The Free High School Science Texts: Textbooks for High School Students Chemistry

The Free High School Science Texts: Textbooks for High School Students Chemistry
FHSST Authors
The Free High School Science Texts:
Textbooks for High School Students
Studying the Sciences
Chemistry
Grades 10 - 12
Version 0
November 9, 2008
ii
Copyright 2007 “Free High School Science Texts”
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FHSST Core Team
Mark Horner ; Samuel Halliday ; Sarah Blyth ; Rory Adams ; Spencer Wheaton
FHSST Editors
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Whitfield
FHSST Contributors
Rory Adams ; Prashant Arora ; Richard Baxter ; Dr. Sarah Blyth ; Sebastian Bodenstein ;
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Daniels ; Sean Dobbs ; Fernando Durrell ; Dr. Dan Dwyer ; Frans van Eeden ; Giovanni
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Andrew Kubik ; Dr. Marco van Leeuwen ; Dr. Anton Machacek ; Dr. Komal Maheshwari ;
Kosma von Maltitz ; Nicole Masureik ; John Mathew ; JoEllen McBride ; Nikolai Meures ;
Riana Meyer ; Jenny Miller ; Abdul Mirza ; Asogan Moodaly ; Jothi Moodley ; Nolene Naidu ;
Tyrone Negus ; Thomas O’Donnell ; Dr. Markus Oldenburg ; Dr. Jaynie Padayachee ;
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iii
iv
Contents
I
II
Introduction
1
Matter and Materials
3
1 Classification of Matter - Grade 10
1.1
1.2
5
Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
1.1.1
Heterogeneous mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.1.2
Homogeneous mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.1.3
Separating mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
Pure Substances: Elements and Compounds . . . . . . . . . . . . . . . . . . . .
9
1.2.1
Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
1.2.2
Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
1.3
Giving names and formulae to substances . . . . . . . . . . . . . . . . . . . . . 10
1.4
Metals, Semi-metals and Non-metals . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.1
Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.2
Non-metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.4.3
Semi-metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.5
Electrical conductors, semi-conductors and insulators . . . . . . . . . . . . . . . 14
1.6
Thermal Conductors and Insulators . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.7
Magnetic and Non-magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . 17
1.8
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2 What are the objects around us made of? - Grade 10
21
2.1
Introduction: The atom as the building block of matter . . . . . . . . . . . . . . 21
2.2
Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.1
Representing molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3
Intramolecular and intermolecular forces . . . . . . . . . . . . . . . . . . . . . . 25
2.4
The Kinetic Theory of Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.5
The Properties of Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.6
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3 The Atom - Grade 10
3.1
35
Models of the Atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.1
The Plum Pudding Model . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1.2
Rutherford’s model of the atom
v
. . . . . . . . . . . . . . . . . . . . . . 36
CONTENTS
3.1.3
3.2
3.3
CONTENTS
The Bohr Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
How big is an atom? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.1
How heavy is an atom? . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.2
How big is an atom? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Atomic structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.3.1
The Electron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.3.2
The Nucleus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4
Atomic number and atomic mass number . . . . . . . . . . . . . . . . . . . . . 40
3.5
Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.6
3.7
3.8
3.9
3.5.1
What is an isotope? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.5.2
Relative atomic mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Energy quantisation and electron configuration . . . . . . . . . . . . . . . . . . 46
3.6.1
The energy of electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.6.2
Energy quantisation and line emission spectra . . . . . . . . . . . . . . . 47
3.6.3
Electron configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.6.4
Core and valence electrons . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.6.5
The importance of understanding electron configuration . . . . . . . . . 51
Ionisation Energy and the Periodic Table . . . . . . . . . . . . . . . . . . . . . . 53
3.7.1
Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.7.2
Ionisation Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
The Arrangement of Atoms in the Periodic Table . . . . . . . . . . . . . . . . . 56
3.8.1
Groups in the periodic table
. . . . . . . . . . . . . . . . . . . . . . . . 56
3.8.2
Periods in the periodic table . . . . . . . . . . . . . . . . . . . . . . . . 58
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4 Atomic Combinations - Grade 11
63
4.1
Why do atoms bond? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.2
Energy and bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.3
What happens when atoms bond? . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.4
Covalent Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.4.1
The nature of the covalent bond . . . . . . . . . . . . . . . . . . . . . . 65
4.5
Lewis notation and molecular structure . . . . . . . . . . . . . . . . . . . . . . . 69
4.6
Electronegativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.7
4.8
4.6.1
Non-polar and polar covalent bonds . . . . . . . . . . . . . . . . . . . . 73
4.6.2
Polar molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Ionic Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.7.1
The nature of the ionic bond . . . . . . . . . . . . . . . . . . . . . . . . 74
4.7.2
The crystal lattice structure of ionic compounds . . . . . . . . . . . . . . 76
4.7.3
Properties of Ionic Compounds . . . . . . . . . . . . . . . . . . . . . . . 76
Metallic bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.8.1
The nature of the metallic bond . . . . . . . . . . . . . . . . . . . . . . 76
4.8.2
The properties of metals . . . . . . . . . . . . . . . . . . . . . . . . . . 77
vi
CONTENTS
4.9
CONTENTS
Writing chemical formulae
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.9.1
The formulae of covalent compounds . . . . . . . . . . . . . . . . . . . . 78
4.9.2
The formulae of ionic compounds . . . . . . . . . . . . . . . . . . . . . 80
4.10 The Shape of Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.10.1 Valence Shell Electron Pair Repulsion (VSEPR) theory . . . . . . . . . . 82
4.10.2 Determining the shape of a molecule . . . . . . . . . . . . . . . . . . . . 82
4.11 Oxidation numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
5 Intermolecular Forces - Grade 11
91
5.1
Types of Intermolecular Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.2
Understanding intermolecular forces . . . . . . . . . . . . . . . . . . . . . . . . 94
5.3
Intermolecular forces in liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.4
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6 Solutions and solubility - Grade 11
101
6.1
Types of solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6.2
Forces and solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.3
Solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.4
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7 Atomic Nuclei - Grade 11
107
7.1
Nuclear structure and stability . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.2
The Discovery of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.3
Radioactivity and Types of Radiation . . . . . . . . . . . . . . . . . . . . . . . . 108
7.4
7.3.1
Alpha (α) particles and alpha decay . . . . . . . . . . . . . . . . . . . . 109
7.3.2
Beta (β) particles and beta decay . . . . . . . . . . . . . . . . . . . . . 109
7.3.3
Gamma (γ) rays and gamma decay . . . . . . . . . . . . . . . . . . . . . 110
Sources of radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
7.4.1
Natural background radiation . . . . . . . . . . . . . . . . . . . . . . . . 112
7.4.2
Man-made sources of radiation . . . . . . . . . . . . . . . . . . . . . . . 113
7.5
The ’half-life’ of an element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
7.6
The Dangers of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7.7
The Uses of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.8
Nuclear Fission
7.9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
7.8.1
The Atomic bomb - an abuse of nuclear fission . . . . . . . . . . . . . . 119
7.8.2
Nuclear power - harnessing energy . . . . . . . . . . . . . . . . . . . . . 120
Nuclear Fusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
7.10 Nucleosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
7.10.1 Age of Nucleosynthesis (225 s - 103 s) . . . . . . . . . . . . . . . . . . . 121
7.10.2 Age of Ions (103 s - 1013 s) . . . . . . . . . . . . . . . . . . . . . . . . . 122
7.10.3 Age of Atoms (1013 s - 1015 s) . . . . . . . . . . . . . . . . . . . . . . . 122
7.10.4 Age of Stars and Galaxies (the universe today) . . . . . . . . . . . . . . 122
7.11 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
vii
CONTENTS
CONTENTS
8 Thermal Properties and Ideal Gases - Grade 11
125
8.1
A review of the kinetic theory of matter . . . . . . . . . . . . . . . . . . . . . . 125
8.2
Boyle’s Law: Pressure and volume of an enclosed gas . . . . . . . . . . . . . . . 126
8.3
Charles’s Law: Volume and Temperature of an enclosed gas . . . . . . . . . . . 132
8.4
The relationship between temperature and pressure . . . . . . . . . . . . . . . . 136
8.5
The general gas equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
8.6
The ideal gas equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
8.7
Molar volume of gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
8.8
Ideal gases and non-ideal gas behaviour . . . . . . . . . . . . . . . . . . . . . . 146
8.9
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
9 Organic Molecules - Grade 12
151
9.1
What is organic chemistry? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
9.2
Sources of carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
9.3
Unique properties of carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
