chaPTEr 1: The Earth: a mixture of chemicals 2
• Chemistry deals with the composition, structure, properties and reactions of matter.
• Chemistry is concerned with:
– the study of substances that occur in the natural world
– the extraction and separation of useful substances from naturally occurring materials and the synthesis of new substances
– the analysis of substances to determine their composition.
• When chemicals are used, careful consideration needs to be given to both the benefits and possible undesirable effects of their manufacture and use.
• Matter is anything that has mass and occupies space.
• Matter can exist in the solid, liquid and gaseous states.
• The characteristics of solids, liquids and gases are shown in the table and diagram.
Characteristics of solids, liquids and gases
solid liquid gas
Shape definite variable variable
Volume definite definite variable
Compressibility almost
incompressible very slightly
compressible highly compressible
Diffusion negligible slow fast
Particle
arrangement closely packed, fixed positions
fairly closely packed, not fixed positions
widely spaced, move independently
Thermal expansion small moderate large
• The kinetic theory of gases explains the physical properties of gases. The main ideas are as follows.
– Gases are composed of molecules.
– The molecules are widely spaced.
– The molecules move rapidly.
– The forces of attraction between molecules are negligible.
Gas
Particles far apart and move independently of one another.
Liquid
Particles close to one another but free to move.
Solid
Particles closely packed and held in position.
2 moDulE 1: The chemical Earth
– Collisions between molecules are perfectly elastic.
– The average kinetic energy of the molecules increases with temperature.
• Kinetic energy is energy due to motion.
• The kinetic theory can be extended to solids and liquids.
• In solids the particles are closely packed and vibrate about fixed positions.
• In liquids the particles are fairly closely packed but can move past each other.
• Intermolecular forces are important in solids and liquids.
• Matter can be classified into the two categories of pure substances (elements and compounds) and mixtures.
– Elements are pure substances that cannot be separated by chemical change into simpler substances.
– Compounds are pure substances composed of two or more elements chemically combined in fixed proportions.
– Mixtures are two or more substances mixed together in variable proportions.
• Homogeneous mixtures or solutions have uniform composition and properties throughout.
• Heterogeneous mixtures are not uniform in composition and properties throughout.
• The Earth has a layered structure consisting of the crust, mantle and core.
• The Earth’s crust includes a solid lithosphere, liquid hydrosphere and gaseous atmosphere, and a living biosphere which overlaps the other three components of the crust.
• The abundance of elements on the earth is essentially fixed; chemical reactions that occur on the Earth change the way these elements are combined.
• The biosphere, lithosphere, hydrosphere and atmosphere consist of mixtures of elements and compounds.
• Mixtures that occur naturally and are used by humans include granite, sand, sandstone, coal, water and natural dyes.
• Mixtures can be separated by the following processes:
– Filtration—undissolved solid particles are separated from a liquid or gas by passing the mixture through a filter.
– Solution—one or more component(s) of a mixture dissolve more readily in a solvent (often water).
– Evaporation—one of the components of a mixture evaporates more readily.
– Crystallisation—components in a mixture have different solubilities in water (or other liquid solvents) and crystallise separately from solution.
– Sedimentation—solid particles settle out from a liquid or gas mixture in which they occur.
– Centrifugation—a spinning motion is used to separate components of a mixture that have different densities or exist in different states.
– Decantation—liquid is separated from a solid by pouring off the liquid.
– Sieving—solid particles are separated according to their sizes by passing them through a perforated barrier.
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chaPTEr 1: The Earth: a mixture of chemicals 2
– Distillation (and fractional distillation)—components with different boiling points in a liquid mixture are separated by boiling and condensing the mixture.
– Chromatography—components in a liquid or gas mixture are separated by passing the mixture over the surface of a substance to which the components of the mixture adsorb with different strengths.
– Magnetic separation—components with different magnetic properties are separated by passing the mixture through a magnetic field.
• Qualitative analysis is the identification of substances present in a sample.
• Quantitative analysis is the determination of how much of each substance is present in a sample.
• Gravimetric analysis is a method of analysis that involves the measurement of masses.
• Volumetric analysis is a method of analysis that involves the measurement of solution volumes.
• The percentage composition of a chemical compound specifies the percentage mass of each of the different elements in the compound.
1 Choose one chemical substance familiar to you and investigate the advantages and disadvantages of its use.
2 a Recall the essential differences between solids and liquids and between liquids and gases.
b Identify three examples of substances for each of the solid, liquid and gaseous states.
3 Explain why the incompressibility of liquids is important in the functioning of car brakes.
4 Explain the following observations.
a Gases are compressible, whereas solids and liquids are essentially incompressible.
b Solids have a definite shape, whereas liquids and gases take the shape of their container.
c There is usually a small increase in volume when a solid changes to a liquid and a large increase when a liquid changes to a gas.
5 Identify which of the assumptions of the kinetic theory of gases cannot be applied to liquids and solids.
6 Investigate the fourth state of matter, plasma. How is it formed? Does it occur anywhere on Earth?
7 Outline the essential difference between a pure substance and a mixture.
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2 moDulE 1: The chemical Earth
8 Using diagrams (such as those shown in Figure 1.8, for example) construct representations of:
a an element a in which the atoms are combined as diatomic molecules b a compound in which one atom of a is combined with three atoms of b c a mixture of the element and compound in a and b above.
9 When magnesium, a grey metal, is burnt in oxygen it burns with a brilliant flame to form a white powder, magnesium oxide.
a Classify magnesium, oxygen and magnesium oxide as elements, compounds or mixtures.
b Explain why the properties of magnesium oxide are quite different from those of magnesium and oxygen.
10 Classify each of the following as either an element or a compound:
N2, He, C2H5OH, KI, C, NH4NO2 and Cl2.
11 Briefly describe the composition of the Earth’s crust, mantle and core and explain how the separation of these layers is thought to have occurred.
12 Identify an example of an element, a compound and a heterogeneous mixture found naturally in the lithosphere.
13 Discuss how the atmosphere could be considered as both a homogeneous mixture and a heterogeneous mixture.
14 Investigate our current understanding of how the element iron is formed in stars and why it is relatively abundant in the universe.
15 Explain why it is difficult to totally separate two or more soluble solids by filtration and crystallisation.
16 Propose how the following separations could be achieved:
a iron filings from an iron filings–charcoal mixture b sugar from a sugar solution
c small amounts of insecticide from fruit d oxygen from air
e red blood cells from blood plasma f water from sea water.
17 Identify eight common but useful materials and classify these as elements, compounds, homogeneous mixtures or heterogeneous mixtures. Identify which of these materials are used in a naturally occurring form and which have been processed in some way.
18 Identify the substances that are used to make glass. Explain how glasses of different colours are produced.
19 Investigate and summarise the synthesis of either aspirin or paracetamol.
20 Calculate the percentage of salt in a 50 g packet of potato crisps if the crisps are crushed and mixed with water, then filtered and the solution evaporated, leaving 2.13 g of solid salt.
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the universe
Living cell (representing biosphere)
oxygen
Figure 2.1 The distribution, as mass percentages, of the most abundant elements in the universe and Earth
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Figure 2.2 The periodic table of elements: white indicates metals (except H), pale green semi-metals, grey non-metals and dark green noble gases.
2.1 Abundance of elements on Earth
The abundance of the most common elements in all their various forms in the universe and Earth are compared in Figure 2.1.