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Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
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PHYSICAL SCIENCE
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Paper 8780/01
Multiple Choice
Question
Number
Key
Question
Number
Key
1
2
3
4
5
B
A
C
A
A
16
17
18
19
20
D
A
D
A
B
6
7
8
9
10
D
D
A
B
C
21
22
23
24
25
B
A
C
A
D
11
12
13
14
15
D
A
C
B
D
26
27
28
29
30
A
D
A
C
C
General Comments
The time allowance for each question is typical for this level of Multiple Choice paper. This will produce
some time pressure and it is therefore important that candidates have a sound knowledge of the basic facts
and equations, so that they can quickly get into the mechanics of answering the question rather than spend
time retrieving the basics.
There is plenty of space on the paper for candidates to carry out rough work, and this is essential for some
questions. It is a good idea for candidates to note down any relevant equations and facts, including powers
of ten, as they read through the question. Where a mathematical calculation is required it is worthwhile to
write out the calculation neatly; this will help to avoid careless errors and gives an immediate check on the
validity of the answer.
The paper contains two styles of question. Performance on the multiple-response questions, Questions 2130, was generally weaker than on the single response questions, Questions 1-20. Candidates might benefit
from practising strategies for tackling multiple-response style questions.
Candidates found Questions 7, 8, 9, 13, 17 and 25 to be the most straightforward. Questions 2, 16, 21, 22,
23, 28 and 30 were found to be particularly challenging.
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
Comments on Individual Questions
The comments below relate to those questions where many candidates were making the same mistake.
Question 1
Although the majority of candidates recognised that the resultant force on the sphere was vertically
downwards, there was a significant proportion who were under the false impression that the force was in the
direction in which the sphere was moving at that instant.
Question 2
The range of answers shows that there was a lot of guessing for this item. Candidates need to think through
the problem more thoroughly. In the first part of the curve the velocity is increasing, therefore the gradient of
the displacement–time graph increases, limiting the answer to A or D. The second part of the motion shows
the velocity in the same direction albeit decreasing in magnitude; the displacement therefore continues to
increase which rules out option D.
Question 3
The question showed good discrimination. The one force not clearly indicated in the diagram is the weight,
which must act vertically downwards. The body is in equilibrium, and hence all the arrows on the vector
diagram must go round the triangle in the same sense.
Question 4
This is a straight test on the understanding of the concept and calculation of a couple. A significant number
of candidates did not recall that a couple is equal to the magnitude of one force and the perpendicular
distance between the two forces. Care needs to be taken to ensure that the units are consistent.
Question 5
A complete cycle is from any point on one wave, to an identical point on the successive wave. A large
number of candidates did not appreciate this, and chose option B.
Question 6
The majority of candidates recognised that the frequency of light does not change when it moves from one
medium to another; a small number, however, thought that the wavelength is longer in glass than in air.
Question 7
The question was answered well; the most common error was to think that circuit 1 had a smaller resistance
than circuit 3.
Question 8
Most candidates recognised that the charge passing was equal to the current multiplied by the time and they
successfully coped with the powers of ten involved in the question.
Question 10
Many candidates erroneously thought that the emission of a β-particle would reduce the proton number by
one.
Question 16: The most common incorrect responses were B and C. The question requires candidates to
identify the one incorrect statement out of the four options; candidates need to be alert to the use of
2–
2–
negatives in questions. Candidates needed to recognise that, in both CrO4 and Cr2O7 , the chromium is
in an oxidation state of +6 and so no change in oxidation state occurs. The statements in A and in B are
correct and so do not fulfil the rubric of the question.
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
Question 18: The most common incorrect responses were B and C. This question requires candidates to
be familiar with a mainstream part of the syllabus, namely the thermal decomposition of carbonates and
nitrates (section C8 part (c)). The poor performance in this question can only be attributed to insufficient or
ineffective learning.
Question 20: Many candidates seemed to be guessing at the answer, as all of the incorrect responses were
chosen by significant numbers of candidates. The question requires candidates to deduce the number of
possible cis-trans isomers of an alkadiene. One of the C=C bonds carries different substituents on both of its
carbon atoms and so can show isomerism. The other C=C bond carries two methyl groups on the same
carbon and so cannot show isomerism. So, there can only be two cis-trans isomers, which makes B the
correct response. This is a question where candidates might benefit from drawing out the structure in the
working space provided.
Question 21
Candidates need to know the meanings of the prefixes and then to work carefully through the possibilities.
There was clearly a lot of guessing with this question.
Question 22
The question caused many candidates a great deal of difficulty. The spread of answers indicates that few
candidates thought the question through, but merely guessed at the answer. The point here is that the
electron is negative and is therefore attracted towards the positive plate. It therefore has low potential
energy and high kinetic energy when near the positive plate.