9.4
Representing organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . 152
9.4.1
Molecular formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
9.4.2
Structural formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
9.4.3
Condensed structural formula . . . . . . . . . . . . . . . . . . . . . . . . 153
9.5
Isomerism in organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . 154
9.6
Functional groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
9.7
The Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
9.7.1
The Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
9.7.2
Naming the alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
9.7.3
Properties of the alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . 163
9.7.4
Reactions of the alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . 163
9.7.5
The alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
9.7.6
Naming the alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
9.7.7
The properties of the alkenes . . . . . . . . . . . . . . . . . . . . . . . . 169
9.7.8
Reactions of the alkenes
9.7.9
The Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
. . . . . . . . . . . . . . . . . . . . . . . . . . 169
9.7.10 Naming the alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
9.8
9.9
The Alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
9.8.1
Naming the alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
9.8.2
Physical and chemical properties of the alcohols . . . . . . . . . . . . . . 175
Carboxylic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
9.9.1
Physical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
9.9.2
Derivatives of carboxylic acids: The esters . . . . . . . . . . . . . . . . . 178
9.10 The Amino Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
9.11 The Carbonyl Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
9.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
viii
CONTENTS
CONTENTS
10 Organic Macromolecules - Grade 12
185
10.1 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
10.2 How do polymers form? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
10.2.1 Addition polymerisation . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
10.2.2 Condensation polymerisation . . . . . . . . . . . . . . . . . . . . . . . . 188
10.3 The chemical properties of polymers . . . . . . . . . . . . . . . . . . . . . . . . 190
10.4 Types of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
10.5 Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
10.5.1 The uses of plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
10.5.2 Thermoplastics and thermosetting plastics . . . . . . . . . . . . . . . . . 194
10.5.3 Plastics and the environment . . . . . . . . . . . . . . . . . . . . . . . . 195
10.6 Biological Macromolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
10.6.1 Carbohydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
10.6.2 Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
10.6.3 Nucleic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
10.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
III
Chemical Change
209
11 Physical and Chemical Change - Grade 10
211
11.1 Physical changes in matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
11.2 Chemical Changes in Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
11.2.1 Decomposition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 213
11.2.2 Synthesis reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
11.3 Energy changes in chemical reactions . . . . . . . . . . . . . . . . . . . . . . . . 217
11.4 Conservation of atoms and mass in reactions . . . . . . . . . . . . . . . . . . . . 217
11.5 Law of constant composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
11.6 Volume relationships in gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
11.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
12 Representing Chemical Change - Grade 10
223
12.1 Chemical symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
12.2 Writing chemical formulae
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
12.3 Balancing chemical equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
12.3.1 The law of conservation of mass . . . . . . . . . . . . . . . . . . . . . . 224
12.3.2 Steps to balance a chemical equation
. . . . . . . . . . . . . . . . . . . 226
12.4 State symbols and other information . . . . . . . . . . . . . . . . . . . . . . . . 230
12.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
13 Quantitative Aspects of Chemical Change - Grade 11
233
13.1 The Mole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
13.2 Molar Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
13.3 An equation to calculate moles and mass in chemical reactions . . . . . . . . . . 237
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13.4 Molecules and compounds
CONTENTS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
13.5 The Composition of Substances . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
13.6 Molar Volumes of Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
13.7 Molar concentrations in liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
13.8 Stoichiometric calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
13.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
14 Energy Changes In Chemical Reactions - Grade 11
255
14.1 What causes the energy changes in chemical reactions? . . . . . . . . . . . . . . 255
14.2 Exothermic and endothermic reactions . . . . . . . . . . . . . . . . . . . . . . . 255
14.3 The heat of reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
14.4 Examples of endothermic and exothermic reactions . . . . . . . . . . . . . . . . 259
14.5 Spontaneous and non-spontaneous reactions . . . . . . . . . . . . . . . . . . . . 260
14.6 Activation energy and the activated complex . . . . . . . . . . . . . . . . . . . . 261
14.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
15 Types of Reactions - Grade 11
267
15.1 Acid-base reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
15.1.1 What are acids and bases? . . . . . . . . . . . . . . . . . . . . . . . . . 267
15.1.2 Defining acids and bases . . . . . . . . . . . . . . . . . . . . . . . . . . 267
15.1.3 Conjugate acid-base pairs . . . . . . . . . . . . . . . . . . . . . . . . . . 269
15.1.4 Acid-base reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
15.1.5 Acid-carbonate reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 274
15.2 Redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
15.2.1 Oxidation and reduction
. . . . . . . . . . . . . . . . . . . . . . . . . . 277
15.2.2 Redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
15.3 Addition, substitution and elimination reactions . . . . . . . . . . . . . . . . . . 280
15.3.1 Addition reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
15.3.2 Elimination reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
15.3.3 Substitution reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282
15.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
16 Reaction Rates - Grade 12
287
16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
16.2 Factors affecting reaction rates . . . . . . . . . . . . . . . . . . . . . . . . . . . 289
16.3 Reaction rates and collision theory . . . . . . . . . . . . . . . . . . . . . . . . . 293
16.4 Measuring Rates of Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
16.5 Mechanism of reaction and catalysis . . . . . . . . . . . . . . . . . . . . . . . . 297
16.6 Chemical equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
16.6.1 Open and closed systems . . . . . . . . . . . . . . . . . . . . . . . . . . 302
16.6.2 Reversible reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302
16.6.3 Chemical equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
16.7 The equilibrium constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
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CONTENTS
16.7.1 Calculating the equilibrium constant . . . . . . . . . . . . . . . . . . . . 305
16.7.2 The meaning of kc values . . . . . . . . . . . . . . . . . . . . . . . . . . 306
16.8 Le Chatelier’s principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
16.8.1 The effect of concentration on equilibrium . . . . . . . . . . . . . . . . . 310
16.8.2 The effect of temperature on equilibrium . . . . . . . . . . . . . . . . . . 310
16.8.3 The effect of pressure on equilibrium . . . . . . . . . . . . . . . . . . . . 312
16.9 Industrial applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
16.10Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
17 Electrochemical Reactions - Grade 12
319
17.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
17.2 The Galvanic Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320
17.2.1 Half-cell reactions in the Zn-Cu cell . . . . . . . . . . . . . . . . . . . . 321
17.2.2 Components of the Zn-Cu cell . . . . . . . . . . . . . . . . . . . . . . . 322
17.2.3 The Galvanic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323
17.2.4 Uses and applications of the galvanic cell . . . . . . . . . . . . . . . . . 324
17.3 The Electrolytic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
17.3.1 The electrolysis of copper sulphate . . . . . . . . . . . . . . . . . . . . . 326
17.3.2 The electrolysis of water . . . . . . . . . . . . . . . . . . . . . . . . . . 327
17.3.3 A comparison of galvanic and electrolytic cells . . . . . . . . . . . . . . . 328
17.4 Standard Electrode Potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
17.4.1 The different reactivities of metals . . . . . . . . . . . . . . . . . . . . . 329
17.4.2 Equilibrium reactions in half cells . . . . . . . . . . . . . . . . . . . . . . 329
17.4.3 Measuring electrode potential . . . . . . . . . . . . . . . . . . . . . . . . 330
17.4.4 The standard hydrogen electrode . . . . . . . . . . . . . . . . . . . . . . 330
17.4.5 Standard electrode potentials . . . . . . . . . . . . . . . . . . . . . . . . 333
17.4.6 Combining half cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337
17.4.7 Uses of standard electrode potential . . . . . . . . . . . . . . . . . . . . 338
17.5 Balancing redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342
17.6 Applications of electrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . 347
17.6.1 Electroplating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
17.6.2 The production of chlorine . . . . . . . . . . . . . . . . . . . . . . . . . 348
17.6.3 Extraction of aluminium
. . . . . . . . . . . . . . . . . . . . . . . . . . 349
