The views expressed in this Answers and Comments document are those of the author who had nothing to do with setting or marking the paper. The answers are not in any way 'official'. They are simply the responses of a competent chemist familiar with the syllabus and the textbooks in common use. The answers are probably  more detailed than would be required to gain full marks from HSC examiners but are a worthy goal for students to aspire to. Some general comments The official examiners' comments and a marking scheme (of sorts) are available on the  Board of Studies web site, www.boardofstudies.nsw.edu.au (Click on it to go there; find the required year (towards the bottom of the page if not the current one), select Chemistry in the alphabetical list, left or right click on the 'yes' in the exam report column (right column) to open or save it.) The 2006 exam paper continues the practice of putting heavy emphasis upon recall of factual knowledge (often with some interpretation of the recalled material required) as opposed to problem-solving. Again it includes some recall of experimental procedures, this year with heavy emphasis upon risk assessment. The table below gives a breakdown of marks versus type of exam questions over recent years. Type of question Marks 2003 2004 2005 2006 1 numerical calculationa  14 to 17b 8 19 to 23 22 2 non-numerical problem-solvinga   9 to 11 8 to 10 11 to 14 14 to 16 3 straight recall of factual materialc (describe, outline, summarise ...) 23 to 30 34 to 38 32 to 34 30 to 36 4 recall of factual material with some interpretation (evaluate, assess, analyse, discuss, compare ...) 38 to 46 41 to 45 28 to 31 29 to 35 5 descriptions of experiments (including risk assessments) 8 3 5 6 number of extended response questionsd 6 4 5 6 marks for extended response questions (included in categories 3 and 4 above)      31 23 30 22 a Includes chemical equations if required b There are ranges because different options distribute marks somewhat differently c Includes writing an equation or drawing a structure not associated with categories 1 and 2 d Parts (a), (b), (c) and (d) in options questions count as separate questions Averaged over the four years, about 30% of the marks have been for problem-solving (qualitative and quantitative) and 70% for recall of information with some interpretation of it, not a situation that those of us who think that chemistry is a problem-solving discipline are happy with. Extended response questions Extended response questions are the ones that require the most careful attention from students to make sure that they give the information asked for. With no guidance about how much emphasis to place on each aspect of the question, it is easy to give too much information about one aspect and completely neglect another equally important one. In tackling extended response-questions (questions that are just one or two sentences but worth 5 to 7 marks) you should start from the rule of thumb that the question requires one significant piece of information per mark, so a six mark question will require six pieces of relevant information; that probably means at least six sentences, often more. And it is going to require about 1.8 minutes per mark to answer as fully as the examiners want (11 minutes for a 6 mark question). Analyse the question carefully before you start writing to make sure you understand exactly what the question is asking for. If it contains two verbs such as saying 'describe' something and 'discuss' its importance or role in ..., then you need to split the marks between the two parts, say 3 for 'describe' and 3 for 'discuss'. If you feel that it is difficult to give 3 significant facts for the 'discuss' part, perhaps you could do a 4/2 split but a 5/1or 6/0 split would not be answering the question as asked. Even if you could easily give 6 significant facts for the 'describe' part, you must give some for the 'discuss' part even though that might be a struggle. Note that verbs such as evaluate and assess require you to make some judgment, so even if you cannot give a deep and insightful one, at least give some judgment even if it seems trivial  – it's a good thing, it's really important or it's of no great significance – because the examiners are looking for some sort of judgment and they do not have to agree with you to give you the mark (but they cannot give you the mark if the judgment is not there).  Only then are you ready to write your answer in the answer booklet. Note that point form presentation is acceptable (20001 HSC Notes from the Examination Centre Chemistry page 5). Questions from the 2006 exam paper that fit into this extended-response category are Questions 20, 22, 24 and part (c) in each option. Although Question 19 carries 7 marks, it is not an extended-response question, because it consists of several bits that contain quite specific instructions.  If you finish a question with specific instructions ahead of time, then you have extra time to give to questions that are causing you difficulties. However if you seem to have finished an extended-response question in much less time than was allocated to it, and have used only a small proportion of the space allocated to the question in the exam book, then look back over it very carefully; you may have omitted an aspect of the question. If not, then it is very likely that you have not given as much detail as the examiner is really expecting.  With extended-response questions be very careful that you answer the question asked and not answer a related question that you know a good answer for. Try to answer the question exactly as asked. If you cannot give enough detail about that question to fill the time allocation, then perhaps you could provide related but not strictly asked-for information, but do that only as a last resort. One reason for suggesting this is that sometimes it is not possible to spend the allocated time on the question asked; it was not a good question.  