Question 23
This question also caused great difficulties. Many candidates suggested that the kinetic energy of the
molecules increases when a solid changes to liquid without change in temperature.
Question 24
Although the question was done reasonably well done, many candidates thought there were more molecules
in one sample than the other, despite the question stating that both samples consist of 1 mole.
Question 26: The most common incorrect response was C. The question requires candidates to consider
the effect of changes to the conditions of a reaction under equilibrium. As the correct response is A,
statements 1, 2 and 3 are all correct. Almost three-quarters of candidates correctly deduced statements 1
and 3 to be correct – leaving responses A and C as the only options. Those candidates who opted for
response C, incorrectly, decided that statement 2 was wrong. As the reaction involves the combining of
three gaseous molecules to form a single gaseous molecule of methanol, an increase in pressure would lead
to an increase in yield. This may be deduced by a consideration of Le Chatelier’s principle – an increase in
pressure favours the direction producing fewer gaseous moles. Thus, statement 2 is, in fact, true.
Question 27: The question requires candidates to consider the effects that NaBH4 would have on a range of
organic compounds. A majority of candidates correctly deduced that this reagent could not reduce propane.
However, a large proportion of the candidates selected responses B and C. The target molecule is
propan-2-ol, which can only be obtained by reduction of propanone. Propanoic acid and propanal are both
oxidation products of propan-1-ol; so, on reduction they would form this alcohol rather than the target
alcohol. With this type of question, if response A is wrong, response B is automatically wrong as well. The
choice, therefore, should be between responses C and D. Propanal and propanone are produced from
different alcohols, so they cannot both be correct (C); so, D is the only possible correct answer.
Question 28: The most common incorrect response was B. The question requires candidates to consider
four different statements each of which were related in some way to sulfur. As the majority of candidates
selected responses A or B, it is clear that correct deductions were made concerning statements 1 and 3 in
the majority of instances. It would seem, therefore, that candidates struggled with statement 2 which is
correct. When treated with concentrated sulfuric acid, iodide ions are oxidised to iodine element and sulfuric
acid produces three reduction products; these are sulfur dioxide, sulfur element and hydrogen sulfide gas. A
proportion of candidates did recognise this fact but incorrectly assumed statement 4 to be correct and so
selected response C. Statement 4 is incorrect as sulfuric acid is manufactured by the Contact process; the
Haber process is used to produce ammonia.
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
Question 29: The most common incorrect response was B. The question requires candidates to consider
the changes in oxidation number associated with four different chemical equations. In equation 1, no atoms
show an oxidation number of +6 (or of +4); so, responses A and B must both be wrong. This automatically
means that equation 3 can be ignored as well. In equation 3, the chromium in CrO32– has an oxidation
number of +6; this changes to +3 in the Cr3+ ion. In equation 2, the oxidation number of manganese
changes from +6, in MnO42–, to +4, in MnO2. Similarly, in equation 4, the oxidation number of sulfur changes
from +6, in SO42–, to +4 in SO2.
Question 30: The most common incorrect response was A. The question requires candidates to show an
understanding of the Brønsted-Lowry theory of acids and bases. This question was poorly answered,
suggesting that this theory is not well understood by many candidates. Statement 1, that a Brønsted-Lowry
acid contains H+(aq) ions, is incorrect. The Arrhenius theory of acids and bases stipulates that acids must
increase the concentration of H+(aq) ions and bases increase the concentration of OH–(aq) ions in water.
Brønsted-Lowry acids-bases do not require water as solvent; acids are defined as proton donors and bases
as proton acceptors. Thus, statement 2 is correct. This means that response C has to be correct (i.e. as
statement 2 is correct, only responses A and C need be considered; response A has already been
eliminated, so C must be correct). In statement 3, it is the H2SO4 that donates a proton and acts as an acid –
the HNO3 accepts the proton and so behaves as a base. Statement 4 is correct, HCl donates a proton to
NH3 and so acts as an acid.
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
PHYSICAL SCIENCE
Paper 8780/02
Short Response Questions
Key Messages
This is the first examination in this syllabus, which replaces the Physical Science HIGCSE Syllabus
(1253). The syllabus material covered in 8780 is, in some areas, different from that covered in
Syllabus 1253, and the question style and level of understanding demanded at AS are also
different. Whilst some candidates were well prepared across the syllabus, many candidates
seemed to lack specific knowledge in particular topic areas, and would benefit from more thorough
preparation.
General Comments
This component comprises a series of short response questions which are designed to assess
candidates’ understanding over a wide range of syllabus material. Questions covering both the
physics and the chemistry content of the syllabus are intermingled. The questions from the two
strands proved to be of very similar demand; although a minority of candidates performed
significantly better in one strand than the other there was no obvious bias towards either physics or
chemistry overall.
