Nonelectrolytes

Mercury(II) chloride, a weak electrolyte, is only partially dissociated in water.

To (+) electrode

To (–) electrode

In order for the bulb to light, an electric current must flow between the two electrodes that are immersed in the solution.

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Electrolytes and Nonelectrolytes

Glucose, a nonelectrolyte, does not dissociate in water.
A nonelectrolyte is a compound that does not conduct an electric current in either an aqueous solution or the molten state.

To (+) electrode

To (–) electrode

In order for the bulb to light, an electric current must flow between the two electrodes that are immersed in the solution.

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Electrolytes and Nonelectrolytes

Some polar molecular compounds are nonelectrolytes in the pure state but become electrolytes when they dissolve in water.

This change occurs because such compounds ionize in solution.

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Electrolytes and Nonelectrolytes

Some polar molecular compounds are nonelectrolytes in the pure state but become electrolytes when they dissolve in water.

For example, ammonia (NH₃(g)) is not an electrolyte in the pure state.
Yet an aqueous solution of ammonia conducts an electric current because ammonium ions (NH₄⁺) and hydroxide ions (OH^–) form when ammonia dissolves in water.

NH₃(g) + H₂O(l) → NH₄⁺(aq) + OH^–(aq)

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Electrolytes and Nonelectrolytes

Not all electrolytes conduct electric current to the same degree.

In a solution that contains a strong electrolyte, all or nearly all of the solute exists as ions.
A weak electrolyte conducts an electric current poorly because only a fraction of the solute in the solution exists as ions.

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Electrolytes and Nonelectrolytes

Your cells use electrolytes, such as sodium and potassium ions, to carry electrical impulses across themselves and to other cells.

An electrolyte imbalance can occur if you become dehydrated.
When you exercise, you can lose water and electrolytes from your body through perspiration.

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CHEMISTRY & YOU

Pickles contain table salt. Why can electric current flow through a pickle, causing it to glow?

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Electrolytes conduct an electric current when they are in an aqueous solution. Table salt, or NaCl, is a strong electrolyte. The water and salt in the pickle form a solution that conducts an electric current. The electric current causes the pickle to glow.

CHEMISTRY & YOU

Pickles contain table salt. Why can electric current flow through a pickle, causing it to glow?

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Explain why you must be extremely careful when using electricity near a swimming pool.

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Explain why you must be extremely careful when using electricity near a swimming pool.

The chlorinated water in a swimming pool is a solution that can conduct an electric current. If a current is introduced into the water, any swimmers could be shocked.

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Hydrates

Why do hydrates easily lose and regain water?

Hydrates

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Hydrates

The water contained in a crystal is called the water of hydration or water of crystallization.

A compound that contains water of hydration is called a hydrate.

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Hydrates

The forces holding the water molecules in hydrates are not very strong, so the water is easily lost and regained.

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Hydrates

The forces holding the water molecules in hydrates are not very strong, so the water is easily lost and regained.

A substance that is anhydrous does not contain water.

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Hydrates

Heating of a sample of blue CuSO₄·5H₂O begins.

After a time, much of the blue hydrate has been converted to white anhydrous CuSO₄.

CuSO₄·5H₂O(s) CuSO₄(s) + 5H₂O(g)

– heat

+ heat

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Hydrates

A piece of filter paper that has been dipped in an aqueous solution of cobalt(II) chloride and then dried is blue in color (anhydrous CoCl₂).
When the paper is exposed to moist air, it turns pink because of the formation of the hydrate cobalt(II) chloride hexahydrate (CoCl₂·6H₂O).

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Hydrates

Each hydrate contains a fixed quantity of water and has a definite composition.

Some Common Hydrates

Formula

Chemical name

Common name

MgSO₄·7H₂O

Magnesium sulfate heptahydrate

Epsom salt

Ba(OH)₂·8H₂O

Barium hydroxide octahydrate

CaCl₂·2H₂O

Calcium chloride dihydrate

CuSO₄·5H₂O

Copper(II) sulfate pentahydrate

Blue vitriol

Na₂SO₄·10H₂O

Sodium sulfate decahydrate

Glauber’s salt

KAl(SO₄)₂·12H₂O

Potassium aluminum sulfate dodecahydrate

Alum

Na₂B₄O₇·10H₂O

Sodium tetraborate decahydrate

Borax

FeSO₄·7H₂O

Iron(II) sulfate heptahydrate

Green vitriol

H₂SO₄·H₂O

Sulfuric acid hydrate (mp 8.6^oC)

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Hydrates

First determine the mass of water in one mole of hydrate.
Then determine the molar mass of the hydrate.
The percent by mass of water can be calculated using the following equation:

Percent by mass H₂O = × 100%

mass of water

mass of hydrate

To determine what percent by mass of a hydrate is water:

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Hydrates

If a hydrate has a vapor pressure higher than the pressure of water vapor in the air, the hydrate will lose its water of hydration, or effloresce.

