Examiner's tips for Pure Chemistry
EXAMINER TIPS for Pure Chemistry
General Advice
1. It is important that you write your answers in black or dark blue ink. This is because the papers are now
marked ‘on-line’ by the examiners [note : on trial for some papers]. For a similar reason, please use a dark black pencil for your drawings.
2. It is very important that you understand the words or phrases used by examiners to get you
to answer questions in a particular way. These terms are described in the syllabus in the
section Glossary of Terms. You may sometimes lose marks because you do not understand
what to write in response to the words ‘explain’, ‘describe’, ‘suggest’ etc. If you are unsure,
ask your teacher to tell you what each of these terms means. For example the term ‘explain’
means that you have to refer to some idea or theory and you have to write in detail to say
why something happens. For example, if you are asked ‘Explain why rate of a chemical
reaction increases when the temperature increases’, you have to use the idea of particle
collisions in your answer.
3. Make sure that you read the question fully, picking out the key words. For example in a
question such as ‘Give a use of graphite that depends on the ability of its layers of atoms to
move over each other’, it is common to find incorrect answers referring to properties rather
than uses. Therefore the answer slippery is incorrect but the answer ‘as a lubricant’ is
correct. It is useful to underline the key words in a question as you read it.
4. Take careful note of how many marks there are for a question. If there are 3 marks, you will
need to think of 3 different points that you can write down to answer the question. Take for
example, the question ‘Explain how increasing the concentration of acid affects the rate of
reaction between magnesium and hydrochloric acid’. The examiners are looking for:
a. the rate of reaction increases (1st mark) because the more concentrated the acid,
the closer together are the acid particles (2nd mark) the closer the particles, the more
frequent are the collisions (3rd mark)
5. Make sure that you keep referring back to the beginning of the question or main subsection
to get some vital points of information. This is especially important with calculations
6. Don’t be afraid to write down or choose the answer ‘no reaction’ if you feel that nothing
should happen. The lack of reaction still gives valuable information. This is most commonly
seen in questions such as ‘Write down the products of the reactions, if any, between (i)
chlorine and potassium bromide (ii) iodine and potassium chloride.’ The phrase ‘if any’
suggests a possibility that one of these pairs may not react. But it does not mean that this is
definitely the case!
Spelling
7. The correct spelling of chemical names is not always essential as long as they cannot be
mistaken for other chemicals. However, in simple questions where you are asked to select
the names of chemicals from a list, you are expected to get the spelling correct. Writing
ammonium for ammonia or chlorine for chloride will not be given credit because this is a
chemical mistake.
General Tips
8. Look for exactly what the question is asking. Many mistakes are made by not reading the
question correctly. If the question says give two observations apart from temperature
change’, then obviously you shouldn’t write ‘temperature change’.
9. Read over your answers and ask yourself ‘have I contradicted myself?’ This generally refers
to things written in the same sentence. A common error is to write something like – ‘On
adding ammonia a soluble blue precipitate is formed‘. There is confusion here because if
precipitate is not soluble. The correct answer to a question about adding excess ammonia to
copper ions would be: ‘On adding ammonia a light blue precipitate is formed. The precipitate
dissolves in excess ammonia solution.’ Notice that splitting it up into 2 sentences has altered
the meaning.
10. Significant figures are not the same as decimal places. For example 123.08 is to 2 decimal
places but to 5 significant figures. Zeros before any definite numbers do not count as
significant figures. So 0.000045 is to 2 significant figures.
11. When doing calculations, your final answers to each section should be to the correct number
of significant figures. Generally, it should be to the same number of significant figures as the
data. You may get penalised if you write your answer with too many significant figures e.g.
1.257487 instead of 1.26.
12. Make sure that you round up your answers correctly. For example, if an answer to 4
significant figures is to be rounded to 3 significant figures, the answer 22.56 should be
rounded up to 22.6 not down to 22.5.
13. Always show your working – even if your answer is wrong, you may get some marks for your
method. It is not sensible to work everything out on your calculator then just put down the
answer. If you make one slip you risk getting no marks for that question.
14. Make sure that you know your syllabus statements and definitions exactly. For example, the
use of naphtha as a feedstock for the chemical industry. This is because the Principal
Examiner has to use the syllabus as a basis for the exam questions.
15. Look out for the phrase ‘what you would observe’. This means that you must write down
what you see/ hear or feel. For example, ‘the test tube gets hot’. It is a very common error to
write something like ‘a gas is given off’ or ‘copper is deposited’. These are not observations.
16. Don’t get caught out by the phrase ‘describe what you would see’. Some students put down
observations about pops (sound) or heat given off.
17. When drawings diagrams
a.
Make sure they are large enough to fill the space given on the paper
b.
Make sure that you draw apparatus for gas measurement without any places for the
gas to escape. Don’t draw gas syringes with the plunger much smaller than the
syringe barrel. This is a common error.
c.