While some candidates had thoroughly prepared themselves for this examination, others seemed
to lack specific knowledge of some of the topics covered in these questions. This was particularly
noticeable in Questions 7, 8, 10, 11 and 14. To do well on this paper, candidates need to have a
good knowledge of the entire syllabus, an understanding of basic concepts and the ability to
communicate their thoughts in a clear and concise manner.
Comments on Specific Questions
Question 1
The question required candidates to recognise that although an instrument might be designed to
measure to a very precise degree, other factors come into play. In this example the major source
in the uncertainty is the reaction time of the operator. It is possible, using a hand held stopwatch,
to measure a time interval to about one tenth of a second – in this example times of 0.05 to 0.5 s
were accepted. Uncertainties are usually only quoted to one significant figure.
Question 2
The question required an understanding of Newton’s second law and the ability to communicate
that understanding. No credit was available for the first part - this was simply to enable Examiners
to study the candidate’s answer with reference to a clear starting point. In practice the loaded van
would travel further than the unloaded van, although well-argued cases of the van travelling the
same distance or less than the unloaded van were given some credit. The answer looked for the
idea that the increased mass (or, better, increased momentum) meant that the deceleration was
less for the same sized force. Generally candidates had some idea that the increased mass had
an effect, but they did not produce a logical argument stating that, for the same braking force,
decreasing the mass would mean a decrease in the deceleration. Some candidates did quote F =
ma as part of their answer.
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
Question 3
Precise wording is expected at this level. The majority of candidates had some idea of the physics
of the situation, but the use of terminology was poor. It is not the Earth’s gravity or the Earth’s
gravitational force (unless it is acting on identical masses) which is greater than the Moon’s, it is
the Earth’s gravitational field strength. A good answer would have explained that the weight is the
gravitational force on an object and that the Earth’s gravitational field (strength) is greater than that
of the Moon.
Question 4
This was a relatively straightforward question which required candidates to define the term relative
atomic mass. This term may be defined in terms of the average mass of an atom, or in terms of
the mass of a mole of atoms. The mass must then be related to one twelfth of the mass of a 12C
atom or to one twelfth of the mass of a mole of 12C atoms.
While some candidates had clearly learned an appropriate definition, and were able to quote it, the
majority of candidates were not so well prepared. Some candidates were aware that it related to
the mass of an atom, and possibly that 12C was also involved, but their answers tended to be
incomplete and ambiguous.
In many cases, the mass number was defined (sum of protons and neutrons) rather than the
relative atomic mass.
Question 5
The question required the basic knowledge of the atomic structure of different isotopes. Generally
the question was done quite well although some candidates referred to the properties rather than
to the structure. For example, the comment that the two isotopes have identical chemical
properties, although correct, does not describe the structure of the atom.
Question 6
This question required candidates to deduce the electron arrangement of a germanium atom and
then to use the data provided to calculate the relative atomic mass of a sample of germanium.
(a)
Only a minority of candidates correctly completed the required electron arrangement.
Some candidates started but did not complete the arrangement. In some instances,
orbitals were either omitted completely from the arrangement or showed incorrect
numbers of electrons.
The most frequently omitted orbitals were the 2s2 and the 3d10; the most common errors
were 3p8, 3d8 and 3d12. A number of arrangements showing 5d orbitals were seen.
(b)
This part was, in the main, very well answered. Most candidates used an appropriate
calculation method, although some attempts were spoiled by arithmetical errors. A few
candidates simply averaged the mass numbers or divided the percentages by the mass
numbers; a small number made no attempt at this question.
Question 7
This question required candidates to explain the increase in ionisation energy across a period in
the Periodic Table. This is a required element in the syllabus and many candidates were able to
offer acceptable explanations; however, errors were numerous. Many candidates described an
increasing atomic radius across the period or attributed the increase in ionisation energy to an
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
increase in the number of outer-shell electrons, rather than to an increase in the number of
protons.
There was considerable confusion as to the level of shielding experienced by outer-shell electrons
across the period. In many cases, an increase in shielding was proposed as an explanation for the
increase in ionisation energy. In reality there is no real change in shielding across a period but,
had there been an increase, this would have led to a decrease in ionisation energy, not to an
increase.
Question 8
The question required an understanding of potential difference, knowledge of the definition of the
volt and the ability to use it in a particular situation. A surprising number of candidates were unable
to say what is meant by potential difference and many just rehashed the question; others confused
potential difference with current. Of those who had some idea that it was something to do with the
energy transferred between the points, few recognised that it is the energy per unit charge (or per
coulomb).
Question 9
This apparently simple question required thought. Too often candidates did not think the situation
through; they might have recognised that the golfer would not be going downhill all the time but
they did not develop the idea either that even when travelling on the level, work would have to be
done against friction. Likewise few considered that the conversion of gravitational potential energy
to electrical energy would not be 100 % efficient.
