Conquering Chemistry Preliminary Course Module 1

Sept/Oct 2002 Syllabus changes

The revised syllabus and/or a version of the original syllabus with the changes marked in it can be downloaded from the Board of studies web site (www.boardofstudies.nsw.edu.au).

Conquering Chemistry is still a very close match to the revised syllabus though certain sections and exercises, detailed below, can now be omitted. The first of the two sub-sections below is for students: it tells you what you can omit from CCPC. The second sub-section is primarily for teachers: it gives more detail about what the changes are and how they affect CCPC.

In both sub-sections LC, MC and RC mean left, middle and right column respectively and DP means dot point. The references are to the original syllabus (1999 or March/April 2001)

1. For students

As a result of these syllabus changes you should omit the following:

In Chapter 1

·	You could omit Section 1.5 on page 12, though it is worth reading anyway. However you will not be expected to recall information in Table 1.4.
·	Omit Sections 1.21, 1.22 and 1.23 on pages 27 to 31. Omit Exercises 27, 28 and 29.
·	You could omit Section 1.27 on pages 34 and 35 though it is still a good place to introduce the Periodic Table. You should not omit Physical states of the elements on page 36.

In Chapter 2

·	Although there have been some syllabus changes that affect Chapter 2, I believe that you should not omit any sections of this chapter

In Chapter 3

·	No omissions from this chapter.

In Revision Test for Module 1

Omit Questions 4, 6, 13 and 20. Allow 70 minutes for the test and mark it out of 39.

2. For teachers

The changes to the syllabus and their consequences for Conquering Chemistry are set out in the following table.

Syllabus change	Consequence for using CCPC
Section 8.2.1 In MC Delete DP 2 Delete DP 4 Delete DP 5 Change DP 9 In RC Insert new DP 1 Partially delete old DP 2	None Omit Section 1.5 Omit Section 1.23; omit Exercise 29 on page 32 None None None
Section 8.2.2 In MC Delete DP 1 Delete DP 2 Delete DP 6	Omit Sections 1.21 and 1.22; omit Exercises 27 and 28 on page 32 None Could omit Section 1.27 though this is a good place for a first introduction to the Periodic Table.
Section 8.2.3 In MC Delete DP 2 Insert new DP 3 (after old DP 3) Other changes and the one in RC are 'editorial' rather than substantive	Puzzling deletion: students still need to know the basics of atomic structure to understand other syllabus topics Treated in Section 2.9 None
Section 8.2.4 In MC Delete DP 3, 6 and 7 In RC delete DP 4	None None
Section 8.2.5 In MC Delete DP 2 Delete DP 3 Delete old DP 8 Insert new DP 8	None (At first sight this may suggest that Table 1.2 on page 7-8 can be omitted, but really that table is there to illustrate mixtures, compounds and elements so still is needed.) Was delayed until Chapter 5 in CCPC At first sight could omit Section 2.21 and related part of Table 2.6, but covalent lattices are required in RC DP 2 (and for graphite and diamond in Module 3) so this material is still required. Already in CCPC (bottom of page 60)

The (original) syllabus and the book

Although Conquering Chemistry Preliminary Course (CCPC) covers all the material in Module 1 of the original syllabus, it takes a slightly different approach.

CCPC begins similarly to the syllabus by starting (Syllabus Section 1) with the idea that different parts of the Earth are made up of different mixtures, describing some important ones, and discussing separation of mixtures (including some gravimetric analysis of mixtures). It then (Syllabus Section 2) looks at the five most abundant elements in the different 'spheres' of the Earth and at the forms in which they occur (Sections 1.21 to 1.23 which now can be omitted), introducing the ideas of constancy of elemental composition and that only non-reactive elements occur in uncombined states (Sections 1.24 and 1.25, still in the syllabus). Classification into metals and non-metals is introduced as is the Periodic Table. That makes up Chapter 1.

CCPC then combines Syllabus Sections 3 and 5 into Chapter 2 to present a more chemically coherent presentation of atomic structure and chemical bonding and its consequences for properties.

Syllabus Section 4, basically physical and chemical changes (reactions), is treated in the first part of Chapter 3. The second part is a treatment of formulae and naming of simple inorganic compounds (Syllabus Sections 1, ninth dot point, and 4, sixth dot point).

CCPC also includes significant amounts of revision of Stages 4 and 5 material (see below).

For all modules CCPC attempts to cover all items in the Students learn column, to cover some items in the Students do column and to include exercises on as many of the other Students do items as possible. Exercises based on experiments are also common.