17.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
IV
Chemical Systems
353
18 The Water Cycle - Grade 10
355
18.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
18.2 The importance of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
18.3 The movement of water through the water cycle . . . . . . . . . . . . . . . . . . 356
18.4 The microscopic structure of water . . . . . . . . . . . . . . . . . . . . . . . . . 359
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18.4.1 The polar nature of water . . . . . . . . . . . . . . . . . . . . . . . . . . 359
18.4.2 Hydrogen bonding in water molecules . . . . . . . . . . . . . . . . . . . 359
18.5 The unique properties of water . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
18.6 Water conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363
18.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366
19 Global Cycles: The Nitrogen Cycle - Grade 10
369
19.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
19.2 Nitrogen fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
19.3 Nitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
19.4 Denitrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372
19.5 Human Influences on the Nitrogen Cycle . . . . . . . . . . . . . . . . . . . . . . 372
19.6 The industrial fixation of nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . 373
19.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
20 The Hydrosphere - Grade 10
377
20.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
20.2 Interactions of the hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
20.3 Exploring the Hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
20.4 The Importance of the Hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . 379
20.5 Ions in aqueous solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379
20.5.1 Dissociation in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
20.5.2 Ions and water hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . 382
20.5.3 The pH scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382
20.5.4 Acid rain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
20.6 Electrolytes, ionisation and conductivity . . . . . . . . . . . . . . . . . . . . . . 386
20.6.1 Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386
20.6.2 Non-electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
20.6.3 Factors that affect the conductivity of water . . . . . . . . . . . . . . . . 387
20.7 Precipitation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
20.8 Testing for common anions in solution . . . . . . . . . . . . . . . . . . . . . . . 391
20.8.1 Test for a chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
20.8.2 Test for a sulphate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
20.8.3 Test for a carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392
20.8.4 Test for bromides and iodides . . . . . . . . . . . . . . . . . . . . . . . . 392
20.9 Threats to the Hydrosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393
20.10Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394
21 The Lithosphere - Grade 11
397
21.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397
21.2 The chemistry of the earth’s crust . . . . . . . . . . . . . . . . . . . . . . . . . 398
21.3 A brief history of mineral use . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399
21.4 Energy resources and their uses . . . . . . . . . . . . . . . . . . . . . . . . . . . 400
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21.5 Mining and Mineral Processing: Gold . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.2 Mining the Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.3 Processing the gold ore . . . . . . . . . . . . . . . . . . . . . . . . . . . 401
21.5.4 Characteristics and uses of gold . . . . . . . . . . . . . . . . . . . . . . . 402
21.5.5 Environmental impacts of gold mining . . . . . . . . . . . . . . . . . . . 404
21.6 Mining and mineral processing: Iron . . . . . . . . . . . . . . . . . . . . . . . . 406
21.6.1 Iron mining and iron ore processing . . . . . . . . . . . . . . . . . . . . . 406
21.6.2 Types of iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
21.6.3 Iron in South Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408
21.7 Mining and mineral processing: Phosphates . . . . . . . . . . . . . . . . . . . . 409
21.7.1 Mining phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409
21.7.2 Uses of phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409
21.8 Energy resources and their uses: Coal . . . . . . . . . . . . . . . . . . . . . . . 411
21.8.1 The formation of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411
21.8.2 How coal is removed from the ground . . . . . . . . . . . . . . . . . . . 411
21.8.3 The uses of coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
21.8.4 Coal and the South African economy . . . . . . . . . . . . . . . . . . . . 412
21.8.5 The environmental impacts of coal mining . . . . . . . . . . . . . . . . . 413
21.9 Energy resources and their uses: Oil . . . . . . . . . . . . . . . . . . . . . . . . 414
21.9.1 How oil is formed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414
21.9.2 Extracting oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414
21.9.3 Other oil products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415
21.9.4 The environmental impacts of oil extraction and use . . . . . . . . . . . 415
21.10Alternative energy resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415
21.11Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417
22 The Atmosphere - Grade 11
421
22.1 The composition of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . 421
22.2 The structure of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.2.1 The troposphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.2.2 The stratosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.2.3 The mesosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
22.2.4 The thermosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
22.3 Greenhouse gases and global warming . . . . . . . . . . . . . . . . . . . . . . . 426
22.3.1 The heating of the atmosphere . . . . . . . . . . . . . . . . . . . . . . . 426
22.3.2 The greenhouse gases and global warming . . . . . . . . . . . . . . . . . 426
22.3.3 The consequences of global warming . . . . . . . . . . . . . . . . . . . . 429
22.3.4 Taking action to combat global warming . . . . . . . . . . . . . . . . . . 430
22.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431
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23 The Chemical Industry - Grade 12
435
23.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
23.2 Sasol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
23.2.1 Sasol today: Technology and production . . . . . . . . . . . . . . . . . . 436
23.2.2 Sasol and the environment . . . . . . . . . . . . . . . . . . . . . . . . . 440
23.3 The Chloralkali Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442
23.3.1 The Industrial Production of Chlorine and Sodium Hydroxide . . . . . . . 442
23.3.2 Soaps and Detergents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446
23.4 The Fertiliser Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
23.4.1 The value of nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
23.4.2 The Role of fertilisers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450
23.4.3 The Industrial Production of Fertilisers . . . . . . . . . . . . . . . . . . . 451
23.4.4 Fertilisers and the Environment: Eutrophication . . . . . . . . . . . . . . 454
23.5 Electrochemistry and batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . 456
23.5.1 How batteries work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456
23.5.2 Battery capacity and energy . . . . . . . . . . . . . . . . . . . . . . . . 457
23.5.3 Lead-acid batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457
23.5.4 The zinc-carbon dry cell . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
23.5.5 Environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . 460
23.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461
A GNU Free Documentation License
467
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Chapter 20
The Hydrosphere - Grade 10
20.1
Introduction
As far as we know, the Earth we live on is the only planet that is able to support life. Among
other things, Earth is just the right distance from the sun to have temperatures that are suitable
for life to exist. Also, the Earth’s atmosphere has exactly the right type of gases in the right
amounts for life to survive. Our planet also has water on its surface, which is something very
unique. In fact, Earth is often called the ’Blue Planet’ because most of it is covered in water.
This water is made up of freshwater in rivers and lakes, the saltwater of the oceans and estuaries,
groundwater and water vapour. Together, all these water bodies are called the hydrosphere.
20.2
Interactions of the hydrosphere
It is important to realise that the hydrosphere interacts with other global systems, including the
atmosphere, lithosphere and biosphere.
• Atmosphere
When water is heated (e.g. by energy from the sun), it evaporates and forms water vapour.
When water vapour cools again, it condenses to form liquid water which eventually returns
to the surface by precipitation e.g. rain or snow. This cycle of water moving through the
atmosphere, and the energy changes that accompany it, is what drives weather patterns
on earth.
• Lithosphere
In the lithosphere (the ocean and continental crust at the Earth’s surface), water is an
important weathering agent, which means that it helps to break rock down into rock
fragments and then soil. These fragments may then be transported by water to another
place, where they are deposited. This is called erosion. These two process i.e. weathering
and erosion, help to shape the earth’s surface. You can see this for example in rivers.
In the upper streams, rocks are eroded and sediments are transported down the river and
deposited on the wide flood plains lower down. On a bigger scale, river valleys in mountains
have been carved out by the action of water, and cliffs and caves on rocky beach coastlines,
are also the result of weathering and erosion by water.
• Biosphere
In the biosphere, land plants absorb water through their roots and then transport this
through their vascular (transport) system to stems and leaves. This water is needed in
photosynthesis, the food production process in plants. Transpiration (evaporation of water
from the leaf surface) then returns water back to the atmosphere.