Question 10
This question required candidates to deduce the identities of two halogenoalkane isotopes, C and
D, and to give the systematic name of isomer B. Overall, this question was poorly answered.
(a)
Some candidates were able to draw acceptable structures for the required isotopes.
However, in the majority of cases, structures were either flawed or were duplicates of
isomers A or B. In some cases, structures showing either more or less than four carbon
atoms were drawn; isomers must, by definition, contain the same number of atoms as are
present in the parent structure, C4H9Br.
(b)
This part was also poorly done. A minority of candidates correctly named the compound
as 1-bromo(-2-)methylpropane. In many instances, the location of the bromine atom was
omitted from the name, or it was placed on carbon atom number 3.
In some cases it is not necessary to assign a position on the chain to a particular
substituent, as the omission of a reference number does not result in there being any
ambiguity about the structure. For this reason, (-2-)methylpropane may be written either
with, or without, the ‘-2-‘. However, the bromine atom must be assigned to carbon 1 as 2bromo(-2-)methylpropane is a viable alternative.
A few candidates gave
1-bromo(-2-)methylbutane or similar.
Question 11
This question was very poorly done. Only a very small number of candidates seemed to be
familiar with addition polymerisation and were able to write an equation in an appropriate form. In
many instances, equations either showed a double bond in the polymer or did not show any double
bonds at all. Those candidates who did show the correct structure for propene on the left-hand
side of the equation, frequently showed all three carbon atoms in a linear chain in the repeat unit
on the right.
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
Question 12
The first part of the question tested whether candidates understood that the graph, although
apparently ‘transverse’, represented a longitudinal wave. The second part required candidates to
interpret that the movement of the particle at that particular moment was away from the equilibrium
position. Some candidates gave excellent answers, however many interpreted the graph as one of
a transverse wave. Others just drew the position at an angle below W as though it was sliding
down the wave shape.
Question 13
A basic understanding of the formula p = hρg, was tested by this question. Although many
candidates were able to give good answers, there were many who showed much less
understanding. A disappointing number thought that the total mass of water above the point
determined the pressure in B. The knowledge that the density of sea water is greater than that of
fresh water is not required by the syllabus, consequently for the second part, credit was awarded
for a correctly reasoned argument related to the candidates’ positions of R, regardless as to
whether it was placed below, above or at the same level as P.
Question 14
This question required candidates to draw a standard reaction pathway diagram, as detailed in the
syllabus. Some candidates were familiar with this diagram, drew an appropriate diagram and
scored well. Other candidates, however, seemed to have little experience with this type of diagram
and struggled here.
A comparison of the magnitudes of the two activation energy values should have led candidates to
conclude that the reaction was exothermic. While some candidates made this deduction, many did
not. It was not uncommon to see diagrams showing either an endothermic change or no enthalpy
change at all; some candidates simply drew random lines and labelled these Ea(F), Ea(R) and ΔH.
Some diagrams showed either multiple ‘intermediate’ peaks or multiple reactant/product energy
lines.
In addition, there was considerable confusion regarding the labelling of Ea(F), Ea(R) and ΔH. In
many instances, the arrows drawn showed incorrect directions for these enthalpy changes. The
arrow for Ea(F) should point from the reactant line to the top of the intermediate ‘hump’. The arrow
for Ea(R) should point from the product line to the top of the intermediate ‘hump’. The arrow for ΔH
should point from the reactant line to the product line. Many candidates seemed to be unaware of
this and drew arrows which were either double headed or which pointed in the wrong direction.
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
PHYSICAL SCIENCE
Paper 8780/03
AS Structured Question
Key Messages
This is the first examination in this syllabus, which replaces the Physical Science HIGCSE Syllabus
(1253). The syllabus material covered in 8780 is, in some areas, different from that covered in
Syllabus 1253, and the question style and level of understanding demanded at AS are also
different. Whilst some candidates were well prepared across the syllabus, many candidates
seemed to lack specific knowledge in particular topic areas, and would benefit from more thorough
preparation.
General Comments
Questions covering both the physics and the chemistry content of the syllabus are intermingled.
The questions from the two strands proved to be of very similar demand. It was felt that this
component differentiated effectively between candidates on the basis of ability.
Whilst some candidates had clearly appreciated the level of demand of this paper and had
thoroughly prepared themselves for the task, many other candidates had not done so. There was
a clear distinction between those candidates who were comfortable with the material and the
question style used in this paper and those who struggled.
Comments on Specific Questions
Question 1
This question explores candidates’ understanding of the techniques in calibrating instruments.
(a)
Careful inspection shows that the calibration curve does not quite reach the 5 V mark;
candidates needed to use their skill to extend the line to the appropriate point.
(b)
This part required candidates to recognise that the scale was reversed and non-linear.