For a more detailed comparison of CCPC with the syllabus click on
Charts relating C.C. sections to syllabus items. These tables show the Students learn column of the syllabus (in abbreviated form) along with the sections of CCPC that treat individual items.

Some comments

1. Revision
At first sight Module 1 in C.C. appears very long (100 pages compared with an average of 80 per module in the Preliminary Course). This is mainly because the book revises a lot of material that the syllabus takes as assumed knowledge, namely

mixtures, compounds and elements
atoms and molecules
some relationships between elements in the Periodic Table
particle theory of matter
atomic structure (nucleus, electron cloud, protons, neutrons)
word equations and qualitative descriptions of reactants and products in decomposition reactions
common names and formulae for common compounds

This material is all treated in Conquering Chemistry so that if teachers feel the need to revise it, it is easily accessible, and if students need to look up the meaning of a basic terms or concepts, they can use this text.

2. Biosphere
There may be a problem with the word biosphere. All standard texts – biological, geological, chemical – define it as the portion of the Earth inhabited by or used by living matter (and so it is the hydrosphere, atmosphere and part of the lithosphere). See page 10. However from the contexts in which the word is used in the syllabus, the meaning appears to be living matter. It is perhaps necessary to treat both the proper biosphere and living matter (that is what C.C. does on pages 27 to 30).

3. How many naturally occurring elements?
Page 27 claims that there are 92 naturally occurring elements. I have been told that this is incorrect. Apparently the 'official' answer to a question in a past HSC paper was that there are 89 – atomic numbers 1 to 92, but excluding technetium (At No 43), promethium (61) and astatine (210). The CRC Handbook of Physics and Chemistry claims that there are 91– atomic numbers 1 to 94 less technetium and promethium (and presumably another one). That source claims that both neptunium (At No 93) and plutonium (94) have been found in trace amounts in certain uranium-containing rocks. They are believed to have been formed by reaction of neutrons with uranium in natural transmutation processes.

Actually there are several other elements such as francium and astatine which have not strictly been found in nature, but which are conceptually present on Earth. These are elements with only short-lived isotopes that are formed in the radioactive decay series of various naturally-occurring uranium and thorium isotopes. Because the U and Th isotopes are naturally present, and because in the laboratory we have identified their decay products, we must conclude that these decay products are present on Earth, even if in extremely small quantities. One radioactive decay series is discussed on page 72 of Conquering Chemistry HSC Course.

This question of how many naturally occurring elements raises an important point about the methods of scientific discovery. Because we have not found technetium and promethium, does this prove that they are not present on Earth? Because you cannot find the needle in the haystack, does that prove it is not there? Of course the short half lives of all the known isotopes of promethium (less than 18 years) is added evidence for its non-occurrence on Earth; however technetium does have a couple of quite long-lived isotopes (10⁶years). On balance the evidence for the non-occurrence of Tc and Pm on Earth is strong, but not absolute.

Supplementary material

1. Properties of elements and compounds (Section 1.2)
Table 1.2 on pages 7-8 contrasts the properties of a mixture, a compound and one of the elements making up the compound. Further contrasts between the properties of compounds and the elements that make them up are shown in the following tables.

Table S1.1
Properties of the compound sodium chloride and the elements forming it
Sodium	Chlorine	Sodium chloride
lustrous silvery solid	pale yellow-green gas	white crystalline solid
soft and pliable		hard and brittle
melts at 98^oC	condenses to liquid at –35^oC	melts at 800^oC
conducts electricity		solid does not conduct electricity
combines rapidly with atmospheric oxygen	unaffected by air	unaffected by air
violent chemical reaction with water	dissolves slightly in water	readily dissolves in water; solution conducts electricity
reacts chemically with chlorine, sulfur, phosphorus	reacts chemically with aluminium, zinc, copper	does not react with any elements

Table S1.2
Properties of the compound carbon disulfide and the elements forming it

Carbon (graphite)	Sulfur	Carbon disulfide
black powdery solid	yellow solid	colourless liquid
odourless	odourless	unpleasant odour
melts at 3727^oC	melts at 113^oC	boils at 46^oC melts (freezes) at –111^oC
conducts electricity	does not conduct electricity	does not conduct electricity
burns in air	burns in air	does not burn in air
insoluble in hexane	insoluble in hexane	soluble in hexane