377
20.3
20.3
CHAPTER 20. THE HYDROSPHERE - GRADE 10
Exploring the Hydrosphere
The large amount of water on our planet is something quite unique. In fact, about 71% of
the earth is covered by water. Of this, almost 97% is found in the oceans as saltwater, about
2.2% occurs as a solid in ice sheets, while the remaining amount (less than 1%) is available as
freshwater. So from a human perspective, despite the vast amount of water on the planet, only a
very small amount is actually available for human consumption (e.g. drinking water). Before we
go on to look more closely at the chemistry of the hydrosphere, we are going to spend some time
exploring a part of the hydrosphere, in order to start appreciating what a complex and beautiful
part of the world it is.
Activity :: Investigation : Investigating the hydrosphere
1. Choosing a study site:
For this exercise, you can choose any part of the hydrosphere that you would
like to explore. This may be a rock pool, a lake, river, wetland or even just a
small pond. The guidelines below will apply best to a river investigation, but
you can ask similar questions and gather similar data in other areas. When
choosing your study site, consider how accessible it is (how easy is it to get
to?) and the problems you may experience (e.g. tides, rain).
2. Collecting data:
Your teacher will provide you with the equipment you need to collect the following data. You should have at least one study site where you will collect
data, but you might decide to have more if you want to compare your results
in different areas. This works best in a river, where you can choose sites down
its length.
(a) Chemical data
Measure and record data such as temperature, pH, conductivity and dissolved oxygen at each of your sites. You may not know exactly what
these measurements mean right now, but it will become clearer later in the
chapter.
(b) Hydrological data
Measure the water velocity of the river and observe how the volume of water
in the river changes as you move down its length. You can also collect a
water sample in a clear bottle, hold it to the light and see whether the
water is clear or whether it has particles in it.
(c) Biological data
What types of animals and plants are found in or near this part of the
hydrosphere? Are they specially adapted to their environment?
Record your data in a table like the one shown below:
Site 1
Site 2
Site 3
Temperature
pH
Conductivity
Dissolved oxygen
Animals and plants
3. Interpreting the data:
Once you have collected and recorded your data, think about the following
questions:
• How does the data you have collected vary at different sites?
• Can you explain these differences?
• What effect do you think temperature, dissolved oxygen and pH have on
animals and plants that are living in the hydrosphere?
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CHAPTER 20. THE HYDROSPHERE - GRADE 10
20.4
• Water is seldom ’pure’. It usually has lots of things dissolved (e.g. Mg,
Ca and NO−
3 ions) or suspended (e.g. soil particles, debris) in it. Where
do these substances come from?
• Are there any human activities near this part of the hydrosphere? What
effect could these activities have on the hydrosphere?
20.4
The Importance of the Hydrosphere
It is so easy sometimes to take our hydrosphere for granted, and we seldom take the time to
really think about the role that this part of the planet plays in keeping us alive. Below are just
some of the very important functions of water in the hydrosphere:
• Water is a part of living cells
Each cell in a living organism is made up of almost 75% water, and this allows the cell
to function normally. In fact, most of the chemical reactions that occur in life, involve
substances that are dissolved in water. Without water, cells would not be able to carry
out their normal functions, and life could not exist.
• Water provides a habitat
The hydrosphere provides an important place for many animals and plants to live. Many
−
+
gases (e.g. CO2 , O2 ), nutrients e.g. nitrate (NO−
3 ), nitrite (NO2 ) and ammonium (NH4 )
2+
2+
ions, as well as other ions (e.g. Ca and Mg ) are dissolved in water. The presence of
these substances is critical for life to exist in water.
• Regulating climate
You may remember from chapter ?? that one of water’s unique characteristics is its high
specific heat. This means that water takes a long time to heat up, and also a long time
to cool down. This is important in helping to regulate temperatures on earth so that they
stay within a range that is acceptable for life to exist. Ocean currents also help to disperse
heat.
• Human needs
Humans use water in a number of ways. Drinking water is obviously very important, but
water is also used domestically (e.g. washing and cleaning) and in industry. Water can
also be used to generate electricity through hydropower.
These are just a few of the very important functions that water plays on our planet. Many of the
functions of water relate to its chemistry and to the way in which it is able to dissolve substances
in it.
20.5
Ions in aqueous solution
As we mentioned earlier, water is seldom pure. Because of the structure of the water molecule,
it is able to dissolve substances in it. This is very important because if water wasn’t able to do
this, life would not be able to survive. In rivers and the oceans for example, dissolved oxygen
means that organisms are still able to respire (breathe). For plants, dissolved nutrients are also
available. In the human body, water is able to carry dissolved substances from one part of the
body to another.
Many of the substances that dissolve are ionic, and when they dissolve they form ions in solution.
We are going to look at how water is able to dissolve ionic compounds, and how these ions
maintain a balance in the human body, how they affect water hardness, and how specific ions
determine the pH of solutions.
379
20.5
20.5.1
CHAPTER 20. THE HYDROSPHERE - GRADE 10
Dissociation in water
You may remember from chapter 5 that water is a polar molecule (figure 20.1). This means
that one part of the molecule has a slightly positive charge and the other part has a slightly
negative charge.
δ+
H2 O
δ−
Figure 20.1: Water is a polar molecule
It is the polar nature of water that allows ionic compounds to dissolve in it. In the case of
sodium chloride (NaCl) for example, the positive sodium ions (Na+ ) will be attracted to the
negative pole of the water molecule, while the negative chloride ions (Cl− ) will be attracted to
the positive pole of the water molecule. In the process, the ionic bonds between the sodium
and chloride ions are weakened and the water molecules are able to work their way between the
individual ions, surrounding them and slowly dissolving the compound. This process is called
dissociation. A simplified representation of this is shown in figure 20.2.
Definition: Dissociation
Dissociation in chemistry and biochemistry is a general process in which ionic compounds
separate or split into smaller molecules or ions, usually in a reversible manner.
Cl−
δ+
H2 O
δ−
Cl− δ + H2 O δ − N a+ δ − H2 O δ + Cl−
δ−
H2 O
δ+
Cl−
Figure 20.2: Sodium chloride dissolves in water
The dissolution of sodium chloride can be represented by the following equation:
N aCl(s) → N a+ (aq) + Cl− (aq)
The symbols s (solid), l (liquid), g (gas) and aq (material is dissolved in water) are written after
the chemical formula to show the state or phase of the material. The dissolution of potassium
sulphate into potassium and sulphate ions is shown below as another example:
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CHAPTER 20. THE HYDROSPHERE - GRADE 10
20.5
K2 SO4 (s) → 2K + (aq) + SO42− (aq)
Remember that molecular substances (e.g. covalent compounds) may also dissolve, but most
will not form ions. One example is sugar.
C6 H12 O6 (s) ⇔ C6 H12 O6 (aq)
There are exceptions to this and some molecular substances will form ions when they dissolve.
Hydrogen chloride for example can ionise to form hydrogen and chloride ions.
HCl(g) → H + (aq) + Cl− (aq)
teresting The ability of ionic compounds to dissolve in water is extremely important in
Interesting
Fact
Fact
the human body! The body is made up of cells, each of which is surrounded
by a membrane. Dissolved ions are found inside and outside of body cells, in
different concentrations. Some of these ions are positive (e.g. Mg2+ ) and some
are negative (e.g. Cl− ). If there is a difference in the charge that is inside and
outside the cell, then there is a potential difference across the cell membrane.
This is called the membrane potential of the cell. The membrane potential
acts like a battery and affects the movement of all charged substances across the
membrane. Membrane potentials play a role in muscle functioning, digestion,
excretion and in maintaining blood pH, to name just a few. The movement of
ions across the membrane can also be converted into an electric signal that can
be transferred along neurons (nerve cells), which control body processes. If ionic
substances were not able to dissociate in water, then none of these processes
would be possible! It is also important to realise that our bodies can lose ions
such as Na+ , K+ , Ca2+ , Mg2+ , and Cl− , for example when we sweat during
exercise. Sports drinks such as Lucozade and Powerade are designed to replace
these lost ions so that the body’s normal functioning is not affected.