Question 2
This is a relatively straightforward question designed to test candidates’ understanding of oxidation
numbers and of the use of the ideal gas equation. Performance varied considerably across the
candidate body.
(a)
Many candidates correctly deduced the oxidation number of antimony to be +3; some of
them, however, wrote this as 3+. Technically, writing the value followed by the sign, 3+,
indicates the charge on an ion, not the oxidation number of an atom. On this occasion 3+
was accepted and credit was awarded, but candidates need to be aware of the
convention.
Many incorrect attempts were also seen. Among the more common errors was +6 (the
sum of the oxidation numbers of two antimony atoms).
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
(b)
While correct answers were reasonably common, there were many errors made in this
calculation. Candidates were required by the rubric of the question to use the ideal gas
equation; a few still tried to use the GMV value of one mole of gas occupying 24 dm3.
In very many instances, candidates used the amount of stibnite (10 mol) rather than the
amount of CO2 (15 mol) in their calculations.
Frequently seen errors included the incorrect conversion of the gas pressure and the
incorrect rearrangement of the gas law formula. The pressure quoted in the question is
100 kPa. Candidates needed to multiply this by 1000 to obtain 10 000 Pa for use in the
equation; the equation needed to be rearranged into the form V = nRT/p. In addition,
arithmetical errors spoiled some answers, while some candidates, who did not appreciate
that the equation generates a volume in m3, attempted to convert their answers into m3.
Question 3
The question took candidates through a demonstration of Archimedes’ Principle. No knowledge of
the principle was required; the question was aimed to test the candidates’ ability to follow through
several steps in a mathematical proof.
(a)
This tested whether candidates realised that the upward force on an object in equilibrium
is equal to the downward force, in this case the weight of the object. It should be noted
that at this level candidates are expected not to use the approximation of the Earth’s
gravitational field of 10 N kg-1, but the more precise figure of 9.81 N kg-1.
(b)
This part required candidates to make a choice of formulae (P = F/A or P = hρg) to initially
calculate the pressure on the base of the block of ice and then to find the depth of the ice
below the surface of the water. The majority of candidates did not take sufficient care in
choosing the correct formula in the first part and consequently became muddled in the
second part.
(c)
The final part completed the exercise and done correctly gave the original mass of the
block – rounding errors excepted.
None of the steps in the process is intrinsically difficult, however, relatively few candidates
were able to work their way through the whole exercise.
Question 4
This question is designed to assess understanding of bonding and molecular shape.
examples used are relatively simple. Performance here, however, varied considerably.
The
(a)
(i)
Some fully correct diagrams were seen; however, many candidates drew dot-andcross diagrams rather than diagrams showing the relative positions of the bonds. It
is necessary when drawing diagrams such as these, and also in displayed formulae,
that all bonds are represented by lines. Dot-and-cross diagrams are not acceptable.
Many of the line diagrams seen were of poor quality. It was not unusual for BF3 to
be represented by a ‘T’ shape and for BF4– to be shown as a cross/square shape. It
was expected that candidates would draw diagrams similar to those shown below;
some did, many did not and a significant number made no attempt to answer this
question.
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
A fair number of candidates correctly identified the shape of BF3 as being trigonal
planar and the bond angle in BF4– as being 109(½)° but errors and blank spaces
were quite common.
(ii)
Some correct answers were seen but there were also many poor attempts. All that
was required was a reference to there being equal repulsion between three bonding
pairs. In many cases, equal was omitted and bonding pairs was replaced with a
reference to F atoms or ions.
(i)
This part was quite well answered; candidates seemed to be quite familiar with the
concept of dative/coordinate bonding.
(ii)
A significant number of candidates correctly deduced that the bond would result
from the donation of a lone pair from the fluoride ion to the boron atom. However, a
fair number either suggested that the lone pair would be provided by the boron atom
or made no attempt at the question.
(b)
Question 5
This question tested knowledge and understanding of diffraction and superposition of waves.
(a)
This looked at diffraction around an object. Candidates should have an awareness of the
approximate wavelengths of the radiations in the electromagnetic spectrum. Microwaves
have a wavelength in the centimetre region whilst long wave radio waves have
wavelengths of upwards of a kilometre. Consequently the radio waves will diffract around
the mountain whilst the microwaves will not. Candidates need to be able to apply basic
concepts in non-familiar situations.
(b)
Many candidates recognised that this was an interference effect and stated that at P there
is constructive interference whilst at Q destructive interference. However, although this
states what happens at the points, it does not explain it. To do this the candidates needed
to recognise that the path difference of the contributions from the two slits at P was a
whole number of wavelengths, whereas at Q it was an odd number of half wavelengths.