Table S1.3
Properties of haematite (iron(III) oxide) and the elements that make it up
Iron(III) oxide, Fe₂O₃ haematite	Iron, Fe	Oxygen, O₂
red powder (solid)^a	shiny grey solid	colourless gas
melting point 1565^oC	melting point 1535^oC	boiling point –183^oC
density 5.2 g/mL	density 7.9 g/mL	——
does not conduct electricity; poor conductor of heat	good conductor of electricity and heat	does not conduct electricity; poor conductor of heat
hard and brittle	malleable and ductile	——
can be converted to a simpler substance, iron by heating with carbon so is a compound	cannot be decomposed into simpler substances so is an element	cannot be decomposed into simpler substances so is an element
fairly unreactive	burns when heated in oxygen to form Fe₂O₃; rusts in moist air	reacts with many elements, both metals and non-metals
^a often mined as a brown-black rock

2. Other examples of elements, compounds and mixtures
(to supplement the example on pages 8(bottom)–9)

(b) Copper is a pure substance. How do we show that it is an element? First it does not decompose when we heat it in the absence of air or when we pass an electric current through it. Secondly it undergoes a variety of chemical reactions with oxygen, chlorine, sulfur, nitric and sulfuric acids, silver nitrate solution, and so on. And in all of these reactions the copper-containing substance has a greater mass than the starting copper had. This shows that copper is an element, since it cannot be split directly or indirectly into two or more simpler substances.

Similar arguments were used for all the elements to prove that they were, in fact, elements.

(c) Lead nitrate is a white solid. It is soluble in water. It is homogeneous and its properties do not change after repeated purification procedures. Lead nitrate is therefore a pure substance. When lead nitrate is heated, a brown gas is evolved and a white solid remains. This solid is insoluble in water. Hence it is not just left-over lead nitrate. This new white solid always has a smaller mass than the sample of lead nitrate originally taken. Lead nitrate can therefore be decomposed into two other substances, a brown gas and another white solid. Lead nitrate is thus a compound. When heated it decomposes into nitrogen dioxide (the gas) and lead oxide, an insoluble white solid.

3. Other separation methods (Sections 1.6 to 1.13)
Sections 1.6 to 1.13 treat the separation methods that are specifically mentioned in the syllabus. Some other methods that are widely used will now be described.

(a) Centrifuging
Sedimentation occurs quickly if the solid particles are relatively big or dense. For smaller particles it can take an inconveniently long time. Sedimentation can be speeded up by centrifuging the mixture. This means putting the mixture in a suitable container and spinning it so that the solid particles get subjected to centrifugal forces which are much stronger than the force of gravity. This pushes the solid particles outwards and so away from the liquid. The machine that does this is called a centrifuge. In a laboratory centrifuge the mixture is placed in a large test tube which is then spun so that the solids are forced to the bottom of the tube; the clear liquid can then be decanted or sucked off. The photo below shows a laboratory centrifuge and a sample before and after centrifuging: it comes from Conquering Chemistry HSC Course (CCHSC), page 201.

Paints are dispersions of small solid particles in liquids – water for water-based paints and hydrocarbons (petrol-like liquids) for oil paints. We would need to let paint stand for several weeks for the solids to settle to the bottom of the container. However if we centrifuge samples of paints, we can force the particles to settle out much more quickly. Blood is a dispersion of solids including red blood cells in an aqueous solution called plasma. We can separate the solid matter from the plasma by centrifuging.

Centrifuging is widely used in industry to separate solids from liquids. Sometimes the centrifuge is designed to fling the water or solution away from the solids as in domestic clothes washing machines. In sugar mills and refineries crystalline sugar is separated from the syrup from which it formed in a similar way: the syrup is flung outwards through holes in the spinning drum to leave almost dry sugar crystals inside.

(b) Coagulation and decanting
When a suspension of very fine particles in water is boiled, the particles often collide with each other with such force (at the higher temperature) that they stick together and form much bigger (heavier) particles. These bigger particles then more readily settle to the bottom of the container and so allow the clear liquid or solution to be decanted off. This process of small particles combining to form bigger ones is called coagulation. A combination of coagulation and decanting (page 13) is commonly used to obtain better quality drinking water on extended wilderness camping trips: muddy water is boiled for half an hour or so (to coagulate the particles) then let stand overnight. By morning the clay (mud) has settled to the bottom and the clear water can be decanted off for drinking and cooking.

(c) Magnetism
If one substance in a mixture is magnetic while the others are not, then we can separate out the magnetic substance with a magnet. A mixture of iron filings and sulfur can be separated in this way.

Magnetic separations are widely used to separate magnetic materials (mainly iron and steel) from municipal garbage.

