Chapter P1:
Practical skills 1
Practical skills 1
Introduction
The analytical skills of chemists are still important
despite the development of new increasingly rapid and
sensitive instrumental techniques (see Chapter 22).
Your practical skills make an important contribution to
the grade you achieve in your chemistry qualification.
Figure P1.1 Chemist performing a titration.
Review of practical knowledge
and understanding
In scientific investigations we are often interested in
finding out how one variable affects another. For example,
we might want to investigate a precipitation reaction
to find out how the concentration of a reactant affects
the rate at which the precipitate forms. You might have
seen the reaction between sodium thiosulfate and dilute
hydrochloric acid, which is a commonly investigated
precipitation reaction. Sulfur is the precipitate formed:
Na2S2O3(aq) + 2HCl(aq)
2NaCl(aq) + S(s) + H2O(l) + SO2(g)
This type of investigation involves changing only the
variable under investigation (in this case the concentration
of a reactant) and keeping all other relevant variables
constant. We can judge the effect of changing the
concentration by devising a way to measure how quickly
the precipitate forms, such as timing how long it takes for
the solution to become opaque.
We now have the question we are investigating and the
structure of the investigation in terms of its key variables:
In a precipitation reaction, how does the concentration of
a reactant affect the rate of precipitation (as measured by
the time it takes for the solution to become opaque)?
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The independent variable is the one under investigation,
which is changed systematically and for which we can
choose different values (in this case, the concentration
of reactant).
The dependent variable is the one we measure to judge the
effect of changing the independent variable (in this case,
the time it takes for the solution to become opaque).
The control variables are those that we must keep
constant to ensure a fair test is carried out (in this case,
we should control the temperature and total volume of
reactants used).
Note that we can express the question in this type of
investigation generally as:
How does the independent variable affect the dependent
variable?
When asked to plan and/or carry out an investigation, it is
important that you state the question under investigation
clearly and list the independent, dependent and control
variables before you start writing down your proposed
method or planning how to record and present your results.
The type of variable under investigation will determine
whether you display the data collected in a table as a line
graph or as a bar chart. To decide which type of graph
to draw, you need to know the difference between
continuous variables – which are measured so can have any
numerical value within the range of results – and categoric
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Cambridge International AS Level Chemistry
thermometer with fine line divisions every 1 °C should be
read to the nearest 0.5 °C (see Figure P1.3).
However, if a measuring instrument has very fine
calibration (tightly grouped marks), it should be read to
the nearest calibrated mark.
25
value is 25.65 cm3 ± 0.05 cm3
eye must be level
with the bottom of
the meniscus (curved
surface at the top of
the solution)
26
27
burette
30
value is 28.5 °C ± 0.5 °C
20
10
thermometer
Figure P1.3 Taking readings from a magnified burette scale
and a thermometer with an analogue scale.
Rate / cm3 s–1
Rate / cm3 s–1
248
variables – which are described by words. We can assume
that the dependent variable is continuous, as it measures
the effect of varying the independent variable. Then if the
independent variable is continuous, we draw a line graph.
If the independent variable is categoric, we draw a bar
chart. So if you investigate the effect of temperature on
rate of reaction, the data can be presented as a line graph,
whereas if you investigate the rate of different metals
reacting with dilute acid, the data can be presented as a bar
chart (as there are no values between those chosen for the
independent variable). See the graphs in Figure P1.2.
In the Cambridge International AS and A Level
Advanced Practical Skills examination (Paper 3), you will
need to follow instructions to carry out an investigation into
an unknown substance or mixture of substances. Always
read through all of the instructions before carrying out the
tests. Testing for unknown substances will require you to
describe your observations in detail. You will be able to refer
to tables of tests for cations, anions and gases in the
Qualitative Analysis Notes in your examination paper to
draw your conclusions.
You will also carry out a quantitative task (based on
measurements) rather than a qualitative task (based on
observations). Examples of problems that need you to
collect quantitative data could be a titration (a volumetric
analysis) or an enthalpy change experiment. This type
of task will require you to read scales on measuring
instruments such as burettes, measuring cylinders, gas
syringes and balances. For instruments with an analogue
scale, such as a burette, you should be able to read
measurements to within half the value of the fine line
divisions on the scale. So a burette with fine line divisions
every 0.10 cm3 should be read to the nearest 0.05 cm3. A
Temperature / °C
(continuous variable)
Figure P1.2 A continuous independent variable
Zn
Fe
Al
Mg
Type of metal (categoric variable)
Sn
a line graph; a categoric independent variable
a bar chart.