Always label your diagrams as fully as possible.
18. Stick to the number of examples requested by the examiners. For example, if a question
asks for two examples of a transition element, do not write three down. If one of the three is
incorrect, you will lose a mark. If a question asks for a single use for a substance, stick to
one – if you write long lists, the examiner will think that you are ‘playing safe’ and you won’t
get the mark. Take for example in the question ‘State two properties of transition elements
that are not shown by other metals’ (2 marks). The answer – ‘they are good catalysts, they
form ions with different charges, and they do not conduct electricity’ – will only gain one
mark. This is because, although the first two answers are correct, the last one is incorrect.
19. In chemistry, the examiners want you to draw graphs of reaction rates by joining up the
points with a curve of best fit. If you draw lines with a ruler from point to point, you will not get
the mark.
20. Make sure that you draw the curve on a graph so that it touches or is a close to each correct
point as possible. If one point is clearly incorrect, ignore it when drawing the curve.
21. On a graph, make sure that you draw the line in pencil so that you can rub it out if you make
a mistake.
Paper 1 (Multiple Choice) Tips
22. If you are unsure of the answer to a multiple choice question, don’t spend too long on it. Put
a star by it and return to it later.
23. Within a single multiple choice question, use a pencil to cross out the statements which are
clearly incorrect, then choose between those left.
24. In a multiple choice question don’t be swayed by one of the choices just because it has got a
longer (or shorter) statement than the others.
25. Don’t make any assumptions about the order of responses – just because there have been
two answers ‘D’ in sequence, it does not mean that the next answer cannot be ‘D’.
26. Take care to read the whole question word by word. For example, in the question ‘what is the
ratio of the volumes of 2g of H2 and 16g of methane, CH4, at r.t.p?’ Many Candidates will
focus on the numbers and ignore the word ‘volume’. Just a quick look at the figures gives the
incorrect answer 1:8 (using the molar gas volumes gives 1:1 as the correct choice)
27. When given a choice of picking out a noble gas from a group of electronic structures, don’t
jump to the conclusion that noble gases always have 8 electrons in their outer shell.
Remember that helium has 2!
28. When given a choice about electrical conductivity of ionic structures remember that the
conduction is due to IONS moving (not electrons). The ions can only move when the ionic
compound is molten or when dissolved in water.
29. When given choices of why alloys are hard, it’s not the mass of the atoms which is important
but their size. Remember that metals have layers which slip over each other. A different
sized atom will distort the layers and stop them slipping over each other. This makes the
alloy harder than the pure metals.
30. When given choices about the rate of diffusion of gases, remember that the rate of diffusion
depends on the mass of the molecules. Heavier molecules (lower relative molecular mass)
move and diffuse slower than lighter molecules. If you are unsure which molecule is heavier,
use your Periodic Table to calculate the relative molecular masses.
31. If you are given a choice of tap water and several other substances as examples from which
to select a pure compound, it’s not going to be tap water. It is a common error to think that
tap water is pure. It’s a mixture. (Don’t be fooled by the adverts of the mineral water
companies which say ‘pure mineral water’!)
32. If you are given choices of electronic structures of atoms to select to make a compound of
type XY2, first check the type of compound that the examiner wants e.g. ionic or covalent. If
it is ionic, then you can choose an atom with one or two electrons in its outer shell and
combine it with a non-metal atom. If it is covalent look for the structures of two non-metal
atoms i.e. those with 4 to 7 electrons in their outer shell. Remember that the number of
electrons in the outer shell is equal to the group number.
33. Remember that the valencies (combining powers) of the elements in Groups V to VII are
found by taking the group number away from 8. For example, the valency of oxygen in an
oxide is 8-6 = 2. (Oxygen is in Group VI)
Paper 2 (Theory) Tips
34. Reading the question thoroughly and noting the number of marks available is important. In
response to a question such as ‘Use your knowledge of the structure of metals to explain
how they conduct electricity’, many Candidates just write down that ‘metals have a sea of
electrons’, thinking that this answers everything. What the examiners are looking for is
a. The idea of positive ions in a sea of electrons and
b.
The fact that the electrons move.
35. Remember that it is the moving valence electrons which are responsible for the conduction
of electricity in metals. If you just state that ‘the electrons move’ it could refer to any electrons
in the atoms. It is also a common error to suggest that ions are responsible for this
conduction.
36. If you are asked to compare things in the question, your answer must also make the
comparison obvious. In response to the question ‘How do different proportions of carbon
affect the properties of steel?’ the answer ‘mild steel has a low % of carbon’ will not receive
any marks because nothing has been said about steel with a higher % of carbon. An answer
such as ‘the higher the % of carbon the more brittle the steel’ gets the marks because this is
a comparison.