Exercise: Ions in solution
1. For each of the following, say whether the substance is ionic or molecular.
(a)
(b)
(c)
(d)
potassium nitrate (KNO3 )
ethanol (C2 H5 OH)
sucrose sugar (C12 H22 O11
sodium bromide (NaBr)
2. Write a balanced equation to show how each of the following ionic compounds
dissociate in water.
(a)
(b)
(c)
(d)
sodium sulphate (Na2 SO4 )
potassium bromide (KBr)
potassium permanganate (KMNO4 )
sodium phosphate (Na3 PO4 )
381
20.5
20.5.2
CHAPTER 20. THE HYDROSPHERE - GRADE 10
Ions and water hardness
Definition: Water hardness
Water hardness is a measure of the mineral content of water. Minerals are substances such
as calcite, quartz and mica that occur naturally as a result of geological processes.
Hard water is water that has a high mineral content. Water that has a low mineral content
is known as soft water. If water has a high mineral content, it usually contains high levels of
metal ions, mainly calcium (Ca) and magnesium (Mg). The calcium enters the water from either
CaCO3 (limestone or chalk) or from mineral deposits of CaSO4 . The main source of magnesium
is a sedimentary rock called dolomite, CaMg(CO3 )2 . Hard water may also contain other metals
as well as bicarbonates and sulphates.
teresting The simplest way to check whether water is hard or soft is to use the lather/froth
Interesting
Fact
Fact
test. If the water is very soft, soap will lather more easily when it is rubbed
against the skin. With hard water this won’t happen. Toothpaste will also not
froth well in hard water.
A water softener works on the principle of ion exchange. Hard water passes through a media
bed, usually made of resin beads that are supersaturated with sodium. As the water passes
through the beads, the hardness minerals (e.g. calcium and magnesium) attach themselves to
the beads. The sodium that was originally on the beads is released into the water. When the
resin becomes saturated with calcium and magnesium, it must be recharged. A salt solution is
passed through the resin. The sodium replaces the calcium and magnesium, and these ions are
released into the waste water and discharged.
20.5.3
The pH scale
The concentration of specific ions in solution, affects whether the solution is acidic or basic. You
will learn about acids and bases in chapter 15. Acids and bases can be described as substances
that either increase or decrease the concentration of hydrogen (H+ or H3 O+) ions in a solution.
An acid increases the hydrogen ion concentration in a solution, while a base decreases the
hydrogen ion concentration. pH is used to measure whether a substance is acidic or basic
(alkaline).
Definition: pH
pH is a measure of the acidity or alkalinity of a solution. The pH scale ranges from 0 to 14.
Solutions with a pH less than seven are acidic, while those with a pH greater than seven
are basic (alkaline). pH 7 is considered neutral.
pH can be calculated using the following equation:
pH = −log[H + ]
or
pH = −log[H3 O+ ]
The brackets in the above equation are used to show concentration in mol.dm−3 .
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CHAPTER 20. THE HYDROSPHERE - GRADE 10
20.5
Worked Example 93: pH calculations
Question: Calculate the pH of a solution where the concentration of hydrogen ions
is 1 × 10−7 mol.dm−3 .
Answer
Step 1 : Determine the concentration of hydrogen ions in mol.dm−3
In this example, the concentration has been given and is 1 × 10−7 mol.dm−3
Step 2 : Substitute this value into the pH equation and calculate the pH
value
pH = -log[H+]
= -log(1 × 10−7 )
=7
Worked Example 94: pH calculations
Question: In a solution of ethanoic acid, the following equilibrium is established:
CH3 COOH(aq) + H2 O ⇔ CH3 COO− (aq) + H3 O+
The concentration of CH3 COO− ions is found to be 0.003 mol.dm−3 . Calculate the
pH of the solution.
Answer
Step 1 : Determine the concentration of hydrogen ions in the solution
According to the balanced equation for this reaction, the mole ratio of CH3 COO−
ions to H3 O+ ions is the same, therefore the concentration of these two ions in the
solution will also be the same. So, [H3 O+ ] = 0.003 dm−3 .
Step 2 : Substitute this value into the pH equation and calculate the pH
value
pH = -log[H3O+ ]
= -log(0.003)
= 2.52
Understanding pH is very important. In living organisms, it is necessary to maintain a constant
pH so that chemical reactions can occur under optimal conditions.
Important: It may also be useful for calculations involving the pH scale, to know that the
following equation can also be used:
[H3 O+ ][OH− ] = 1 × 10−14
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20.5
CHAPTER 20. THE HYDROSPHERE - GRADE 10
teresting A build up of acid in the human body can be very dangerous. Lactic acidosis
Interesting
Fact
Fact
is a condition caused by the buildup of lactic acid in the body. It leads to
acidification of the blood (acidosis) and can make a person very ill. Some of
the symptoms of lactic acidosis are deep and rapid breathing, vomiting, and
abdominal pain. In the fight against HIV, lactic acidosis is a problem. One of
the antiretrovirals (ARV’s) that is used in anti-HIV treatment is Stavudine (also
known as Zerit or d4T). One of the side effects of Stavudine is lactic acidosis,
particularly in overweight women. If it is not treated quickly, it can result in
death.
In agriculture, farmers need to know the pH of their soils so that they are able to plant the
right kinds of crops. The pH of soils can vary depending on a number of factors such as
rainwater, the kinds of rocks and materials from which the soil was formed and also human
influences such as pollution and fertilisers. The pH of rain water can also vary and this too has
an effect on agriculture, buildings, water courses, animals and plants. Rainwater is naturally
acidic because carbon dioxide in the atmosphere combines with water to form carbonic acid.
Unpolluted rainwater has a pH of approximately 5.6. However, human activities can alter the
acidity of rain and this can cause serious problems such as acid rain.
Exercise: Calculating pH
1. Calculate the pH of each of the following solutions:
(a) A 0.2 mol.dm−3 KOH solution
(b) A 0.5 mol.dm−3 HCl solution
2. What is the concentration (in mol.dm−3 ) of H3 O+ ions in a NaOH solution
which has a pH of 12?
3. The concentrations of hydronium and hydroxyl ions in a typical sample of
seawater are 10−8 mol.dm−3 and 10−6 mol.dm−3 respectively.
(a) Is the seawater acidic or basic?
(b) What is the pH of the seawater?
(c) Give a possible explanation for the pH of the seawater.
(IEB Paper 2, 2002)
20.5.4
Acid rain
The acidity of rainwater comes from the natural presence of three substances (CO2 , NO, and
SO2 ) in the lowest layer of the atmosphere. These gases are able to dissolve in water and
therefore make rain more acidic than it would otherwise be. Of these gases, carbon dioxide
(CO2 ) has the highest concentration and therefore contributes the most to the natural acidity
of rainwater. We will look at each of these gases in turn.
Definition: Acid rain
Acid rain refers to the deposition of acidic components in rain, snow and dew. Acid rain
occurs when sulfur dioxide and nitrogen oxides are emitted into the atmosphere, undergo
chemical transformations, and are absorbed by water droplets in clouds. The droplets then
fall to earth as rain, snow, mist, dry dust, hail, or sleet. This increases the acidity of the
soil, and affects the chemical balance of lakes and streams.
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CHAPTER 20. THE HYDROSPHERE - GRADE 10
20.5
1. Carbon dioxide
Carbon dioxide reacts with water in the atmosphere to form carbonic acid (H2 CO3 ).
CO2 + H2 O → H2 CO3
The carbonic acid dissociates to form hydrogen and hydrogen carbonate ions. It is the
presence of hydrogen ions that lowers the pH of the solution, making the rain acidic.