Practical skills 1
Useful definitions to know, because you may need to
decide upon or recognise these in a task, are:
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Range: The minimum and maximum values for the
independent or the dependent variable. For example,
in the rate of precipitation investigation, the range of
the independent variable (the concentration) might be
0.2 mol dm–3 to 1.0 mol dm–3.
Interval: The difference chosen between consecutive
values of the independent variable. For example, in the
rate of precipitation investigation you might choose to test
concentrations of 0.2, 0.4, 0.6, 0.8 and 1.0 mol dm–3, giving
an interval of 0.2 mol dm–3.
Anomalous result: A result that does not follow an
established pattern.
Precise results: Results in which each set of repeat
readings are grouped closely together.
Accurate results: Results that reflect the true value of
a quantity.
ques ion
1 A student was investigating how temperature affects
the rate of a reaction between magnesium and
dilute hydrochloric acid. The student decided to
measure the volume of gas given off in 30 seconds for
different concentrations of acid. She decided to use
temperatures of 10, 20, 30, 40 and 50 °C.
a Name the independent variable.
b Name the dependent variable.
c
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Quality of measurements or observations
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Which type of graph would you use to display the
results of an investigation to find out how different
transition metal oxides affect the rate of reaction?
Points to remember
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In order to address the Cambridge International AS and A
Level Advanced Practical Skills examination (Paper 3), you
will need to master the expectations set out throughout
this chapter.
Manipulation, measurement
and observation
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Expectations
You should be able to:
Successful collection of data and observations
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set up apparatus correctly
follow instructions given in the form of written instructions
or diagrams
decide how many tests or observations to perform
make measurements that span a range and have a
distribution appropriate to the experiment
decide how long to leave experiments running before
making readings
identify where repeated readings or observations
are appropriate
replicate readings or observations as necessary
identify where confirmatory tests are appropriate and the
nature of such tests
choose reagents to distinguish between given ions.
249
d Give the range of the independent variable.
f
make accurate and consistent measurements and
observations.
Decisions relating to measurements or
observations
List two control variables.
e What is the value of the interval chosen for the
independent variable?
use the apparatus to collect an appropriate quantity of
data or observations, including subtle differences in colour,
solubility or quantity of materials
make measurements using pipettes, burettes,
measuring cylinders, thermometers and other common
laboratory apparatus.
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When describing a liquid or solution that is not coloured
and is transparent, always use the word ‘colourless’. Some
people make the mistake of just writing ‘clear’ – but a
solution of copper(II) sulfate is clear (i.e. transparent) but
blue in colour.
A solution that appears white and opaque in a chemical test
probably contains a fine suspension of a white precipitate,
for example when testing for carbon dioxide gas.
When carrying out a titration, you should repeat the test
until you have two titres that are within 0.1 cm3 of each
other. Ideally you should be aiming for two concordant titres
with the same values – but judging the end-point can be
tricky. That is why we carry out repeat sets of each test in
many investigations – to make our results more accurate by
reducing experimental error.
The first titre measured is always a rough value to
establish approximately where the actual end-point lies.
When obtaining subsequent values, you should be able to
add the solution from the burette one drop at a time near
the end-point.
Sometimes a result is clearly incorrect. For example, it
might be very different from the others in a repeat set of
readings or does not follow a well-established pattern in a
series of tests. If you have time, try it again. If not, discard it
– do not include it in your calculation of the mean or ignore
the point when drawing a line of best fit on a graph.
Cambridge International AS Level Chemistry
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When plotting a line graph of the data collected, a minimum
of five values of the independent variable (which is plotted
along the horizontal axis) must be recorded to be confident
of any pattern observed.