(d) Sublimation
While most substances pass from solid to liquid to gas, there are some such as dry ice (solid carbon dioxide), iodine and ammonium chloride which change directly from solid to gas. This is called sublimation. If some iodine crystals are placed in a conical flask with a test tube containing ice water suspended in it as in photo on page 78 and gently heated, purple vapour will be seen to form from the crystals and it will condense back to crystals on the cool surface of the test tube. This is sublimation followed by condensation back to solid.

Sublimation is the process in which a solid changes directly to a gas without passing through the liquid state.

Sublimation can be used to separate mixtures such as ammonium chloride and sodium chloride (both soluble in water).

(e) Paper chromatography
Paper chromatography is a technique for separating mixtures. Separation occurs because the substances to be separated have different solubilities in two solvents. In school laboratories paper chromatography is often used to separate the components of various inks. A spot of ink is placed on a strip of filter paper which is then suspended in a suitable liquid. As the liquid creeps up the paper (by capillary attraction), it ‘washes’ the components of the ink upwards at different rates. After a few minutes different-coloured, separated spots can be seen as is shown in the diagram below (from page 477 of CCHSC).

The separation comes about because the different substances have different solubilities in the two liquids involved. One liquid is water trapped in the cellulose fibres of the paper (called the stationary phase) and the other is the liquid which moves up the paper (called the mobile phase). Substances with low solubility in the stationary phase and high solubility in the mobile phase move up the paper quickly. Those with high solubility in the stationary phase and low solubility in the mobile phase move slowly. Hence a separation occurs.

There is a whole range of techniques for separating or analysing mixtures which go under the name of chromatography. Paper chromatography is discussed further in CCHSC on pages 476-8 with gas chromatography briefly introduced on page 189 (of that book).

4. The Periodic Table and electron configuration (Section 2.13)
It is instructive to see how the Periodic Table relates to the filling of electron energy levels. We can use Figure 2.9(b) on page 53 to obtain the order in which energy levels are filled, and by working through the Periodic Table (see inside front cover) we get the schematic table shown below.

The first (extremely short) period (hydrogen and helium) corresponds to the filling of the first energy level.
The second period corresponds to filling the second energy level (Li to Ne). The big horizontal gap has been left between Be and Al because in later periods we have to fit in extra elements.
The third period (Na to Ar) corresponds to semi-filling (going up to 8 electrons) the third energy level; this takes us to the stable argon configuration, 2, 8, 8.

The fourth period (the first long period) corresponds to first putting two electrons in the fourth level (K and Ca), then completing the third level (scandium through to zinc), and finally semi-filling the fourth level (to krypton 2, 8, 18, 8).
The fifth period is formed by putting two electrons into the fifth level (Rb abd Sr), then building the fourth level from 8 to 18 in what is another transition series (yttrium to cadmium), and finally semi-filling the fifth level to 8, ending with xenon (2, 8, 18, 18, 8).

Similar patterns apply for Periods 6 and 7.

5. Another example of ionic bonding (Section 2.15)
Magnesium and oxygen combine to form magnesium oxide.
Magnesium, with electron configuration (2, 8, 2), (Table 2.4), loses two electrons to become like neon (2, 8). Oxygen (2, 6) gains two electrons, also to become like neon.

By losing two electrons, the neutral magnesium atom becomes the doubly charged positive ion, Mg²⁺. Similarly, by gaining two electrons, the neutral oxygen atom becomes the doubly charged negative ion, O^2–, called the oxide ion.

As each magnesium atom loses two electrons and each oxygen atom gains two electrons, the compound formed will consist of one oxygen atom per magnesium atom. Its formula will be MgO, which we may write as Mg²⁺O^2– to emphasise the ionic bonding. Again there are no discrete molecules of MgO — just an infinite lattice of positive and negative ions very tightly bound together by electrostatic attraction.

6. More everyday applications (Section 3.5, page 85)
Several examples of the use of decomposition and direct combination reactions in everyday life are given in Section 3.5. Some additional ones are:

sodium chloride solution is electrolysed to form chlorine and sodium hydroxide, two very important industrial chemicals (to be discussed in CCHSC Chapter 10); this same electrolysis is used in some home swimming pools for chlorination (sanitisation) of pool water
in some industries water is electrolysed as a source of hydrogen gas (though this is not a major industrial source of hydrogen)
decomposition of silver salts particularly silver bromide by light is the basis of photography
ultraviolet light is used to decompose certain molecules in bacteria in some commercial sterilisers.
the burning of sulfur (combination with oxygen) is used industrially to make sulfur dioxide which is then used to make sulfuric acid, the most widely used industrial chemical