H2 CO3 → H + + HCO3−
2. Nitric oxide
Nitric oxide (NO) also contributes to the natural acidity of rainwater and is formed during
lightning storms when nitrogen and oxygen react. In air, NO is oxidised to form nitrogen
dioxide (NO2 ). It is the nitrogen dioxide which then reacts with water in the atmosphere
to form nitric acid (HNO3 ).
3N O2 (g) + H2 O → 2HN O3 (aq) + N O(g)
The nitric acid dissociates in water to produce hydrogen ions and nitrate ions. This again
lowers the pH of the solution, making it acidic.
HN O3 → H + + N O3−
3. Sulfur dioxide
Sulfur dioxide in the atmosphere first reacts with oxygen to form sulfur trioxide, before
reacting with water to form sulfuric acid.
2SO2 + O2 → 2SO3
SO3 + H2 O → H2 SO4
Sulfuric acid dissociates in a similar way to the previous reactions.
H2 SO4 → HSO4− + H +
Although these reactions do take place naturally, human activities can greatly increase the concentration of these gases in the atmosphere, so that rain becomes far more acidic than it would
otherwise be. The burning of fossil fuels in industries, vehicles etc is one of the biggest culprits.
If the acidity of the rain drops below 5, it is referred to as acid rain.
Acid rain can have a very damaging effect on the environment. In rivers, dams and lakes, increased acidity can mean that some species of animals and plants will not survive. Acid rain can
also degrade soil minerals, producing metal ions that are washed into water systems. Some of
these ions may be toxic e.g. Al3+ . From an economic perspective, altered soil pH can drastically
affect agricultural productivity.
Acid rain can also affect buildings and monuments, many of which are made from marble and
limestone. A chemical reaction takes place between CaCO3 (limestone) and sulfuric acid to
produce aqueous ions which can be easily washed away. The same reaction can occur in the
lithosphere where limestone rocks are present e.g. limestone caves can be eroded by acidic
rainwater.
H2 SO4 + CaCO3 → CaSO4 .H2 O + CO2
385
20.6
CHAPTER 20. THE HYDROSPHERE - GRADE 10
Activity :: Investigation : Acid rain
You are going to test the effect of ’acid rain’ on a number of substances.
Materials needed:
samples of chalk, marble, zinc, iron, lead, dilute sulfuric acid, test tubes, beaker,
glass dropper
Method:
1. Place a small sample of each of the following substances in a separate test
tube: chalk, marble, zinc, iron and lead
2. To each test tube, add a few drops of dilute sulfuric acid.
3. Observe what happens and record your results.
Discussion questions:
• In which of the test tubes did reactions take place? What happened to the
sample substances?
• What do your results tell you about the effect that acid rain could have on each
of the following: buildings, soils, rocks and geology, water ecosystems?
• What precautions could be taken to reduce the potential impact of acid rain?
20.6
Electrolytes, ionisation and conductivity
Conductivity in aqueous solutions, is a measure of the ability of water to conduct an electric
current. The more ions there are in the solution, the higher its conductivity.
Definition: Conductivity
Conductivity is a measure of a solution’s ability to conduct an electric current.
20.6.1
Electrolytes
An electrolyte is a material that increases the conductivity of water when dissolved in it.
Electrolytes can be further divided into strong electrolytes and weak electrolytes.
Definition: Electrolyte
An electrolyte is a substance that contains free ions and behaves as an electrically conductive
medium. Because they generally consist of ions in solution, electrolytes are also known as
ionic solutions.
1. Strong electrolytes
A strong electrolyte is a material that ionises completely when it is dissolved in water:
AB(s,l,g) → A+ (aq) + B − (aq)
This is a chemical change because the original compound has been split into its component ions and bonds have been broken. In a strong electrolyte, we say that the extent
of ionisation is high. In other words, the original material dissociates completely so that
there is a high concentration of ions in the solution. An example is a solution of potassium
nitrate:
KN O3 (s) → K + (aq) + N O3− (aq)
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CHAPTER 20. THE HYDROSPHERE - GRADE 10
20.6
2. Weak electrolytes
A weak electrolyte is a material that goes into solution and will be surrounded by water
molecules when it is added to water. However, not all of the molecules will dissociate into
ions. The extent of ionisation of a weak electrolyte is low and therefore the concentration
of ions in the solution is also low.
AB(s,l,g) → AB(aq) ⇔ A+ (aq) + B − (aq)
The following example shows that, in the final solution of a weak electrolyte, some of the
original compound plus some dissolved ions are present.
C2 H3 O2 H(l) → C2 H3 O2 H ⇔ C2 H3 O2− (aq) + H + (aq)
20.6.2
Non-electrolytes
A non-electrolyte is a material that does not increase the conductivity of water when dissolved
in it. The substance goes into solution and becomes surrounded by water molecules, so that the
molecules of the chemical become separated from each other. However, although the substance
does dissolve, it is not changed in any way and no chemical bonds are broken. The change is a
physical change. In the oxygen example below, the reaction is shown to be reversible because
oxygen is only partially soluble in water and comes out of solution very easily.
C2 H5 OH(l) → C2 H5 OH(aq)
O2 (g) ⇔ O2 (aq)
20.6.3
Factors that affect the conductivity of water
The conductivity of water is therefore affected by the following factors:
• The type of substance that dissolves in water
Whether a material is a strong electrolyte (e.g. potassium nitrate, KNO3 ), a weak electrolyte (e.g. acetate, C2 H3 O2 H) or a non-electrolyte (e.g. sugar, alcohol, oil) will affect
the conductivity of water because the concentration of ions in solution will be different in
each case.
• The concentration of ions in solution
The higher the concentration of ions in solution, the higher its conductivity will be.
• Temperature
The warmer the solution the higher the solubility of the material being dissolved, and
therefore the higher the conductivity as well.
Activity :: Experiment : Electrical conductivity
Aim:
To investigate the electrical conductivities of different substances and solutions.
Apparatus:
solid salt (NaCl) crystals; different liquids such as distilled water, tap water,
seawater, benzene and alcohol; solutions of salts e.g. NaCl, KBr; a solution of
an acid (e.g. HCl) and a solution of a base (e.g. NaOH); torch cells; ammeter;
conducting wire, crocodile clips and 2 carbon rods.
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20.6
CHAPTER 20. THE HYDROSPHERE - GRADE 10
Method:
Set up the experiment by connecting the circuit as shown in the diagram below.
In the diagram, ’X’ represents the substance or solution that you will be testing.
When you are using the solid crystals, the crocodile clips can be attached directly to
each end of the crystal. When you are using solutions, two carbon rods are placed
into the liquid, and the clips are attached to each of the rods. In each case, complete
the circuit and allow the current to flow for about 30 seconds. Observe whether the
ammeter shows a reading.
battery
Ammeter
A
test substance
X
crocodile clip
Results:
Record your observations in a table similar to the one below:
Test substance
Ammeter reading
What do you notice? Can you explain these observations?
Remember that for electricity to flow, there needs to be a movement of charged
particles e.g. ions. With the solid NaCl crystals, there was no flow of electricity
recorded on the ammeter. Although the solid is made up of ions, they are held
together very tightly within the crystal lattice, and therefore no current will flow.
Distilled water, benzene and alcohol also don’t conduct a current because they are
covalent compounds and therefore do not contain ions.
The ammeter should have recorded a current when the salt solutions and the acid
and base solutions were connected in the circuit. In solution, salts dissociate into
their ions, so that these are free to move in the solution. Acids and bases behave
in a similar way, and dissociate to form hydronium and oxonium ions. Look at the
following examples:
KBr → K+ + Br−
NaCl → Na+ + Cl−
HCl + H2 O → H3 O+ + Cl−
NaOH → Na+ + OH−
Conclusions:
Solutions that contain free-moving ions are able to conduct electricity because of
the movement of charged particles. Solutions that do not contain free-moving ions
do not conduct electricity.