Note that it is possible to have precise results that are not
particularly accurate. For example, if you measure the
mass of a product formed three times and the results are all
the same, they are precise. However, if the balance was not
set to zero for any of the measurements, the mass will not
be accurate.
ques ion
2 a A student carried out a titration four times and got
results for the titre of 13.25, 12.95, 12.65 and then
12.65 cm3. What is the most accurate value of the
titre to use in any calculations?
b What do we call a mixture of water and fine
particles of a insoluble solid dispersed throughout
the liquid?
c
Describe any similarities and differences you
observe when looking at a test tube of dilute
sulfuric acid and a test tube of 0.05 mol dm–3
copper(II) sulfate solution.
Display of calculation and reasoning
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Data layout
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e In question 1, what piece of apparatus could
the student use to measure:
i
the independent variable
ii the dependent variable.
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Presentation of data and
observations
Expectations
You should be able to:
Recording data or observations
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present numerical data, values or observations in a single
table of results
draw up the table in advance of taking readings/making
observations so that you do not have to copy up your results
include in the table of results, if necessary, columns for raw
data, for calculated values and for analyses or conclusions
use column headings that include both the quantity and the
unit and that conform to accepted scientific conventions
record raw readings of a quantity to the same degree of
precision and observations to the same level of detail.
choose a suitable and clear method of presenting
the data, e.g. tabulations, graphs or a mixture of
presentation methods
select which variables to plot against which and decide
whether a graph should be drawn as a straight line or
a curve
plot appropriate variables on clearly labelled x- and y-axes
choose suitable scales for graph axes
plot all points or bars to an appropriate accuracy
follow the Association for Science Education (ASE)
recommendations for putting lines on graphs (see the
‘Points to remember’ referring to graphs, below).
Points to remember
d Name the white precipitate formed in the test for
carbon dioxide gas.
250
show your working in calculations, and the key steps in
your reasoning
use the correct number of significant figures for
calculated quantities.
There are certain conventions to observe when designing
and drawing a table to use to record your experimental
data. Generally, the independent variable goes in the first
column and the dependent variable goes in the second
column. Sometimes, if space on the page is an issue, a table
can be organised horizontally. In this case, the independent
variable again goes first but at the start of the first row in
the table, with the dependent variable beneath it, not next
to it as in a conventional table.
When recording quantitative data you will often need
columns for repeat results and calculations of the mean. This
is achieved by subdividing the column for the dependent
variable into the required number of columns. For example,
in the rate of precipitation investigation described at the
beginning of this chapter, the table to record three repeat
readings and their means would be organised as:
Concentration /
mol dm–3
Time for reaction mixture to turn
opaque / s
1st test
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2nd test 3rd test
Mean
Note that the headings in the table have their units included
– therefore you do not need to record the units for each
entry you make in the table.
On graphs, always plot the independent variable along
the horizontal (x) axis and the dependent variable up the
vertical (y) axis.
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Draw the lines in tables and graphs in pencil, labelling the
axes as in the corresponding table headings with their units.
In the table above, there could be an extra column on the
right-hand side for values of the reciprocal of the mean
time taken for the reaction mixture to become opaque
(headed ‘1 / time’). This could be plotted on a graph of
1/time against concentration to see how the rate of reaction
varies with temperature (as rate is proportional to 1/time,
so the greater the time, the slower the rate). See the graphs
in Figure P1.4.
The labelled axes must be longer than half the size of the
graph grid in both directions, selecting a sensible scale (e.g.
1, 2 or 5 units per 20 mm square on the grid – not 3 units).
The points should be plotted as small, neat x’s with a sharp
pencil. The line drawn through the points should not be
‘dot-to-dot’ but should be a line of best fit – either drawn
with a ruler for a straight line or a smooth free-hand line for
a curve. Think of the best-fit line as the ‘average’ line though
the points.
Always show your working out in calculations.
Only give answers produced by calculation to correspond
to the number of significant figures of the least accurate
experimental data used. So if calculating a concentration
using titre volumes such as 15.35 cm3, then the value of
the concentration of the unknown solution can be given to
4 significant figures (e.g. 1.244 or 0.9887 mol dm–3).
However, if the known concentration of one of the reactants
is given to three significant figures (e.g. 0.0250 mol dm−3 or
0.200 mol dm−3), then the calculated concentration could be
given to three or four significant figures.