Dodgy questions Sometimes the question asked may seem to contain an error or not seem to make sense. For example in Question 20 you are asked to give a description of new materials and fuels that can be made from ethylene while Question 22 talks about concentrations of acidic oxides of sulfur in the atmosphere ... increasing since the Industrial Revolution. Sometimes you have to twist what you know to try to fit the question; for example for Question 20 by passing off 50 to 60 year old polymers as new materials or of treating ethanol as the fuel because you do not know of any other (neither do I!), but make sure you clarify that ethylene is not the source of the ethanol that is currently being talked about as a substitute motor fuel or fuel additive. In Question 22 state what you know to be the true situation, that there is no convincing evidence for long-term global increases in sulfur dioxide concentrations then talk about what has been increasing – the quantities of sulfur dioxide released into the atmosphere and why this has not lead to increases in global concentrations. Expanding knowledge requirements It seems that every exam paper produces extra factual material that students are required to know – a disturbing trend in what is already a fact-crammed syllabus and in a subject where the emphasis should be on problem-solving. For example, Question 3 requires that students know that zinc-65 is radioactive, a fact that cannot be deduced from the two conditions for stability set out in the textbooks. The sample answer given in the Notes from the Marking Centre for Question 29 (c) implies that students should know the chemical structure of styrene-butadiene rubber (in order to be able to produce a 7 mark answer to the question). Rubber is a better choice of natural product than my soap, but at least my answer on soap relies only on material that is in the main stream of the syllabus. When I wrote CCHSC I had no idea that the one dot point in syllabus section 9.5.1 would ever be the subject of a 7 mark exam question. Key verbs The Board of Studies has produced a list of verbs that examiners are to use in HSC exam papers and has provided definitions of them. You need to know these meanings and make your answers fit the verbs used. The Chemistry exam paper tries to use as many as possible of these key verbs and in order to do so, sometimes uses an inappropriate one. Analyse (identify components and relationships among them; draw out and relate implications) has been used occasionally in recent years but its meaning has been more like explain rather than the one in the brackets. You need to be aware that the meanings of the verbs can occasionally depend upon the context. Experiments As the table above shows there are always a few marks for accounts of compulsory experiments in the course. In your exam preparation you need to prepare brief accounts of the procedures you followed in each of the 22 to 24 compulsory experiments, and a brief summary of the results and conclusions that you should have obtained. When you did these experiments, sometimes things went wrong and you did not get the expected results. When you were writing up these experiments in class you had to report the results you actually obtained and offer explanations for them. In preparing for the exam assume everything worked properly and make sure that you know what the proper results should have been for each experiment, because that is what the examiner is looking for.       You need to know what your procedures were because examiners heavily penalise students who make such serious boo-boos that they reveal that they did not actually do the experiment (such as using propene or butene as the alkene upon which to perform the bromine test, or heating a mercury compound in order to perform a flame test on it.      Another aspect of experimental work is the risk assessment you are required to do for each experiment. Such assessments figured strongly in the 2006 exam. For many experiments there are specific hazards that you should be well aware of, such as proper disposal of heavy metals in cation testing or safe handling of sulfuric acid in testing it as an oxidising and dehydrating agent. For other experiments such as performing an acid base titration or testing water quality there are no particular hazards; in these cases you need to be aware of general laboratory precautions and procedures to ensure laboratory safety, such as wearing goggles, using pipette fillers and keeping flames away from cluttered benches and solvents. Make sure you include brief risk assessments along with your summaries of experimental procedures. Comments relating to specific questions are given beside the sample answers below. Answers and comments   You need a copy of the exam paper to make sense of what follows. If you do not already have one you can get a copy from the Board of Studies web site, www.boardofstudies.nsw.edu.au . Click on 'HSC exam papers', scroll down the page till you find '2005 exam papers', click on it, choose ' Chemistry' in the alphabetical list, click on the 'yes' in the exams column (middle one) then you have the option of opening or saving it. You can also get a copy of the examiners report while you are there: click on the 'yes' in the right hand column . Section I part A (Multiple Choice questions)

Question & Answer		Comment (and relevant page in CCHSC if appropriate)