37. Properties of transition elements often cause problems. Remember that transition elements
themselves are NOT colored, it is their compounds that are colored.
38. If you see the words ‘What observations are made?’ remember that this means what you
39. See, hear or feel and NOT for example, ‘gas given off’.
40. Make sure that you know the use of the various substances stated in the syllabus. Go
through your syllabus and make a note of these. For example, the uses of the naphtha
fraction from petroleum distillation are not well known. Make a list of all the substances in the
syllabus whose uses you need to know and test yourself by making ‘flash cards’ for these.
41. If you are asked to describe the meaning of a term which has two words in it, you must make
sure that the description has included the meaning of both words. So to get full marks for the
question ‘What is the meaning of the term saturated hydrocarbon?
You have to define
a. saturated
b. hydrocarbon.
42. Always check back in the question to see the wording in the stem of the question. You will
not get any marks for repeating words in the stem of the question or putting what is in the
stem into different words. For example, if the stem includes the words ‘explain why the
reaction rate increases as the concentration of acid increases’, you will not get any marks for
putting this in another way e.g. ‘the reaction speeds up because the concentration of acid
increases’.
43. Be on the lookout for questions involving processes e.g. ‘What advantages are there in using
hydrogen as a fuel?’ The section underlined is a process. The answer ‘hydrogen is not
polluting’ is therefore not correct because there is no indication that it is being used as a fuel.
A better answer would be ‘it forms no pollutants when burnt’.
44. Watch out for the words ‘if any’. e.g. ‘State the products in the following reactions if any’.
Sometimes the examiner wants to test your understanding of why a reaction does NOT
occur.
45. If you are asked to draw electronic structures of atoms or ions, it is best to draw full
electronic structures showing all shells, not just the outer shell. If the Examiner wants you to
draw just the outer shell, that will be stated in the question.
46. When drawing the structure of ions, the charge must be put at the top right hand corner. You
will not gain any marks if you put the charge in the nucleus as do some Candidates.
47. Explaining the properties of ionic structures is always a problem area. Never mention atoms,
molecules, covalent bonds or sharing electrons even if you mention ions as well. In answer
to the question ‘Why does sodium chloride conduct electricity when molten? The answer
‘because the ions and the electrons can move’ gets no marks. This is because the incorrect
‘electrons’ negates the mark for ‘ions’.
48. A common error involving the conductivity of molten ionic compounds is to state that the
electrons are responsible for conduction. Remember that it is the IONS which move when
ionic compounds conduct electricity.
49. It is a common mistake to believe that energy is needed to form bonds. In answering
questions about bond making and bond breaking, think about a model of a compound – to
break the bonds, you have to physically pull them apart. In other words you’re putting in
energy. To form bonds, it must be the opposite i.e. energy is given out on bond formation.
50. Look out for the term ‘explain’. This indicates that you have to write in detail about why
something happens. If for example you are asked to explain why the reaction between
copper (I) chloride and chlorine is a redox reaction, you must write about redox in terms of
either electrons or oxidation number changes.
51. Make use of all the information given in a question, including graphs. For example if a graph
of % yield of ammonia against temperature for various pressures is given, it is there for a
purpose. If you use the information given you are less likely to lose marks than if you try to
remember figures from a book.
52. Never write ammonium hydroxide as the product when ammonia dissolves in water –
ammonium hydroxide it does not exist! (Even though you may see it still on bottles in the lab
and even in some books). The correct term is aqueous ammonia. In addition make sure that
you know the difference between ammonia, NH3 and ammonium, NH4+. The latter is an ion
present in ammonium salts.
53. The number of carbon atoms in formulae of carboxylic acids often causes problems. Make
sure that you include the carbon of the –COOH group when you name the acid.
CH3CH2CH2CH2COOH has 5 carbon atoms so it is the 5th member of this homologous
series, pentanoic acid.
54. Take care with the formulae of the metal salts of the carboxylic acids if the metal is from
Group 2. Remember that group 2 metals form 2+ ions and so they need 2 carboxylate ions
to balance. E.g. the formula for magnesium ethanoate is (CH3COO)2 Mg.
55. When doing mole calculations if given an equation such as:
Mg + 2CH3CO2H → (CH3CO2)2 Mg + H2
You ignore the 2 in the equation when calculating the molar mass of ethanoic acid. So the
molar mass of ethanoic acid is 60 not 120. Remember though that when calculating reacting
masses, the 2 needs to be taken into account because two moles of ethanoic acid react with
only one mole of magnesium. The 2 also needs to be taken into account when doing
calculations involving limiting reagents.
56. Try to be as accurate as possible in all your answers. In response to a two mark question
such as ‘Explain why the reaction between ethanoic acid and magnesium much slower than
the reaction between hydrochloric acid and magnesium?’ it is too vague to write ‘Ethanoic
acid is a weak acid’. ‘A weaker acid’ would be a more acceptable answer. Some reference to
the differences in hydrogen ion concentration or degree of dissociation is also needed.