In (iii) quite a lot of candidates recognised that the intensity is proportional to the square
of the amplitude but were unable to correctly calculate that when the intensity is halved
the amplitude will be reduced by a factor of the square root of two. In (iv) many
candidates expressed themselves badly – the frequency does not change in either case,
and the datalogger shows a trace of amplitude against position, not time. Candidates
needed to refer to the distance between fringes (or maxima / /minima) increasing in 1 and
decreasing in 2.
Question 6
This question required candidates to write a balanced equation and to show understanding of Le
Chatelier’s Principle. In essence, the question involved little more than the application of basic
knowledge and understanding. Some candidates were very well prepared for such questions,
others, however, seemed to have little experience in this area. In a significant number of
instances, candidates did not follow the rubric of the question and gave answers to questions
which had not been asked.
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© 2011
Cambridge International Advanced Subsidiary Level
8780 Physical Science November 2011
Principal Examiner Report for Teachers
(a)
Surprisingly, this question was very poorly answered. There were many errors in formula,
both of reactants and of products; for example, CH3, C2H4, C2H6, CO2, CH3OH and
CH4OH.
(b)
(i)
All that was required was for candidates to link the evidence from the table, which
shows a decrease in yield as the temperature increases, to an appropriate argument
based on Le Chatelier’s Principle. Some very clear and accurate explanations were
seen. Some candidates, however, did not refer to the evidence in the table, as
required by the rubric. Others gave explanations based on pressure variation which
is contrary to the rubric of the question.
(ii)
Again, all that was required was an appropriate argument based on Le Chatelier’s
Principle. While some candidates presented such an argument, many attempts
were vague and unconvincing. It was not uncommon for candidates to base their
arguments on the increased collision rate associated with an increase in pressure.
As this is an argument which explains the increase in reaction rate (not the product
yield) as the pressure is increased, it was not accepted here.
(iii)
This part was quite well answered by many candidates. While a few candidates
tried to argue, incorrectly, in terms of pressure versus yield or rate, most candidates
appreciated that the compromise was based on economic factors.
(i)
This question was quite well answered by many candidates. Convincing arguments
in terms of the relative strengths of the hydrogen bonding and van der Waals
attraction present in ammonia and hydrogen/nitrogen respectively were quite
common. In a few instances, candidates either referred to dipole-dipole interactions
in their answers or offered no response.
(ii)
This part was less well done. Many candidates appreciated that use should be
made of the different boiling points of the gases in order to separate out the
ammonia. However, most answers tended to contain little more than a brief
reference to the use of fractional distillation. What was required was a statement to
the effect that the ammonia is removed as a liquid by cooling the gaseous mixture.
Credit was given to candidates who tied this idea into a description of fractional
distillation.
(c)
(d)
Many fully correct answers were seen; however, a large number of attempts were spoiled
by not using 2 × -87 kJ or by reversing the signs for the ΣΔHf(reactants) and
ΣΔHf(products) values.
Question 7
This question related the basic mechanics of collisions and showed its relevance to the interaction
between fundamental particles.
(a)
This looked at the difference in the amount of ionisation caused by a proton and an
α-particle as they travel through air. It required candidates to apply their knowledge of the
different ionisations produced by α, β and γ-radiations to another scenario. It was
pleasing that many candidates were able to do this.
(b)
This section explored the dynamics of the collision between an α-particle and a proton –
the first part looking at conservation of momentum and the second at conservation of
kinetic energy in an elastic collision. Many candidates were able to cope with the basic
mathematics of the situation; however, too often there was a jumble of numbers with little
or no explanation. It is always important to set out work so that others can interpret it, but
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when a question asks the candidate to ‘show that’ a quantity has a certain value (as in (i))
it becomes essential. Similarly in (ii) the candidate needed to explain that for an elastic
collision the kinetic energy is conserved.
Question 8
This question required candidates to shown an understanding of the chemistry associated with
alkenes. Again, the level of understanding displayed by candidates varied considerably.
(a)
(i)
Some candidates produced excellent answers, while others struggled badly. The
concept of end-on and sideways overlap of orbitals, to form σ bonds and π bonds
respectively, was often confused and in some instances non-existent. A number of
candidates thought that σ bonds could only be formed between overlapping ‘s’
orbitals. In some instances, candidates had an idea of what was involved but were
unable to express themselves clearly enough to earn credit.
(ii)
Rather more candidates were able to draw acceptable orbital shapes here than
were able to describe their formation in (i). In some cases, however, it was clear
that candidates had little or no experience in this area. A diagram such as the one
below was all that was required.
(i)
It was clear that relatively few candidates had any real understanding of how to draw
reaction mechanisms and so this question was very poorly done. The electrophilic
mechanism required in the question should have shown the intermediate
carbocation. Also, there should have been three ‘curly arrows’. One from the C=C
bond to the first bromine atom, one from the Br—Br bond to the second bromine and
one from a lone pair of electrons on the Br– ion towards the positive carbon on the
intermediate. A diagram such as the one below would have been sufficient.