Efflorescent Hydrates

The water molecules in hydrates are held by weak forces, so hydrates often have an appreciable vapor pressure.

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Hydrates

These hydrates and other compounds that remove moisture from air are called hygroscopic.

Hygroscopic Hydrates

Hydrated ionic compounds that have low vapor pressure remove water from moist air to form higher hydrates.

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Hydrates

Calcium chloride is used as a desiccant in the laboratory.
A desiccant is a substance used to absorb moisture from the air and create a dry atmosphere.

Hygroscopic Hydrates

Calcium chloride monohydrate spontaneously absorbs a second molecule of water when exposed to moist air.

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Calculate the percent by mass of water in washing soda, sodium carbonate decahydrate (Na₂CO₃·10H₂O).

Sample Problem 15.1

Finding the Percent by Mass of Water in a Hydrate

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KNOWN

formula of hydrate = Na₂CO₃·10H₂O

UNKNOWN

percent H₂O = ? %

Analyze List the known and the unknown.

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Sample Problem 15.1

To determine the percent by mass, divide the mass of water in one mole of the hydrate by the molar mass of the hydrate and multiply by 100%.

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Determine the mass of 10 mol of water.

Calculate Solve for the unknown.

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For every 1 mol of Na₂CO₃·10H₂O, there are 10 mol of H₂O.

Sample Problem 15.1

mass of 10 mol H₂O = 10[(2 × 1.0 g) + 16.0 g] = 180.0 g

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Determine the mass of 1 mol of the hydrated compound.

Calculate Solve for the unknown.

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Sample Problem 15.1

= (2 × 23.0 g) + 12.0 g + (3 × 16.0 g) + 180.0 g

= 286.0 g

molar mass of Na₂CO₃·10H₂O

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Calculate the percent by mass of water in the hydrate.

Calculate Solve for the unknown.

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Sample Problem 15.1

mass of water

mass of hydrate

percent by mass H₂O = × 100%

= 62.94%

180.0 g

286.0 g

= × 100%

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The mass of the water accounts for more than half the molar mass of the compound.
So, a percentage greater than 50% is expected.

Evaluate Does the result make sense?

3

Sample Problem 15.1

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Hydrates

These compounds are deliquescent, which means that they remove sufficient water from the air to dissolve completely and form solutions.

Deliquescent Compounds

Some compounds are so hygroscopic that they become wet when exposed to normally moist air.

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Hydrates

Pellets of sodium hydroxide are deliquescent.

For this reason, containers of NaOH should always be tightly stoppered.

The solution formed by a deliquescent substance has a lower vapor pressure than that of the water in the air.

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Calculate the percent by mass of water in epsom salt, magnesium sulfate heptahydrate (MgSO₄·7H₂O).

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Calculate the percent by mass of water in epsom salt, magnesium sulfate heptahydrate (MgSO₄·7H₂O).

= 24.3 g + 32.1 g + (4 × 16.0 g) + 126.0 g

= 246.4 g

molar mass of

MgSO₄·7H₂O

mass of H₂O = 7 × ( 2 × 1.0 g + 16.0 g) = 126.0 g

126.0 g

246.4 g

percent by mass H₂O = × 100% = 51.14%

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Key Concepts and Key Equation

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

All ionic compounds are electrolytes because they dissociate into ions.

The forces holding the water molecules in hydrates are not very strong, so the water is easily lost and regained.

mass of water

mass of hydrate

percent by mass H₂O = × 100%

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Glossary Terms

aqueous solution: water that contains dissolved substances
solvent: the dissolving medium in a solution
solute: dissolved particles in a solution
solvation: a process that occurs when an ionic solute dissolves; in solution, solvent molecules surround the positive and negative ions

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Glossary Terms

electrolyte: a compound that conducts an electric current when it is in an aqueous solution or in the molten state; all ionic compounds are electrolytes, but most covalent compounds are not
nonelectrolyte: a compound that does not conduct an electric current in aqueous solution or in the molten state
strong electrolyte: a solution in which a large portion of the solute exists as ions
weak electrolyte: a solution that conducts electricity poorly because only a fraction of the solute exists as ions

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Glossary Terms

water of hydration: water molecules that are an integral part of a crystal structure
hydrate: a compound that has a specific number of water molecules bound to each formula unit
anhydrous: a substance that does not contain water
effloresce: to lose water of hydration; the process occurs when the hydrate has a vapor pressure higher than that of water vapor in the air

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Glossary Terms

hygroscopic: a term describing salts and other compounds that remove moisture from the air
desiccant: a hygroscopic substance used as a drying agent
deliquescent: describes a substance that removes sufficient water from the air to form a solution; the solution formed has a lower vapor pressure than that of the water in the air

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Ionic compounds and polar covalent compounds dissolve most readily in water to form aqueous solutions.
Ionic compounds dissolve in water when the polar water molecules attract the ions of the solute, causing the individual solute ions to break away from the ionic crystal.

BIG IDEA

Bonding and Interactions

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