57. When there are questions with large unspecified mark allocations (for example 6 marks) you
need to plan your answers out carefully and need to underline or list on the question paper
all the points that need to be written about. It is very common to miss out important points in
extended questions like this. When you have written a list of the points you need to deal with,
you can cross these out one by one as you complete them. E.g. equations, gas tests, color
changes etc.
58. Answers to questions on ‘chemistry and the environment’ are often answered in a too vague
manner. The word ‘pollution’ is too vague to be given credit as an answer to any question. In
answering a question such as ‘What is a disadvantage of the use of nylon for fishing nets’,
pollution would not score, neither would vague statements such as ‘Dangerous to sea life’.
‘Non-biodegradable’ would score the mark because it is much more exact.
59. In doing calculations, always check that the relative molecular masses are correct. Incorrect
addition or extraction of the relative atomic masses is often a reason for failure in a
calculation. Also, make sure that you double check that you have used atomic masses and
not atomic numbers. If you are unsure, use the key in the Periodic Table at the bottom left to
check which number is which.
60. Look out for the word ‘each’. For example, in the question ‘Explain the purpose of adding
each of calcium hydroxide and ammonium sulphate to soil. You will lose marks if you do not
make it clear exactly which compound you are writing about. To make sure that there is no
ambiguity, start each sentence with the name of the compound. In this case: calcium
hydroxide is added to….. Ammonium sulphate is added because……
61. Avoid missing out connecting processes. For example if you are asked about how
ammonium sulphate helps soil fertility, an answer that ‘ammonium sulphate provides nitrate
ions’ is not good enough. This answer suggests that the ammonium sulphate contains nitrate
ions. A better answer would include the connecting process. For example: ‘Ammonium ions
are converted by micro-organisms in the soil to nitrate ions.’
62. Know the difference between –ides and –ates. Compounds ending with –ide contain only two
types of atoms e.g. magnesium oxide, potassium chloride. Compounds ending with – ate
contain three or more types of atom, one of which is usually oxygen e.g. sodium sulphate,
potassium nitrate. The ions of ides are simple e.g. sulphide, S2- whereas – ates have
compound ions e.g. carbonate CO3263. When writing oxidation numbers, remember that the sign + or – should be included. The
oxidation number is not written like the charge on the ion. For example Cu2+ is a copper (II)
ion. The oxidation number of copper in this ion is +2. (rather than 2+)
64. Make sure that you know the solubility rules for various compounds. These are important
when writing state symbols in equations. If you know that carbonates of group II metals are
insoluble in water, then you know to write CaCO3(s) in an equation rather guessing.
65. When deciding which method to use to make a given salt, you need to be able to remember
to use the solubility rules. For example, to make an insoluble salt from two soluble salts you
use a precipitation reaction. In order to make a soluble salt from two soluble salts, you use
titration
66. Know the difference between (aq) and (l). The state symbol (aq) refers to a substance
dissolved in water. The state symbol (l) refers to a substance as a pure liquid e.g. Br2(l),
H2O(l).
67. Remember that when writing equations for the reactions between aqueous solutions of
halide and aqueous halogens, the state symbols are all (aq). This is because the halogens
are dissolved in water in the first place.
68. When given graphs which read back in time from the present day be careful to remember to
read the graphs in a forward direction if you are asked about the order of a sequence of
events e.g. how carbon dioxide concentration changed over the last 2000 million years.
69. Always read the scales on graphs very carefully especially when very large numbers are
involved. For example if the graph has the figures 1000, 2000, 3000 with the words ‘millions
of years’ underneath, it is all too easy to miss the word ‘millions’ when answering questions.
70. Remember that if you want to separate a particular gas from the air, you can’t just heat the
air up. You have to make it liquid first by lowering the temperature so that all the gases
liquefy. Then you raise the temperature gradually and collect the gases as they evaporate off
one by one. Although the actual process is more complex, this is all that you have to know
for your examination.
71. With questions which require extended answers, especially in part B of the paper do not
write too much. There is often a danger that you will contradict yourself.
72. When asked to draw diagrams, make sure that you have included all the pieces of apparatus
necessary. Go over each point in the question carefully to check. It is quite common for
example, to leave out the test tubes to collect gases when asked to draw the apparatus for
electrolysis and test the gaseous products.
About the Examiner
Dr. Roger Norris has been an examiner for many years. He is currently a Principal Examiner for
Cambridge International ‘O’ level Chemistry as well as IGCSE Chemistry. He has also been involved
in developing and examining OCR Chemistry courses and examinations in Britain. As well as undertaking
several years of research he has 28 years of teaching experience in Chemistry (as well as in Biology and
Physics). He has contributed to a number of publications ranging from General Science for 11-13 year
olds to advanced level Chemistry courses.
