Chapter 1 Mixtures in the Earth

Chapter 2 Elements

Chapter 3 Compounds

Chapter 4 Chemical extraction

Chapter 5 Bonding and structure

THE CHEMICAL EARTH

CORE MODULE 1

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MIXTURES IN THE EARTH Introduction The Earth and all its living and non-living components consist of complex mixtures. The rocks of the Earth are composed of mixtures of many different minerals. The oceans, rivers and lakes contain water in which many substances are dissolved or suspended. The atmosphere is a mixture of many gases including oxygen, nitrogen and carbon dioxide. Living things, from the lowest life forms to the most complex forms, contain mixtures of simple and complex compounds.

In this chapter 1.1 1.2 1.3 1.4 1.5

Figure 1.1 The Earth can be divided into various zones or spheres. The rocks of the cliff face and the sandy beach are part of the lithosphere. The ocean and the water in the sand are part of the hydrosphere. The air and clouds are part of the atmosphere. The living things in the ocean water, in and on the sand of the beach and flying in the air are all part of the biosphere. 2

THE CHEMICAL EARTH

Classification of matter Mixtures and the spheres of the Earth Physical separation techniques Gravimetric analysis Industrial separation of mixtures

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&#& Remember Before beginning this section, you should be able to: • describe the behaviour of matter in terms of particles that are continuously moving and interacting • relate properties of solids, liquids and gases to the particle model of matter • describe the physical changes that occur during observations of evaporation, condensation, boiling, melting and freezing • explain density in terms of a simple particle model.

Key content By the end of this section, you should be able to: • identify the difference between elements, compounds and mixtures in terms of particle theory • present information by using symbols and formulae • present information using a variety of pictorial representations • analyse information using models to explain phenomena and/or make predictions.

# , !3 3 ) & ) # !4 ) / .  / &  - !4 4 % 2 Elements, compounds and mixtures Chemistry is an ancient science. Inquiring minds over the centuries have wondered about the interesting variety of materials that make up our Earth. Humans have used the materials of the Earth to build their homes, monuments and roads. They have used materials such as gold as money and to make ornaments. They have used salt for seasoning and preserving food, and as a form of money. They have used coal and oil as fuels for warmth and as a source of energy to power machines. Chemistry is the study of matter. Chemists classify matter in many ways. One way is to classify matter as solids, liquids or gases at room temperature. Another more useful scheme is to classify matter as pure or impure. In everyday life, we use the term ‘pure’ to describe things that are not contaminated with other substances. Pure water does not contain parasites, salt or mud. Chemists have developed more exact definitions of the terms ‘pure’ and ‘impure’. • Pure substances have a fixed composition and fixed properties. They cannot be decomposed by simple physical separation techniques. • Impure substances are mixtures. They have variable composition and variable properties. They can be separated into their components by various physical separation techniques. Pure substances can be further classified into elements and compounds. • Elements are the simplest pure substances consisting of only one type of atom. They cannot be broken down (or decomposed). • Compounds are also pure substances. They are composed of two or more elements that are chemically bonded together. They are composed of a fixed number of atoms of each component element. They can be decomposed into their component elements or into simpler compounds. Table 1.1 lists some examples of pure and impure substances. The chemical symbols for elements and the chemical formulae for compounds are also included. Table 1.1 Classification of matter

Figure 1.2 Gold is a pure substance, while the rock around it contains mixtures of various compounds.

Elements

Compounds

Mixtures

oxygen, O

water, H2O

air

silver, Ag

silver oxide, Ag2O

brass

magnesium, Mg

magnesium sulfide, MgS

steel

silicon, Si

silicon dioxide, SiO2

sand

The Earth’s crust contains elements, compounds and mixtures. The two most abundant elements are oxygen and silicon. Silicon is present in many compounds and in mixtures of compounds but is rarely found as the free element. Pure, crystalline silicon (figure 1.3) has a greasy feel, like graphite. Sand is a mixture of silicon dioxide and many other substances. Quartz crystals are composed of interlocking silicon dioxide tetrahedra in a relatively pure form. Such large crystals often take thousands or millions of years to grow to such a size deep in the Earth’s crust.