(b)
Relatively few acceptable diagrams were seen.
Many candidates deduced the product to be a bromoalkane but the correct
systematic name for 1,2-dibromoethane was not frequently seen. In many cases,
candidates omitted the location numbers for the bromine atoms, or referred to
bromoethane, rather than to dibromoethane. In a few instances, candidates offered
names which identified the carbon chain to contain an incorrect number of carbon
atoms.
(ii)
Some candidates offered convincing explanations as to why bromine, a non-polar
molecule, is able to undergo electrophilic addition with ethane. However, many
candidates seemed to lack this knowledge and either offered no explanation here or
argued in an unconvincing manner. All that was needed was a simple reference to
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the high electron density on the C=C bond inducing a dipole on the bromine
molecule.
(c)
(i)
This question required candidates to deduce compound D to be a ketone on the
basis of the evidence from the 2,4-dinitrophenylhydrazine and Fehling’s solution
tests. Thus, compounds C and A have to be propan-2-ol and 2-bromopropane
respectively. Some candidates were able to do this but in many instances, the
structure of 1-bromopropane was drawn instead.
(ii)
This part was well done; many candidates deduced C to be an alcohol, some
deduced it to be a secondary alcohol.
(iii)
Rather fewer candidates were successful here. A number of totally incorrect
reagents were suggested. Some candidates recognised that an oxidising agent was
needed here and correctly suggested the use of K2Cr2O7. However, an excess of a
strong acid is required for this reagent to work; many candidates omitted to specify
acidified K2Cr2O7 and so could not be awarded credit. A number suggested the use
of acidified KMnO4 which is too powerful an oxidising agent for use here.
(iv)
Product D was correctly identified to be propanone by many candidates, although
some described it as propone.
Question 9
The development of the atomic model in the first thirty years of the twentieth century is one of the
great examples of scientific understanding being driven by experimental techniques and
discoveries.
(a)
There was clearly some confusion about the different models of the atom – with many
candidates thinking that the Thompson model consisted of protons and electrons, few
recognised that this model consisted of a positive background ‘dough’, with the electrons
embedded representing the plums.
(b)
The basic concept of and observations from the Rutherford-Geiger-Marsden experiment
were reasonably widely known – although some candidates were unsure of the type of
particle that was used. To gain credit candidates needed not only to describe the
Rutherford model but also to link the development of the model to the observations made
in the experiment.
(c)
The points that were being looked for in the Bohr model were that the electrons were in
‘allowed’ fixed, radiationless orbits or ‘shells’. Few candidates had a real knowledge of
the Bohr model, candidates should understand that the basic concept of the Bohr model
of the atom was the first to introduce the idea of ‘allowed’, ‘radiationless’ orbits. The link
between the Bohr model and the Periodic Table in (ii) was done better, with a significant
number of candidates linking the number of shells to the period and the number of
electrons in the outer shell with the Group.
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Question 10
This question caused great problems with few candidates showing any understanding of
Kirchhoff’s Laws – also many candidates, despite being instructed to solve a problem using
Kirchhoff’s Laws, tried to do so without reference to these laws.
(a)
This required a simple application of the second law. The minimum that was expected
was for the equation to be written out clearly, and then its being solved to show the
required result.
(b)
This required the use of the first law at point E, using the information from the previous
section.
(c)
This was more difficult, candidates had to identify a suitable loop and then correctly apply
the second law to this loop. Too many candidates, having chosen a loop, for example
ACEB, assumed that the current was the same all the way round the loop, rather than
using the figures calculated in previous parts.
Candidates need plenty of practice at solving this type of problem; identifying suitable
points at which to use the first law and suitable loops on which to apply the second law.
This will also develop their understanding of circuits and the currents in different parts of
the circuit.
Question 11
This question required candidates to have an understanding of the meaning of, and calculation of,
the empirical formula of a substance, and to perform a mole-based calculation. Overall,
performance was quite good, with relatively few candidates receiving no credit.
(a)
(i)
Very few candidates offered an acceptable definition of the term empirical formula.
An acceptable definition would be the simplest ratio of atoms of each element
present in a compound. Very few attempts contained all of the required elements.
In the majority of instances definitions were confused or incomplete; references to
simplest ratio, atoms and element were frequently omitted. Definitions such as the
simplest ratio of elements in a compound were quite common.
(ii)
This calculation was, in general, well done. There were some errors involving the
use of the proton number in place of the relative atomic mass number, or in the
calculation of the final whole number ratio, but many fully correct answers were
seen.
(i)
Overall, this calculation was encouragingly well done. Many candidates correctly
calculated the Mr value of Q to be 138. Most candidates were able to secure at
least some of the credit available here.