General Advice
1. It is important that you write your answers in black or dark blue ink. This is because the papers are now
marked ‘on-line’ by the examiners [note : on trial for some papers]. For a similar reason, please use a dark black pencil for your drawings.
2. It is very important that you understand the words or phrases used by examiners to get you
to answer questions in a particular way. These terms are described in the syllabus in the
section Glossary of Terms. You may sometimes lose marks because you do not understand
what to write in response to the words ‘explain’, ‘describe’, ‘suggest’ etc. If you are unsure,
ask your teacher to tell you what each of these terms means. For example the term ‘explain’
means that you have to refer to some idea or theory and you have to write in detail to say
why something happens. For example, if you are asked ‘Explain why rate of a chemical
reaction increases when the temperature increases’, you have to use the idea of particle
collisions in your answer.
3. Make sure that you read the question fully, picking out the key words. For example in a
question such as ‘Give a use of graphite that depends on the ability of its layers of atoms to
move over each other’, it is common to find incorrect answers referring to properties rather
than uses. Therefore the answer slippery is incorrect but the answer ‘as a lubricant’ is
correct. It is useful to underline the key words in a question as you read it.
4. Take careful note of how many marks there are for a question. If there are 3 marks, you will
need to think of 3 different points that you can write down to answer the question. Take for
example, the question ‘Explain how increasing the concentration of acid affects the rate of
reaction between magnesium and hydrochloric acid’. The examiners are looking for:
a. the rate of reaction increases (1st mark) because the more concentrated the acid,
the closer together are the acid particles (2nd mark) the closer the particles, the more
frequent are the collisions (3rd mark)
5. Make sure that you keep referring back to the beginning of the question or main subsection
to get some vital points of information. This is especially important with calculations
6. Don’t be afraid to write down or choose the answer ‘no reaction’ if you feel that nothing
should happen. The lack of reaction still gives valuable information. This is most commonly
seen in questions such as ‘Write down the products of the reactions, if any, between (i)
chlorine and potassium bromide (ii) iodine and potassium chloride.’ The phrase ‘if any’
suggests a possibility that one of these pairs may not react. But it does not mean that this is
definitely the case!
Spelling
7. The correct spelling of chemical names is not always essential as long as they cannot be
mistaken for other chemicals. However, in simple questions where you are asked to select
the names of chemicals from a list, you are expected to get the spelling correct. Writing
ammonium for ammonia or chlorine for chloride will not be given credit because this is a
chemical mistake.
General Tips
8. Look for exactly what the question is asking. Many mistakes are made by not reading the
question correctly. If the question says give two observations apart from temperature
change’, then obviously you shouldn’t write ‘temperature change’.
9. Read over your answers and ask yourself ‘have I contradicted myself?’ This generally refers
to things written in the same sentence. A common error is to write something like – ‘On
adding ammonia a soluble blue precipitate is formed‘. There is confusion here because if
precipitate is not soluble. The correct answer to a question about adding excess ammonia to
copper ions would be: ‘On adding ammonia a light blue precipitate is formed. The precipitate
dissolves in excess ammonia solution.’ Notice that splitting it up into 2 sentences has altered
the meaning.
10. Significant figures are not the same as decimal places. For example 123.08 is to 2 decimal
places but to 5 significant figures. Zeros before any definite numbers do not count as
significant figures. So 0.000045 is to 2 significant figures.
11. When doing calculations, your final answers to each section should be to the correct number
of significant figures. Generally, it should be to the same number of significant figures as the
data. You may get penalised if you write your answer with too many significant figures e.g.
1.257487 instead of 1.26.
12. Make sure that you round up your answers correctly. For example, if an answer to 4
significant figures is to be rounded to 3 significant figures, the answer 22.56 should be
rounded up to 22.6 not down to 22.5.
13. Always show your working – even if your answer is wrong, you may get some marks for your
method. It is not sensible to work everything out on your calculator then just put down the
answer. If you make one slip you risk getting no marks for that question.
14. Make sure that you know your syllabus statements and definitions exactly. For example, the
use of naphtha as a feedstock for the chemical industry. This is because the Principal
Examiner has to use the syllabus as a basis for the exam questions.
15. Look out for the phrase ‘what you would observe’. This means that you must write down
what you see/ hear or feel. For example, ‘the test tube gets hot’. It is a very common error to
write something like ‘a gas is given off’ or ‘copper is deposited’. These are not observations.
16. Don’t get caught out by the phrase ‘describe what you would see’. Some students put down
observations about pops (sound) or heat given off.
17. When drawings diagrams
a.
Make sure they are large enough to fill the space given on the paper
b.
Make sure that you draw apparatus for gas measurement without any places for the
gas to escape. Don’t draw gas syringes with the plunger much smaller than the
syringe barrel. This is a common error.
c.