1.	C	Ethylene is obtained either as a byproduct of catalytic cracking of crude oil fractions or by deliberately steam cracking mixtures of alkanes distilled from crude oil. Although ethylene can be made from ethanol, this has not been done industrially since before World War II; today ethanol is commonly made from ethylene. Although ethylene could in principle be made from glucose, first by fermenting glucose to ethanol then dehydrating ethanol, this does not occur industrially. Polyethylene is made from ethylene, not vice versa. (p 5-7)
2.	A	You need to know the catalysts used for these two reactions (p 30-1)
3.	B	A and D are easily eliminated; both isotopes of uranium are radioactive (as are all isotopes with atomic mass > 83) and even if you do not know that C-14 is radioactive, you should recognise that isotopes heavier than the normal ones (C-12 in this case) are radioactive. This same reasoning might lead you to eliminate C by thinking O-18 is radioactive (right elimination but for the wrong reason!) and so conclude that B was correct. Actually in C it is Zn-65 that is radioactive, something I do not believe HSC students should be expected to know. All three isotopes of oxygen are stable. The alternative approach is to recognise that O-16, K-39 and C-12 are the common isotopes of those elements and so decide that they are all stable and hence B the correct answer without bothering to identify the unstable one in C. In my view this question is unfair for HSC students. (p 75-6)
4.	C	Heat absorbed by the water = 0.300 kg x 4.18 x103 J kg–1 K–1 x (45.5 –25.0) = 2.57 x 104 J This is the heat released by the combustion of 1.15 g ethanol so heat released by one mole ethanol (46 g) = 2.57 x 104 x ( 46/1.15) = 1.03 x 103 J  = 1030 kJ mol–-1 (p 36-7)
5.	B	Draw the carbon chain C–C–C then see where you can place the two Cl atoms: 1,1-, 1,2-, 1,3- and 2,2-. Be careful to recognise that 2,3- is the same as 1,2- and that 3,3- is 1,1- (that's where the 5 and 6 answers come from). Note that both Cls can be on the one C atom – otherwise you might choose answer A. (p 250 and references therein)
6.	B	The polymers that glucose forms are condensation polymers, one that form by the elimination of a small molecule between pairs of monomers, in this case water. You need to know that. To join together five glucose molecules four water molecules must be eliminated. Hence Molecular weight of polymer = 5 x (6 x 12 + 12 x 1 + 6 x 16) – 4 x (2 x 1 + 16) = 828   (p 22-3)
7.	A	A fact you need to know  (p 176).
8.	B	You need to know that hydrochloric acid is strong and that acetic acid is weak (that eliminates A and C), and know that strong and weak are quite different from dilute and concentrated: 0.1 is more dilute than 1.0. Hence B is the correct answer.  (p 140-2)
9.	C	D is wrong because it is about chemical equilibrium. A is wrong because the end point is when the colour is half way between starting and finishing colours and is the equivalence point only if the correct indicator has been used. B at first sight looks correct, but if the acid is di- or triprotic while the base is monobasic then B is incorrect. C is correct regardless of whether the acid is mono- di- or triprotic.  (p 151, 157-65)
10.	C	Molar mass of P2O5 = 2 x 31.0 + 5x 16.0  =  142.0 g/mol Number of moles of P2O5 used  =  1.42/142  =  0.010 Moles of H3PO4 formed  =  0.020 Moles of NaOH needed  =  3 x 0.020  =  0.060 mol Volume of NaOH needed  = 0.060/0.30  =  0.20 L    (p 160-2 and CCPC p 136, 217)
11.	D	According to Arrhenius an acid  was a substance that in aqueous solution produced H+ ions. It was used to classify acids as strong or weak based upon their ease of producing H+ ions in solution, so D is the required answer. This definition has nothing to do with pH so C is wrong. Water was the only solvent it dealt with so B is wrong (It was the Lowry-Bronsted definition that took account of the solvent.) The Arrhenius definition says nothing about oxygen content so A is wrong. It was Lavoisier who involved oxygen in a definition of acids but that had been rejected many decades before Arrhenius.  (p 146-7)
12.	A	This is the definition of catalysts: you need to know it. Catalysts do not affect the yield or purity of products; if anything the decrease the activation energy, not increase it. While relevant here, remember that catalysts do not alter the position of equilibrium for a reaction (p 202).    (CCPC p 291, 296-7)
13.	D	That is what a filter does – removes suspended particles from the water (but not dissolved substances). They have no effect upon the pH of the water, they cannot kill bacteria though they may filter them out of the water if the pore size is small enough, and though made of synthetic polymer they are generally not biodegradable. (p 292-4)
14.	D	Analysis 3 is an outlier – far removed from the other values so probably results from a mistake in that measurement or a faulty sample, so reject it and average the other four to get 0.39075, but there are only three significant figures in the results so there can be no more than three in the average, so D is the answer, not C. If you average all five results you get 0.3624 which rounds to 0.362. However with one result so divergent from the other four it is wrong just to average all results, so A and B are wrong answers.
15.	A	The equation for the Haber process is: N2(g) + 3H2(g) 2NH3(g) This shows that 4 moles ® 2 moles which means, by Le Chatelier's principle, that increasing the pressure will push the equilibrium to the right, that is will increase the yield of ammonia, so answers C and D are incorrect. The reaction is exothermic so as temperature increases the reaction moves in the direction that absorbs heat, that is to the left, so as temperature increases yield drops, meaning that A is correct. You need to know that this reaction is exothermic.   (p 200-2)