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CHAPTER 20. THE HYDROSPHERE - GRADE 10
20.7
teresting Conductivity in streams and rivers is affected by the geology of the area where
Interesting
Fact
Fact
the water is flowing through. Streams that run through areas with granite
bedrock tend to have lower conductivity because granite is made of materials
that do not ionise when washed into the water. On the other hand, streams
that run through areas with clay soils tend to have higher conductivity because
the materials ionise when they are washed into the water. Pollution can also affect conductivity. A failing sewage system or an inflow of fertiliser runoff would
raise the conductivity because of the presence of chloride, phosphate, and nitrate
(ions) while an oil spill (non-ionic) would lower the conductivity. It is very important that conductivity is kept within a certain acceptable range so that the
organisms living in these water systems are able to survive.
20.7
Precipitation reactions
Sometimes, ions in solution may react with each other to form a new substance that is insoluble.
This is called a precipitate.
Definition: Precipitate
A precipitate is the solid that forms in a solution during a chemical reaction.
Activity :: Demonstration : The reaction of ions in solution
Apparatus and materials:
4 test tubes; copper(II) chloride solution; sodium carbonate solution; sodium
sulphate solution
CuCl2
CuCl2
Na2 CO3
Na2 SO4
Method:
1. Prepare 2 test tubes with approximately 5 ml of dilute Cu(II)chloride solution
in each
2. Prepare 1 test tube with 5 ml sodium carbonate solution
3. Prepare 1 test tube with 5 ml sodium sulphate solution
4. Carefully pour the sodium carbonate solution into one of the test tubes containing copper(II) chloride and observe what happens
5. Carefully pour the sodium sulphate solution into the second test tube containing
copper(II) chloride and observe what happens
Results:
389
20.7
CHAPTER 20. THE HYDROSPHERE - GRADE 10
1. A light blue precipitate forms when sodium carbonate reacts with copper(II)
chloride
2. No precipitate forms when sodium sulphate reacts with copper(II) chloride
It is important to understand what happened in the previous demonstration. We will look at
what happens in each reaction, step by step.
1. Reaction 1: Sodium carbonate reacts with copper(II) chloride
When these compounds react, a number of ions are present in solution: Cu2+ , Cl− , N a+
and CO32− .
Because there are lots of ions in solution, they will collide with each other and may recombine in different ways. The product that forms may be insoluble, in which case a precipitate
will form, or the product will be soluble, in which case the ions will go back into solution.
Let’s see how the ions in this example could have combined with each other:
Cu2+ + CO2−
3 → CuCO3
Cu2+ + 2Cl− → CuCl2
Na+ + Cl− → NaCl
Na+ + CO2−
3 → Na2 CO3
You can automatically exclude the reactions where sodium carbonate and copper(II) chloride are the products because these were the initial reactants. You also know that sodium
chloride (NaCl) is soluble in water, so the remaining product (copper carbonate) must be
the one that is insoluble. It is also possible to look up which salts are soluble and which
are insoluble. If you do this, you will find that most carbonates are insoluble, therefore the
precipitate that forms in this reaction must be CuCO3 . The reaction that has taken place
between the ions in solution is as follows:
2N a+ + CO32− + Cu2+ + 2Cl− → CuCO3 + 2N a+ + 2Cl−
2. Reaction 2: Sodium sulphate reacts with copper(II) chloride
The ions that are present in solution are Cu2+ , Cl− , N a+ and SO42− .
The ions collide with each other and may recombine in different ways. The possible combinations of the ions are as follows:
Cu2+ + SO2−
4 → CuSO4
Cu2+ + 2Cl− → CuCl2
Na+ + Cl− → NaCl
Na+ + SO2−
4 → Na2 SO4
If we look up which of these salts are soluble and which are insoluble, we see that most
chlorides and most sulphates are soluble. This is why no precipitate forms in this second
reaction. Even when the ions recombine, they immediately separate and go back into
solution. The reaction that has taken place between the ions in solution is as follows:
2N a+ + SO42− + Cu2+ + 2Cl− → 2N a+ + SO42− + Cu2+ + 2Cl−
Table 20.1 shows some of the general rules about the solubility of different salts based on a
number of investigations:
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CHAPTER 20. THE HYDROSPHERE - GRADE 10
20.8
Table 20.1: General rules for the solubility of salts
Salt
Nitrates
Potassium, sodium and ammonium salts
Chlorides
Sulphates
Carbonates
20.8
Solubility
All are soluble
All are soluble
All are soluble except silver chloride, lead(II)chloride and mercury(II)chloride
All
are
soluble
except
lead(II)sulphate,
barium sulphate and calcium sulphate
All are insoluble except those of
potassium, sodium and ammonium
Testing for common anions in solution
It is also possible to carry out tests to determine which ions are present in a solution.
20.8.1
Test for a chloride
Prepare a solution of the unknown salt using distilled water and add a small amount of silver
nitrate solution. If a white precipitate forms, the salt is either a chloride or a carbonate.
−
Cl− + Ag+ + NO−
3 → AgCl + NO3 (AgCl is white precipitate)
−
+
CO2−
3 + 2Ag + 2NO3 → Ag2 CO3 + 2NO2 (Ag2 CO3 is white precipitate)
The next step is to treat the precipitate with a small amount of concentrated nitric acid. If
the precipitate remains unchanged, then the salt is a chloride. If carbon dioxide is formed, and
the precipitate disappears, the salt is a carbonate.
AgCl + HNO3 → (no reaction; precipitate is unchanged)
Ag2 CO3 + 2HNO3 → 2AgNO3 + H2 O + CO2 (precipitate disappears)
20.8.2
Test for a sulphate
Add a small amount of barium chloride solution to a solution of the test salt. If a white precipitate
forms, the salt is either a sulphate or a carbonate.
2+
SO2−
+ Cl− → BaSO4 + Cl− (BaSO4 is a white precipitate)
4 + Ba
2+
CO2−
+ Cl− → BaCO3 + Cl− (BaCO3 is a white precipitate)
3 + Ba
If the precipitate is treated with nitric acid, it is possible to distinguish whether the salt is a
sulphate or a carbonate (as in the test for a chloride).
BaSO4 + HNO3 → (no reaction; precipitate is unchanged)
BaCO3 + 2HNO3 → Ba(NO3 )2 + H2 O + CO2 (precipitate disappears)
391
20.8
20.8.3
CHAPTER 20. THE HYDROSPHERE - GRADE 10
Test for a carbonate
If a sample of the dry salt is treated with a small amount of acid, the production of carbon
dioxide is a positive test for a carbonate.
Acid + CO32− → CO2
If the gas is passed through limewater and the solution becomes milky, the gas is carbon dioxide.
Ca(OH)2 + CO2 → CaCO3 + H2 O (It is the insoluble CaCO3 precipitate that makes the
limewater go milky)
20.8.4
Test for bromides and iodides
As was the case with the chlorides, the bromides and iodides also form precipitates when they
are reacted with silver nitrate. Silver chloride is a white precipitate, but the silver bromide and
silver iodide precipitates are both pale yellow. To determine whether the precipitate is a bromide
or an iodide, we use chlorine water and carbon tetrachloride (CCl4 ).
Chlorine water frees bromine gas from the bromide, and colours the carbon tetrachloride a reddish brown.
Chlorine water frees iodine gas from an iodide, and colours the carbon tetrachloride is coloured
purple.
Exercise: Precipitation reactions and ions in solution
1. Silver nitrate (AgNO3 ) reacts with potassium chloride (KCl) and a white precipitate is formed.
(a) Write a balanced equation for the reaction that takes place.
(b) What is the name of the insoluble salt that forms?
(c) Which of the salts in this reaction are soluble?
2. Barium chloride reacts with sulfuric acid to produce barium sulphate and hydrochloric acid.
(a) Write a balanced equation for the reaction that takes place.
(b) Does a precipitate form during the reaction?
(c) Describe a test that could be used to test for the presence of barium
sulphate in the products.
3. A test tube contains a clear, colourless salt solution. A few drops of silver
nitrate solution are added to the solution and a pale yellow precipitate forms.
Which one of the following salts was dissolved in the original solution?