Always label your diagrams as fully as possible.
18. Stick to the number of examples requested by the examiners. For example, if a question
asks for two examples of a transition element, do not write three down. If one of the three is
incorrect, you will lose a mark. If a question asks for a single use for a substance, stick to
one – if you write long lists, the examiner will think that you are ‘playing safe’ and you won’t
get the mark. Take for example in the question ‘State two properties of transition elements
that are not shown by other metals’ (2 marks). The answer – ‘they are good catalysts, they
form ions with different charges, and they do not conduct electricity’ – will only gain one
mark. This is because, although the first two answers are correct, the last one is incorrect.
19. In chemistry, the examiners want you to draw graphs of reaction rates by joining up the
points with a curve of best fit. If you draw lines with a ruler from point to point, you will not get
the mark.
20. Make sure that you draw the curve on a graph so that it touches or is a close to each correct
point as possible. If one point is clearly incorrect, ignore it when drawing the curve.
21. On a graph, make sure that you draw the line in pencil so that you can rub it out if you make
a mistake.
Paper 1 (Multiple Choice) Tips
22. If you are unsure of the answer to a multiple choice question, don’t spend too long on it. Put
a star by it and return to it later.
23. Within a single multiple choice question, use a pencil to cross out the statements which are
clearly incorrect, then choose between those left.
24. In a multiple choice question don’t be swayed by one of the choices just because it has got a
longer (or shorter) statement than the others.
25. Don’t make any assumptions about the order of responses – just because there have been
two answers ‘D’ in sequence, it does not mean that the next answer cannot be ‘D’.
26. Take care to read the whole question word by word. For example, in the question ‘what is the
ratio of the volumes of 2g of H2 and 16g of methane, CH4, at r.t.p?’ Many Candidates will
focus on the numbers and ignore the word ‘volume’. Just a quick look at the figures gives the
incorrect answer 1:8 (using the molar gas volumes gives 1:1 as the correct choice)
27. When given a choice of picking out a noble gas from a group of electronic structures, don’t
jump to the conclusion that noble gases always have 8 electrons in their outer shell.
Remember that helium has 2!
28. When given a choice about electrical conductivity of ionic structures remember that the
conduction is due to IONS moving (not electrons). The ions can only move when the ionic
compound is molten or when dissolved in water.
29. When given choices of why alloys are hard, it’s not the mass of the atoms which is important
but their size. Remember that metals have layers which slip over each other. A different
sized atom will distort the layers and stop them slipping over each other. This makes the
alloy harder than the pure metals.
30. When given choices about the rate of diffusion of gases, remember that the rate of diffusion
depends on the mass of the molecules. Heavier molecules (lower relative molecular mass)
move and diffuse slower than lighter molecules. If you are unsure which molecule is heavier,
use your Periodic Table to calculate the relative molecular masses.
31. If you are given a choice of tap water and several other substances as examples from which
to select a pure compound, it’s not going to be tap water. It is a common error to think that
tap water is pure. It’s a mixture. (Don’t be fooled by the adverts of the mineral water
companies which say ‘pure mineral water’!)
32. If you are given choices of electronic structures of atoms to select to make a compound of
type XY2, first check the type of compound that the examiner wants e.g. ionic or covalent. If
it is ionic, then you can choose an atom with one or two electrons in its outer shell and
combine it with a non-metal atom. If it is covalent look for the structures of two non-metal
atoms i.e. those with 4 to 7 electrons in their outer shell. Remember that the number of
electrons in the outer shell is equal to the group number.
33. Remember that the valencies (combining powers) of the elements in Groups V to VII are
found by taking the group number away from 8. For example, the valency of oxygen in an
oxide is 8-6 = 2. (Oxygen is in Group VI)
Paper 2 (Theory) Tips
34. Reading the question thoroughly and noting the number of marks available is important. In
response to a question such as ‘Use your knowledge of the structure of metals to explain
how they conduct electricity’, many Candidates just write down that ‘metals have a sea of
electrons’, thinking that this answers everything. What the examiners are looking for is
a. The idea of positive ions in a sea of electrons and
b.
The fact that the electrons move.
35. Remember that it is the moving valence electrons which are responsible for the conduction
of electricity in metals. If you just state that ‘the electrons move’ it could refer to any electrons
in the atoms. It is also a common error to suggest that ions are responsible for this
conduction.
36. If you are asked to compare things in the question, your answer must also make the
comparison obvious. In response to the question ‘How do different proportions of carbon
affect the properties of steel?’ the answer ‘mild steel has a low % of carbon’ will not receive
any marks because nothing has been said about steel with a higher % of carbon. An answer
such as ‘the higher the % of carbon the more brittle the steel’ gets the marks because this is
a comparison.
37. Properties of transition elements often cause problems. Remember that transition elements
themselves are NOT colored, it is their compounds that are colored.