CHAPTER 1 MIXTURES IN THE EARTH

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Figure 1.3 Silicon as (left) pure, crystalline silicon and as silicon dioxide in (centre) sand and (right) quartz

physical properties: the properties characteristic of a chemical substance (such as melting point, boiling point, colour, density and conductivity)

Pure substances have fixed physical properties. Thus, the melting and boiling points of different elements and compounds are unique to that substance. The boiling point of pure water is 100 °C at 100 kPa pressure (standard pressure). This is not true for mixtures. For example, the boiling point of salt water depends on the amount of salt dissolved in the water. The greater the salt concentration, the greater the boiling point. Table 1.2 shows some examples of the fixed physical properties of some common elements and compounds. The properties of the compounds are quite different from their component elements. This shows that compounds are not just mixtures of elements. Table 1.2 Physical properties of some common elements and compounds Substance

copper

oxygen

copper (I) oxide, Cu2O

copper (II) oxide, CuO

Colour

salmon pink

colourless

red

black

solid

gas

solid

solid

Density (g/cm3 at 25 °C/100 kPa)

9.0

1.3 r 10–3

6.0

6.4

Melting point (°C)

1085

–219

1235

1326

State (at 25 °C/100 kPa)

Particle models Chemists use models to understand the differences between elements, compounds and mixtures. The components of matter can be visualised in terms of different types of particle. Atoms are the basic building blocks of matter. Sometimes, atoms of the same element or different elements group together to form more complex particles called molecules. The number of atoms in a molecule is shown using a subscript next to the symbol for the element. For example, N2 represents a nitrogen molecule that has two atoms of nitrogen. Elements are composed of atoms or molecules. Examples include: • helium (single atoms or monatomic molecules, He) • oxygen (diatomic molecules, O2) • phosphorus (tetra-atomic molecules, P4). Compounds are composed of fixed numbers of atoms of different elements. Examples include: • ammonia, NH3 • zinc oxide, ZnO • carbon dioxide, CO2. 4

THE CHEMICAL EARTH

homogeneous mixture: a mixture in which all the particles are uniformly distributed heterogeneous mixture: a mixture in which the particles are not uniformly distributed

Mixtures have various particle types and compositions. The particles of each component in homogeneous mixtures are distributed uniformly; they are not uniformly distributed in heterogeneous mixtures. Sugar solutions in water are homogeneous mixtures as the sugar particles are uniformly mixed with the water particles. Muddy water is a heterogeneous mixture as the heavier mud particles settle under gravity to form a sediment.

Elements

Mixtures

Compounds

Cu Cu Cu Cu Cu Cu Cu

Cu O Cu O Cu O Cu O O Cu O Cu

Cl–

N

Copper oxide

N

OH H

N N

N

Nitrogen gas

HO H

N N

Water

Ne Ne

Neon gas

C OC O

O

N N C O

Ar

O

Ne Figure 1.4 Particle diagrams showing the differences between elements, compounds and mixtures

Salt water OO

N

OH H

Cl–

Na+ Copper metal

Na+

HO H

O

Carbon dioxide

Air

N N

Pb Sn Pb Sn Sn Pb Pb Solder

SYLLABUS FOCUS 1. USING INSTRUCTION TERMS CORRECTLY When answering questions, it is important to know what the instruction terms (verbs) require you to do. Here are some examples. ‘Classify’ This instruction term requires you to group things into categories. Thus, pure substances can be grouped into elements and compounds. Example: Classify barium as an element or compound. Answer: Element

‘Define’ This instruction term requires you to state the meaning of a term or identify the essential features of a concept. Example: Define the term ‘molecule’. Answer: A molecule is the smallest part of a substance that can exist independently but still retain the characteristic properties of that substance.

‘Explain’ This instruction term requires you to provide reasons for why or how a process occurs, or to make the relationship between things evident. Example: Explain why barium is classified as an element. Answer: Barium is classified as an element because it is a simple, pure substance that cannot be broken down into anything simpler.

CHAPTER 1 MIXTURES IN THE EARTH

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1.1 QUESTIONS 1. Classify the following as P mixtures or pure substances. (a) Air (b) Sea water (c) Carbon dioxide (d) Granite (e) Calcium 2. Classify the following pure substances as elements or compounds. (a) Glucose (b) Silver (c) Ammonia (d) Lead (e) Copper (II) oxide 3. Classify the particle models in figure 1.5 as elements, compounds or mixtures. X

Y

Z

Figure 1.5 Particle model diagrams

4. Define the terms: (a) homogeneous mixture (b) heterogeneous mixture. 5. Explain why the properties of copper (II) oxide (see table 1.2) are quite different from the properties of copper and oxygen.

&#' Remember Before beginning this section, you should be able to: • identify some common mixtures • identify the importance of water as a solvent • describe aqueous mixtures in terms of solute, solvent and solution.

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THE CHEMICAL EARTH

6. Classify each of the particle models in figure 1.6 as homogeneous mixtures or heterogeneous mixtures. Q

R

S

Figure 1.6 Particle model diagrams of different mixtures

7. The following information about solutions of sulfuric acid was collected from a chemical data book. Table 1.3 Concentration of sulfuric acid solution (%w/w)

5

10

15

20

25

Density of acid solution at 20 °C (g/cm3)

1.032

1.066

1.102

1.139

1.178

(a) Classify sulfuric acid solutions as homogeneous or heterogeneous mixtures. (b) Explain how the tabulated data is typical of a mixture rather than a pure substance.