When recording qualitative descriptions in a table, if there is
‘no change visible’, write that and do not just put in a dash.
ques ion
3 In an experiment to find the enthalpy change of a
reaction between two solutions, the mass of solutions
mixed together was 50.0 g and the temperature
increased by 7.5 °C. The following equation is used:
energy transferred = mass × specific heat capacity
× change in temperature
Graph a
(negative curve)
Concentration / mol dm–3
1
/ s–1
time taken
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Time for mixture to become opaque / s
Practical skills 1
Graph b
(positive
straight line)
251
Concentration / mol dm–3
Figure P1.4 Graphs that could be drawn from the data in a
table using concentrations and mean times.
Analysis, conclusions and
evaluation
Expectations
You should be able to:
Interpretation of data or observations and
identifying sources of error
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where the specific heat capacity of the solutions was
taken as 4.18 J g–1 °C–1.
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a Calculate the energy transferred in joules (J) to an
appropriate number of significant figures.
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b Explain the number of significant figures chosen in
part a.
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describe the patterns and trends shown by tables
and graphs
describe and summarise the key points of a set
of observations
find an unknown value by using co-ordinates or intercepts
on a graph
calculate other quantities from data, or calculate
the mean from replicate values, or make other
appropriate calculations
determine the gradient of a straight-line graph
evaluate the effectiveness of control variables
Cambridge International AS Level Chemistry
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identify the most significant sources of error in
an experiment
estimate, quantitatively, the uncertainty in
quantitative measurements
express such uncertainty in a measurement as an actual or
percentage error
show an understanding of the distinction between
systematic errors and random errors.
y
read values
from the y-axis
change in y
Drawing conclusions
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draw conclusions from an experiment, giving an outline
description of the main features of the data, considering
whether experimental data support a given hypothesis, and
making further predictions
draw conclusions from interpretations of observations, data
and calculated values
make scientific explanations of the data, observations and
conclusions that have been described.
Suggesting improvements
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252
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suggest modifications to an experimental arrangement that
will improve the accuracy of the experiment or the accuracy
of the observations that can be made
suggest ways in which to extend the investigation to answer
a new question
describe such modifications clearly in words or diagrams.
Points to remember
To measure the gradient (slope) of a straight line on a
graph, choose two points on the line at least half as far
apart as its total length. Then construct a right-angled
triangle, as shown in Figure P1.5. The gradient tells us
the rate of change of y (the dependent variable) per unit
change of x (the independent variable):
change in y
gradient = ​ __________ ​
change in x
When evaluating the quality of the data collected, there
are two types of error to consider: random errors and
systematic errors. Whenever we carry out an experiment
there are always errors involved. They might be due to
the experimenter not reading the scale on a measuring
instrument correctly or choosing a measuring instrument
with an inappropriate scale. These examples of human
error could equally make the values of data too high or
too low, so they are called random errors. Repeating tests
and taking the mean value helps to reduce the effect of
random errors.
However, other errors can result in consistently high
or low values being recorded. These are called systematic
errors. Examples would be reading the volume of liquid
change in x
x
read values
from the x-axis
Figure P1.5 Finding the gradient of a straight-line graph.
Choosing to construct a large triangle reduces the percentage
error in the values read from the axes, which are then used to
calculate the gradient.
in a burette to the upper level of the liquid instead of to
the bottom of the meniscus. It should noted though that
these consistently high measurements of volume would
not result in an incorrect value for the titre, because the
final volume is subtracted from the initial volume. Not
ensuring the measuring instrument is correctly set on
zero is another example of a systematic error, which if not
corrected during an investigation can result in consistently
high or low masses being measured on balances.
Other systematic errors can be caused by errors when
planning an investigation. This might result in data being
collected that does not really answer the question under
investigation. For example, a control variable might not
be kept constant or taken into account, or the dependent
variable chosen might not be a true measure of the effect
of varying the independent variable. Such error will make
the data collected invalid.
You will have to estimate the error inherent in reading
scales, as described at the beginning of this chapter, and
the evaluation is the place to discuss the effect these
measurement errors might have on the validity of the data
and conclusions you can draw from them. In the example
of a thermometer with 1 °C calibration marks, you can
quote values to the nearest 0.5 °C. The actual effect of this
margin of error on confidence levels will depend on the
magnitude of the temperature being measured. When
reading a low temperature of, say, 5.0 °C, plus or minus
Practical skills 1
0.5 °C will have a bigger effect than when reading a higher
temperature, such as 92.5 °C. For this reason it is best to
quote percentage errors, worked out using the equation:
margin of error
​× 100%
percentage error = ​ ________________________
actual or mean measurement
worked exam le
In the case of the two temperatures 5 °C and 92.5 °C, for
5 °C the percentage error will be:
0.5 × 100
​________​= 10%
5
whereas for 92.5 °C the percentage error is:
0.5 × 100
​________ ​= 0.54%
92.5
So there is a significant error in reading 5 °C compared
with reading 92.5 °C.