38. If you see the words ‘What observations are made?’ remember that this means what you
39. See, hear or feel and NOT for example, ‘gas given off’.
40. Make sure that you know the use of the various substances stated in the syllabus. Go
through your syllabus and make a note of these. For example, the uses of the naphtha
fraction from petroleum distillation are not well known. Make a list of all the substances in the
syllabus whose uses you need to know and test yourself by making ‘flash cards’ for these.
41. If you are asked to describe the meaning of a term which has two words in it, you must make
sure that the description has included the meaning of both words. So to get full marks for the
question ‘What is the meaning of the term saturated hydrocarbon?
You have to define
a. saturated
b. hydrocarbon.
42. Always check back in the question to see the wording in the stem of the question. You will
not get any marks for repeating words in the stem of the question or putting what is in the
stem into different words. For example, if the stem includes the words ‘explain why the
reaction rate increases as the concentration of acid increases’, you will not get any marks for
putting this in another way e.g. ‘the reaction speeds up because the concentration of acid
increases’.
43. Be on the lookout for questions involving processes e.g. ‘What advantages are there in using
hydrogen as a fuel?’ The section underlined is a process. The answer ‘hydrogen is not
polluting’ is therefore not correct because there is no indication that it is being used as a fuel.
A better answer would be ‘it forms no pollutants when burnt’.
44. Watch out for the words ‘if any’. e.g. ‘State the products in the following reactions if any’.
Sometimes the examiner wants to test your understanding of why a reaction does NOT
occur.
45. If you are asked to draw electronic structures of atoms or ions, it is best to draw full
electronic structures showing all shells, not just the outer shell. If the Examiner wants you to
draw just the outer shell, that will be stated in the question.
46. When drawing the structure of ions, the charge must be put at the top right hand corner. You
will not gain any marks if you put the charge in the nucleus as do some Candidates.
47. Explaining the properties of ionic structures is always a problem area. Never mention atoms,
molecules, covalent bonds or sharing electrons even if you mention ions as well. In answer
to the question ‘Why does sodium chloride conduct electricity when molten? The answer
‘because the ions and the electrons can move’ gets no marks. This is because the incorrect
‘electrons’ negates the mark for ‘ions’.
48. A common error involving the conductivity of molten ionic compounds is to state that the
electrons are responsible for conduction. Remember that it is the IONS which move when
ionic compounds conduct electricity.
49. It is a common mistake to believe that energy is needed to form bonds. In answering
questions about bond making and bond breaking, think about a model of a compound – to
break the bonds, you have to physically pull them apart. In other words you’re putting in
energy. To form bonds, it must be the opposite i.e. energy is given out on bond formation.
50. Look out for the term ‘explain’. This indicates that you have to write in detail about why
something happens. If for example you are asked to explain why the reaction between
copper (I) chloride and chlorine is a redox reaction, you must write about redox in terms of
either electrons or oxidation number changes.
51. Make use of all the information given in a question, including graphs. For example if a graph
of % yield of ammonia against temperature for various pressures is given, it is there for a
purpose. If you use the information given you are less likely to lose marks than if you try to
remember figures from a book.
52. Never write ammonium hydroxide as the product when ammonia dissolves in water –
ammonium hydroxide it does not exist! (Even though you may see it still on bottles in the lab
and even in some books). The correct term is aqueous ammonia. In addition make sure that
you know the difference between ammonia, NH3 and ammonium, NH4+. The latter is an ion
present in ammonium salts.
53. The number of carbon atoms in formulae of carboxylic acids often causes problems. Make
sure that you include the carbon of the –COOH group when you name the acid.
CH3CH2CH2CH2COOH has 5 carbon atoms so it is the 5th member of this homologous
series, pentanoic acid.
54. Take care with the formulae of the metal salts of the carboxylic acids if the metal is from
Group 2. Remember that group 2 metals form 2+ ions and so they need 2 carboxylate ions
to balance. E.g. the formula for magnesium ethanoate is (CH3COO)2 Mg.
55. When doing mole calculations if given an equation such as:
Mg + 2CH3CO2H → (CH3CO2)2 Mg + H2
You ignore the 2 in the equation when calculating the molar mass of ethanoic acid. So the
molar mass of ethanoic acid is 60 not 120. Remember though that when calculating reacting
masses, the 2 needs to be taken into account because two moles of ethanoic acid react with
only one mole of magnesium. The 2 also needs to be taken into account when doing
calculations involving limiting reagents.
56. Try to be as accurate as possible in all your answers. In response to a two mark question
such as ‘Explain why the reaction between ethanoic acid and magnesium much slower than
the reaction between hydrochloric acid and magnesium?’ it is too vague to write ‘Ethanoic
acid is a weak acid’. ‘A weaker acid’ would be a more acceptable answer. Some reference to
the differences in hydrogen ion concentration or degree of dissociation is also needed.