- ) 8 4 5 2 % 3 ! . $ 4 ( %  3 0 ( % 2 % 3 / & 4 ( %  % ! 2 4 ( The spheres of the Earth The Earth is believed to be about five billion years old. It formed from a swirling gas and dust cloud that orbited the primitive Sun. Gravitational compression and heating, as well as heat from radioactive decay, caused many minerals to melt and form new chemical compounds. Geologists have determined that the Earth eventually formed into a layered structure with dense metals, such as iron, at its centre, and lighter minerals, containing elements such as silicon, oxygen and aluminium, forming a solid crust at the surface. As the Earth cooled, water vapour condensed and formed the oceans and seas. The atmosphere has changed over the eons of time. Some light gases, such as hydrogen, have escaped into space, and the amount of oxygen has changed considerably in response to the evolution of life forms.

Key content By the end of this section, you should be able to: • identify that the biosphere, lithosphere, hydrosphere and atmosphere contain examples of mixtures of elements and compounds • present information clearly and succinctly using a variety of pictorial representations to show relationships • present information by selecting and drawing appropriate graphs to convey information and relationships clearly and accurately.

Figure 1.8 shows the layered structure or spheres of the Earth, starting with the lithosphere, which consists of the rigid, outer crust and the upper mantle. The inner mantle and core are not shown. The hydrosphere consists of the surface oceans, rivers, lakes and glacial water; it also extends into the atmosphere and lithosphere. In the atmosphere, water can exist as water vapour as well as liquid microdroplets in clouds. The biosphere is also an extensive region where living things are found. Scientists have discovered simple life forms deep within the hot crust as well as near volcanic vents on the ocean floor. Microscopic life forms have also been found in the highest levels of the atmosphere.

Figure 1.7 The biosphere is a part of the atmosphere, hydrosphere and lithosphere. ATMOSPHERE

ROSP H

PHER E

¹ ¹ ­ ­ ­ ­ º º ­ ­ ­ ­ » » HYD

¹ ­ ­ º ­ ­ »

) RE SPHE mantle LITHOand upper (crust

Figure 1.8 The spheres of the Earth

Oceanic crust

Continental crust

ERE

Oceans

Rivers

BIOS

Lakes

Upper mantle Crust ASTHENOSPHERE

CHAPTER 1 MIXTURES IN THE EARTH

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Mixtures in each sphere Within each sphere of the Earth there is a complex mixture of chemical substances.

Lithosphere The lithosphere contains many different types of mineral that are combined in different proportions to form sedimentary, igneous and metamorphic rocks. Silicates, oxides, carbonates and sulfides are some of the common minerals found in the lithosphere. The sand that forms the beaches along our coasts consists mainly of silicon dioxide. Humans mine mineral resources to manufacture metals and alloys. Many of these minerals are oxide and sulfide compounds of metals, such as aluminium, iron, lead and silver. Extraction of metals from these minerals requires the removal of the unwanted minerals that make up the rock. The common rock-making minerals and their chemical compositions are listed in table 1.4.

mineral: a naturally occurring, crystalline solid that has a fixed chemical composition or a composition that varies between strict limits. Some minerals are pure elements, such as gold and sulfur, but most minerals are chemical compounds.

Table 1.4 Common rock-making minerals Mineral

Composition

quartz

SiO2

orthoclase feldspar

KAlSi3O8

muscovite mica

KAl3Si3O10(OH)2

olivine

(Mg, Fe)2SiO4

calcite

CaCO3

Tables 1.5 (a) and (b) show the abundance of some of the elements that make up the Earth’s crust, and the abundance of elements in the whole Earth. The high percentage of oxygen and silicon in the crust reveals the abundance and importance of silicate minerals in the crust. Iron is the most abundant element in the whole Earth due to the massive amount of iron in the Earth’s core. Table 1.5 (a) Abundance of elements in the Earth’s crust Element

Abundance in the crust (%w/w*)

oxygen

46.6

iron

35

silicon

27.7

oxygen

30

aluminium

8.2

silicon

15

iron

5.0

magnesium

13

calcium

3.6

nickel

2.4

sodium

2.8

sulfur

1.9

potassium

2.6

calcium

1.1

magnesium

2.1

aluminium

1.1

all others

1.4

*%w/w = percentage by weight

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(b) Abundance of elements in the whole Earth

THE CHEMICAL EARTH

Element

Abundance in the whole Earth (%w/w*)

others *%w/w = percentage by weight