(ii)
This part of the question required candidates to know how to manipulate the Mr
value from (i) to obtain a value for the Ar of metal Q. Many candidates were able to
do this, at least in part. In some instances, candidates obtained a value for the
relative mass of Q2 but forgot to divide by 2 to get the Ar of Q. Candidates who
obtained an incorrect value for the Mr in (i) were given credit for appropriate
calculations here.
(b)
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PHYSICAL SCIENCE
Paper 8780/04
Advanced Practical Skills
Key messages
Some candidates were inconsistent with their use of significant figures and/or decimal points when
recording their measurements in the physics question. Records of raw data should reflect the
precision of the measurement. Calculated quantities should be reported to a precision similar to
that of the data from which they are calculated, that is to the same number of significant figures or
one more.
It is important that candidates take more care when recording their observations and use
appropriate terminology.
General comments
The paper set for the first year of this new AS Level examination was appropriate for the
candidates who were entered for it.
This practical examination proved to be the right level of difficulty in that it enabled all candidates to
attempt all parts of both questions. Candidates had enough time to complete the paper; there is no
evidence that they ran out of time.
The practical skills required proved to be within the capabilities of candidates; where credit could
not be awarded it was often where not enough care was taken with the wet chemical tests or
during the drawing of graphs and processing results.
Comments on specific questions
Question 1
This experiment seems to have worked very well for most candidates. It proved to be well within
the capabilities of most candidates to carry out the experiment and to obtain valid results.
Most candidates followed the instructions well and were able to make a reasonable attempt at the
experiment.
More mistakes were made in the graph drawing and subsequent parts of the question.
(a)
Almost all candidates were able to obtain measurements for h and l. Many, however,
were not able to distinguish between the absolute uncertainty in the measurement and the
smallest scale reading possible, e.g. if the scale on a rule can be read to 0.05 cm the
uncertainty in a length will be 0.1 cm due to reading the rule at both ends. This then needs
to be combined with difficulties in measurement such as the oscillation of the rule, hence
0.2 cm was an acceptable answer. Uncertainties are usually only given to one significant
figure.
(b)
Almost all candidates were able to draw a suitable table. The most common errors were
to omit the units from the headings but to write them after each recorded measurement
(this practice was reluctantly accepted this year but will not be in future years), or to give
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the units of l2 as cm instead of cm2. Values of l2 were usually correct but a few candidates
calculated h2 instead.
(c)
The standard of graph drawing was generally good. It was pleasing to see that few
candidates had used awkward scales or had jointed points instead of a smooth curve or a
straight line. Units were sometimes omitted from the axes labels.
Candidates did not always show how they had attempted to calculate a gradient. Others
had a tiny triangle only (at least half the data line/curve should be included). Where the
graph is a curve then a tangent needs to be drawn. A valid gradient cannot be calculated
directly from the results table; readings from the graph must be used.
(d)
Suggested sources of error must be clear. Answers such as “parallax” are insufficient as
more detail is needed. The improvement proposed must include enough detail to give a
good chance of a more accurate answer: e.g. difficult to ensure that a hand held rule
would be vertical; clamp the rule in a vertical position next to the hacksaw blade.
Very few candidates knew what ‘directly proportional’ means. They needed to say “yes
because it is a straight line through the origin”, “no because it is a curve” or “no because
the straight line does not go through the origin”.
Question 2
The vast majority of candidates were able to carry out the tests and obtain results. Where credit
was not awarded it was because of careless experimentation or inaccurate recording.
Throughout the question candidates need to state “white precipitate” where appropriate. “Milky”,
“chalky”, “white cloudiness”, etc. are not acceptable. These are only acceptable in the limewater
test for carbon dioxide.
(a)
The gas tests were generally carried out well. However, many candidates did not report
on the colour of the solid after it had cooled down; they omitted the fact that it turns white
again on cooling.
(b)
Candidates need to read the question carefully, as here they were asked to record their
temperatures to the nearest 0.1 degree.
(c)
This was generally answered well. A few had a positive chloride test as well as a positive
sulfate one. ‘Slight or faint precipitate’ was accepted in the chloride test because it was
recognised that this is very susceptible to contamination but only as a contrast to a ‘white
precipitate’ in the sulfate test.
(d)
Some candidates did not get the precipitate to dissolve in excess with both reagents.
Care needs to be taken that sufficient excess is added.
(e)
The conclusions were not always consistent with the observations made. Sometimes
there was confusion between the anion in the acid (sulfuric) and those in solid X (zinc
carbonate).
(f)
It was pleasing to see many good attempts at these calculations. Sometimes one or more
incorrect masses were used. Some candidates did not convert the final answer to kJ mol-1
correctly.
(g)
The most significant source of error was heat loss. Other correct suggestions were
accepted as were appropriate suggested improvements.
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In (ii) many candidates simply repeated the question; they stated it would change but did
not state in which direction and therefore could not be awarded credit.
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