57. When there are questions with large unspecified mark allocations (for example 6 marks) you
need to plan your answers out carefully and need to underline or list on the question paper
all the points that need to be written about. It is very common to miss out important points in
extended questions like this. When you have written a list of the points you need to deal with,
you can cross these out one by one as you complete them. E.g. equations, gas tests, color
changes etc.
58. Answers to questions on ‘chemistry and the environment’ are often answered in a too vague
manner. The word ‘pollution’ is too vague to be given credit as an answer to any question. In
answering a question such as ‘What is a disadvantage of the use of nylon for fishing nets’,
pollution would not score, neither would vague statements such as ‘Dangerous to sea life’.
‘Non-biodegradable’ would score the mark because it is much more exact.
59. In doing calculations, always check that the relative molecular masses are correct. Incorrect
addition or extraction of the relative atomic masses is often a reason for failure in a
calculation. Also, make sure that you double check that you have used atomic masses and
not atomic numbers. If you are unsure, use the key in the Periodic Table at the bottom left to
check which number is which.
60. Look out for the word ‘each’. For example, in the question ‘Explain the purpose of adding
each of calcium hydroxide and ammonium sulphate to soil. You will lose marks if you do not
make it clear exactly which compound you are writing about. To make sure that there is no
ambiguity, start each sentence with the name of the compound. In this case: calcium
hydroxide is added to….. Ammonium sulphate is added because……
61. Avoid missing out connecting processes. For example if you are asked about how
ammonium sulphate helps soil fertility, an answer that ‘ammonium sulphate provides nitrate
ions’ is not good enough. This answer suggests that the ammonium sulphate contains nitrate
ions. A better answer would include the connecting process. For example: ‘Ammonium ions
are converted by micro-organisms in the soil to nitrate ions.’
62. Know the difference between –ides and –ates. Compounds ending with –ide contain only two
types of atoms e.g. magnesium oxide, potassium chloride. Compounds ending with – ate
contain three or more types of atom, one of which is usually oxygen e.g. sodium sulphate,
potassium nitrate. The ions of ides are simple e.g. sulphide, S2- whereas – ates have
compound ions e.g. carbonate CO3263. When writing oxidation numbers, remember that the sign + or – should be included. The
oxidation number is not written like the charge on the ion. For example Cu2+ is a copper (II)
ion. The oxidation number of copper in this ion is +2. (rather than 2+)
64. Make sure that you know the solubility rules for various compounds. These are important
when writing state symbols in equations. If you know that carbonates of group II metals are
insoluble in water, then you know to write CaCO3(s) in an equation rather guessing.
65. When deciding which method to use to make a given salt, you need to be able to remember
to use the solubility rules. For example, to make an insoluble salt from two soluble salts you
use a precipitation reaction. In order to make a soluble salt from two soluble salts, you use
titration
66. Know the difference between (aq) and (l). The state symbol (aq) refers to a substance
dissolved in water. The state symbol (l) refers to a substance as a pure liquid e.g. Br2(l),
H2O(l).
67. Remember that when writing equations for the reactions between aqueous solutions of
halide and aqueous halogens, the state symbols are all (aq). This is because the halogens
are dissolved in water in the first place.
68. When given graphs which read back in time from the present day be careful to remember to
read the graphs in a forward direction if you are asked about the order of a sequence of
events e.g. how carbon dioxide concentration changed over the last 2000 million years.
69. Always read the scales on graphs very carefully especially when very large numbers are
involved. For example if the graph has the figures 1000, 2000, 3000 with the words ‘millions
of years’ underneath, it is all too easy to miss the word ‘millions’ when answering questions.
70. Remember that if you want to separate a particular gas from the air, you can’t just heat the
air up. You have to make it liquid first by lowering the temperature so that all the gases
liquefy. Then you raise the temperature gradually and collect the gases as they evaporate off
one by one. Although the actual process is more complex, this is all that you have to know
for your examination.
71. With questions which require extended answers, especially in part B of the paper do not
write too much. There is often a danger that you will contradict yourself.
72. When asked to draw diagrams, make sure that you have included all the pieces of apparatus
necessary. Go over each point in the question carefully to check. It is quite common for
example, to leave out the test tubes to collect gases when asked to draw the apparatus for
electrolysis and test the gaseous products.
About the Examiner
Dr. Roger Norris has been an examiner for many years. He is currently a Principal Examiner for
Cambridge International ‘O’ level Chemistry as well as IGCSE Chemistry. He has also been involved
in developing and examining OCR Chemistry courses and examinations in Britain. As well as undertaking
several years of research he has 28 years of teaching experience in Chemistry (as well as in Biology and
Physics). He has contributed to a number of publications ranging from General Science for 11-13 year
olds to advanced level Chemistry courses.
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