(a)
(b)
(c)
(d)
NaI
KCl
K2 CO3
Na2 SO4
(IEB Paper 2, 2005)
392
CHAPTER 20. THE HYDROSPHERE - GRADE 10
20.9
20.9
Threats to the Hydrosphere
It should be clear by now that the hydrosphere plays an extremely important role in the survival of
life on Earth, and that the unique properties of water allow various important chemical processes
to take place which would otherwise not be possible. Unfortunately for us however, there are
a number of factors that threaten our hydrosphere, and most of these threats are because of
human activities. We are going to focus on two of these issues: overuse and pollution and look
at ways in which these problems can possibly be overcome.
1. Overuse of water
We mentioned earlier that only a very small percentage of the hydrosphere’s water is
available as freshwater. However, despite this, humans continue to use more and more
water to the point where water consumption is fast approaching the amount of water
that is available. The situation is a serious one, particularly in countries such as South
Africa which are naturally dry, and where water resources are limited. It is estimated that
between 2020 and 2040, water supplies in South Africa will no longer be able to meet the
growing demand for water in this country. This is partly due to population growth, but
also because of the increasing needs of industries as they expand and develop. For each
of us, this should be a very scary thought. Try to imagine a day without water...difficult
isn’t it? Water is so much a part of our lives, that we are hardly aware of the huge part
that it plays in our daily lives.
Activity :: Discussion : Creative water conservation
As populations grow, so do the demands that are placed on dwindling water
resources. While many people argue that building dams helps to solve this watershortage problem, the reality is that dams are only a temporary solution, and
that they often end up doing far more ecological damage than good. The only
sustainable solution is to reduce the demand for water, so that water supplies
are sufficient to meet this. The more important question then is how to do this.
Discussion:
Divide the class into groups, so that there are about five people in each.
Each group is going to represent a different sector within society. Your teacher
will tell you which sector you belong to from the following: Farming, industry,
city management or civil society (i.e. you will represent the ordinary ’man on
the street’). In your groups, discuss the following questions as they relate to the
group of people you represent: (Remember to take notes during your discussions,
and nominate a spokesperson to give feedback to the rest of the class on behalf
of your group)
• What steps could be taken by your group to conserve water?
• Why do you think these steps are not being taken?
• What incentives do you think could be introduced to encourage this group
to conserve water more efficiently?
2. Pollution
Pollution of the hydrosphere is also a major problem. When we think of pollution, we
sometimes only think of things like plastic, bottles, oil and so on. But any chemical that
is present in the hydrosphere in an amount that is not what it should be is a pollutant.
Animals and plants that live in the hydrosphere are specially adapted to surviving within a
certain range of conditions. If these conditions are changed (e.g. through pollution), these
organisms may not be able to survive. Pollution then, can affect entire aquatic ecosystems.
The most common forms of pollution in the hydrosphere are waste products from humans
and from industries, nutrient pollution e.g. fertiliser runoff which causes eutrophication
(this will be discussed in a later section) and toxic trace elements such as aluminium,
mercury and copper to name a few. Most of these elements come from mines or from
industries.
393
20.10
CHAPTER 20. THE HYDROSPHERE - GRADE 10
It is important to realise that our hydrosphere exists in a delicate balance with other systems,
and that disturbing this balance can have serious consequences for life on this planet.
Activity :: Group Project : School Action Project
There is a lot that can be done within a school to save water. As a class,
discuss what actions could be taken by your class to make people more aware of how
important it is to conserve water.
20.10
Summary
• The hydrosphere includes all the water that is on Earth. Sources of water include freshwater (e.g. rivers, lakes), saltwater (e.g. oceans), groundwater (e.g. boreholes) and water
vapour. Ice (e.g. glaciers) is also part of the hydrosphere.
• The hydrosphere interacts with other global systems, including the atmosphere, lithosphere and biosphere.
• The hydrosphere has a number of important functions. Water is a part of all living cells,
it provides a habitat for many living organisms, it helps to regulate climate, and it is used
by humans for domestic, industrial and other use.
• The polar nature of water means that ionic compounds dissociate easily in aqueous
solution into their component ions.
• Ions in solution play a number of roles. In the human body for example, ions help to
regulate the internal environment (e.g. controlling muscle function, regulating blood pH).
Ions in solution also determine water hardness and pH.
• Water hardness is a measure of the mineral content of water. Hard water has a high
mineral concentration and generally also a high concentration of metal ions e.g. calcium
and magnesium. The opposite is true for soft water.
• pH is a measure of the concentration of hydrogen ions in solution. The formula used to
calculate pH is as follows:
pH = -log[H3O+ ] or pH = -log[H+ ]
A solution with a pH less than 7 is considered acidic and more than 7 is considered basic
(or alkaline). A neutral solution has a pH of 7.
• Gases such as CO2 , NO2 and SO2 dissolve in water to form weak acid solutions. Rain is
naturally acidic because of the high concentrations of carbon dioxide in the atmosphere.
Human activities such as burning fossil fuels, increase the concentration of these gases in
the atmosphere, resulting in acid rain.
• Conductivity is a measure of a solution’s ability to conduct an electric current.
• An electrolyte is a substance that contains free ions, and is therefore able to conduct an
electric current. Electrolytes can be divided into strong and weak electrolytes, based on
the extent to which the substance ionises in solution.
• A non-electrolyte cannot conduct an electric current because it dooes not contain free
ions.
• The type of substance, the concentration of ions and the temperature of the solution,
affect its conductivity.
394
CHAPTER 20. THE HYDROSPHERE - GRADE 10
20.10
• A precipitate is formed when ions in solution react with each other to form an insoluble
product. Solubility ’rules’ help to identify the precipitate that has been formed.
• A number of tests can be used to identify whether certain anions are present in a solution.
• Despite the importance of the hydrosphere, a number of factors threaten it. These include
overuse of water, and pollution.
Exercise: Summary Exercise
1. Give one word for each of the following descriptions:
(a)
(b)
(c)
(d)
the change in phase of water from a gas to a liquid
a charged atom
a term used to describe the mineral content of water
a gas that forms sulfuric acid when it reacts with water
2. Match the information in column A with the information in column B by writing
only the letter (A to I) next to the question number (1 to 7)
Column A
1. A polar molecule
2. molecular solution
3. Mineral that increases water hardness
4. Substance that increases the hydrogen ion concentration
5. A strong electrolyte
6. A white precipitate
7. A non-conductor of electricity
Column B
A. H2 SO4
B. CaCO3
C. NaOH
D. salt water
E. calcium
F. carbon dioxide
G. potassium nitrate
H. sugar water
I. O2
3. For each of the following questions, choose the one correct answer from the
list provided.
(a) Which one of the following substances does not conduct electricity in the
solid phase but is an electrical conductor when molten?
i. Cu
ii. PbBr2
iii. H2 O
iv. I2
(IEB Paper 2, 2003)
(b) The following substances are dissolved in water. Which one of the solutions
is basic?
i. sodium nitrate
ii. calcium sulphate
iii. ammonium chloride
iv. potassium carbonate
(IEB Paper 2, 2005)
4. The concentration of hydronium and hydroxyl ions in a typical sample of seawater are 10−8 and 10−6 respectively.
(a) Is the seawater acidic or basic?
(b) Calculate the pH of this seawater.
5. Three test tubes (X, Y and Z) each contain a solution of an unknown potassium
salt. The following observations were made during a practical investigation to
identify the solutions in the test tubes:
A: A white precipitate formed when silver nitrate (AgNO3 ) was added to test
tube Z.
395
20.10
CHAPTER 20. THE HYDROSPHERE - GRADE 10
B: A white precipitate formed in test tubes X and Y when barium chloride
(BaCl2 ) was added.
C: The precipitate in test tube X dissolved in hydrochloric acid (HCl) and a gas
was released.
D: The precipitate in test tube Y was insoluble in hydrochloric acid.
(a) Use the above information to identify the solutions in each of the test tubes
X, Y and Z.
(b) Write a chemical equation for the reaction that took place in test tube X
before hydrochloric acid was added.
(DoE Exemplar Paper 2 2007)
396
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