In enthalpy change investigations you often have to
measure temperature differences, subtracting the final
temperature from the initial temperature. In this case,
using the thermometer just described, the error would
be plus or minus 1 °C, as the two 0.5 °C margins of error
should be added together. In this case you should suggest
increasing the temperature change. For example, when
evaluating enthalpies of combustion of alcohols by heating
water in a copper calorimeter, you could use a smaller
volume of water to heat. However, this change would have
to be balanced against the increase in percentage error in
measuring the smaller volume of water, to see which gives
the least percentage error overall.
You might need to suggest how to make your data
more accurate. For example, in the rate of precipitation
investigation you could improve the accuracy of judging
the time when the reaction reaches a certain point in each
test carried out. Whereas judging the moment when a
pencil mark can no longer be seen through the reaction
mixture is subjective, you could use a light source and
lightmeter to make more objective judgements. You could
stop the timer when the level of light passing through the
reaction mixture drops to the same level as measured by
the lightmeter in each test. This will make your repeat (or
replicate) data more precise and improve the accuracy of
your results.
Your evaluation might lead beyond suggestions to
change the method to ideas to investigate new questions.
For example, evaluating the rate of precipitation, you will
have tried to control the temperature – probably by ensuring
both solutions started mixing at the same temperature.
However, if the reaction is exothermic, this might cause the
temperature to change between the different concentrations
investigated. This could lead to a new investigation to
compare the energy transferred in a precipitation reaction at
different concentrations. The question could be phrased as
‘How does the concentration of a reactant solution affect the
energy transferred in the reaction?’.
When drawing conclusions from investigations
involving the manipulation of variables, you should refer to
your graph when commenting on the relationship between
the independent and dependent variables, explaining your
findings using the data and your scientific knowledge and
understanding. When drawing conclusions from qualitative
tests to identify an unknown substance, where possible try
to carry out or suggest a further confirmatory test for the
same substance from the table provided in Paper 3.
Any hypotheses you were testing can only be refuted
(disproved) by a practical investigation you carry out;
they cannot be proved because of the limitations of your
investigation. So hypotheses can only be ‘supported’ by
the data collected. If your hypothesis predicts that the rate
of reaction increases with increasing concentration, with
a justification using collision theory, and data from your
investigation supports this, you cannot say whether this
relationship is true beyond the range of concentrations
tested. There might be a point at higher concentrations
where increasing the concentration of one reactant
will start to have less, or even no, effect because there
is such excess that the rate of collisions with the other
reactant particles is not affected by further increases of
concentration – so this would give you another idea to test!
ques ion
4 a A student measured the average rate of a reaction
by timing how long it took to collect 20 cm3 of the
gas liberated in the reaction. What calculation
would the student do to work out the average rate
in cm3 s–1?
b The student finds that the rate of a reaction is
directly proportional to the concentration of a
reactant, X.
i
Sketch a graph to show this relationship.
ii Explain how to work out the gradient of the line
on the graph.
iii If the student changed the concentration of X
from 0.50 mol dm–3 to 0.25 mol dm–3, what
would happen to the rate of reaction?
c
Explain the quantitative relationship that the
student found in this investigation using your
scientific knowledge and understanding.
253
Cambridge international as level Chemistry
Summary
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The following table summarises the breakdown of skills and the marks allocated to each skill area each skill area
as it is assessed in the Cambridge International AS and A Level Advanced Practical Skills examination (Paper 3).
Skill
Manipulation, measurement and
observation
Presentation of data and observations
Analysis, conclusions and evaluation
Minimum
mark
allocation*
12 marks
6 marks
10 marks
Breakdown of skills
Minimum
mark
allocation*
Successful collection of data and observations
8 marks
Quality of measurements or observations
2 marks
Decisions relating to measurements or
observations
2 marks
Recording data or observations
2 marks
Display of calculation and reasoning
2 marks
Data layout
2 marks
Interpretation of data or observations and
identifying sources of error
4 marks
Drawing conclusions
5 marks
Suggesting improvements
1 mark
* The remaining 12 marks will be allocated across the skills in this grid and their allocation may vary from paper to paper.
254
End-of-chapter questions
1 A student investigated the ease with which
various metal carbonates decompose on
heating. She decide to heat equal masses of
each carbonate and time how long it took for
the carbon dioxide given off to turn limewater
in a test tube cloudy.
metal carbonate
heat
limewater
a i Name the independent variable in the investigation.
ii Name the dependent variable.
iii Name the control variable described at the start of this question.
[1]
[1]
[1]
Practical skills 1
b The student decided to repeat the test on each of five metal carbonates provided three times.
i Why is it a good idea to collect replicate data in investigations?
ii Draw a table that the student could fill in as the investigation was carried out.
c i The test tube contained 10 cm3 of limewater. The student measured this volume in a 10 cm3 measuring
cylinder with calibration marks every 0.1 cm3. What is the margin of error when reading this scale
and what is the percentage error in measuring the volume of limewater for this investigation?
ii Explain what is likely to be the greatest source of error in this investigation.
d What type of graph should the student use to display the data from the investigation?
[1]
[3]
[2]
[2]
[1]
Total = 12
2 The rate of the following reaction between hydrogen peroxide (H2O2) and iodide ions can be monitored using
sodium thiosulfate and starch indicator:
2H+(aq) + H2O2(aq) + 2I–(aq)
2H2O(l) + I2(aq)
A mixture of starch solution, potassium iodide solution, sulfuric acid and sodium thiosulfate is made. This mixture
can then be reacted with varying concentrations of 10-volume hydrogen peroxide, made by making measured
volumes of the peroxide solution up to 25 cm3 with distilled water. When the hydrogen peroxide solution is added
to the original mixture containing starch in a flask, the time for the contents of the flask to turn a blue/black colour
can be measured.
This procedure, using a range of volumes of hydrogen peroxide, can determine how the concentration of
hydrogen peroxide affects the rate of the reaction shown above. Here is a set of results obtained from one such
investigation.
Volume of hydrogen
peroxide used / cm3
Time, t, for blue/black
colour to appear / s
1
300
2
200
4
90
6
60
8
44
10
37
12
28
a A student wants to use these results to draw a graph that will show how the concentration of hydrogen
peroxide affects the rate of reaction. Record the heading and values that the student could use to
complete the third column of the table (to 2 significant figures).
b What piece of measuring equipment would be used to make up the volumes of hydrogen peroxide solution
to 25 cm3?
c The student was provided with a stopclock measuring to the nearest second to measure the time taken
for the solution to turn blue/black but asked for a stopwatch measuring to one-hundredth of a second.
The teacher said that would not be necessary. Explain the teacher’s response.
d The original mixture was made up using a solution of 40 cm3 of 0.10 mol dm–3 potassium iodide.
How many moles of iodide ions are in the reaction mixture?
e What role does the sodium thiosulfate play in this investigation?
[3]
[1]
[2]
[2]
[3]
Total = 11
Cambridge international as level Chemistry
3 You have to identify an unknown compound, x.
Test
Observations made
To a small spatula measure of sodium carbonate in a test
tube, add enough distilled water to make a solution.
Add 1 cm depth of x solution.
White ppt
To a small spatula measure of sodium sulfate in a test
tube, add enough distilled water to make a solution.
Add 1 cm depth of x solution.
White ppt
To 1 cm depth of x solution in a test tube, add aqueous
sodium hydroxide.
White ppt that is soluble in excess sodium hydroxide
Carefully heat the solid x in the test tube provided.
Note: two gases are released.
Brown gas is given off (nitrogen dioxide is a brown gas).
The gas re-lights a glowing splint (showing oxygen
is present).
The solid turns yellow and crackles as it is heated.
a From the results of the tests above, and the Tables of Qualitative Analysis notes, identify
the cation present in x.
b Suggest another reagent to confirm the cation present in x giving the predicted observation.
c Suggest the identity of x.
[1]
[2]
[1]
Total = 4