IGCSE chemistry
Preparing for the practical questions
You will be assessed on your ability to:
1. Devise and plan fair investigations or practical procedures such as
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Fair investigations such as comparing reactivity of metals, factors which affect the rate of a reaction,
comparing enthalpy changes in the combustion of alcohols or comparing enthalpy changes in
displacement reactions, investigating conditions for rusting.
Practical procedures such:
o measuring the percentage of oxygen in an oxide e.g. copper oxide and magnesium oxide
o measuring enthalpy changes
o making soluble and insoluble salts
o making carbon dioxide, oxygen and hydrogen
o decomposing metal carbonates
o testing for unsaturation in hydrocarbons
o determining formula of copper oxide by reduction with hydrogen
2. Recognise and explain the use of common items of laboratory apparatus
Examples of type of investigations for which you need to know the basic laboratory apparatus and the
reason for their use:
Type of experiment
Separation technique
Titration: making soluble salt using an
acid and alkali
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Main processes and items of laboratory apparatus
Step 1: add solute to solvent
Step 2: dissolving: warm to speed up dissolving
Step 3: filtration: filter funnel, filter paper
Step 4: crystallization: evaporating basin
To improve accuracy and precision of a titration you should use:
 a burette for the acid
 use graduated pipette to measure out alkali which is to be
placed in conical flask
To know when neutralization is complete:
 add indicator to solution in conical flask or
 use a thermometer and stop adding acid when the
temperature stops rising.
If the salt is needed repeat the experiment with the same
volumes of acid and alkali but without the indicator.
Energy changes
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Polystyrene cups with lids to reduce heat loss as they are better
insulators than glass beakers.
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Making salts: excess base (making a
soluble salt with a base)
Preparation for gases
Tests for ions
Chromatography
Add base to warm acid (speed up reaction) until no more
dissolves. Filter off excess base and heat and crystallize filtrate,
wash and dry.
 If gas is collected over water you rely on the gas being
insoluble in water
 Use burette or gas syringe to measure volume more
accurately
 Relate density of gas to method of collection
o hydrogen (less denser than air) use inverted test tube
(downward displacement of air)
o carbon dioxide (denser than air) use test tube open at
the top (upward displacement of air; it pushes the air
up!!)
Know the test and test result for the anions and cations – see
section 2g.
 Draw pencil line
 Place sample of mixture on pencil line
 Add solvent to a level below pencil line
 Allow solvent to move up paper
 Relies on difference in solubility
In example to the right sample H is a mixture of F and G.
Making dry pure salt crystals
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Heat solution until saturation
Allow to cool/crystallize
Wash with distilled water
Dry
Others experiments you need to know (these are also listed on the wiki) :
 experiments (using iron, copper and phosphorus) used to provide evidence that air has 21% oxygen
 thermal decomposition of copper carbonate: the green stuff turning black!!!
 cracking alkanes
 dehydration of ethanol
 fermentation of sugar using yeast
3. Make observations and measurements with appropriate precision, record these methodically, and
present them in a suitable form.
This means reading scales to an appropriate degree of accuracy and designing your own results tables.
Measuring tools you need to be able to read and record their values in a suitable form i.e. correct number of
significant figures and decimal places (you should read the scale of an instrument to  half of the smallest
scale division)
 a digital balance,
 thermometer,
 measuring cylinder,
 burette (you need two decimal places when recording data obtained from a burette e.g. 25.50 cm3).
Mathematical operations:
 Example: calculation of the mean or average
 When calculating a mean only use concordant values i.e. values within 0.2 and do not include any
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anomalous results.
Express your final result in the correct significant figures; this means your answer should have the
same number of decimals as the numbers you used in the calculation.
4. Plot graphs.
 Draw graphs and bar charts.
 Ignoring anomalous results
 Plot and use graphs and draw straight lines and curves of best fit e.g. energy changes, thermometric
titrations.
5. Analyse and interpret data to draw conclusions from experimental activities which are consistent
with the evidence.
Identify relationships from graphs, identify anomalous results and explain how they may have arisen.
This involves:
 interpreting intersections of graph lines, finding values.
 recognizing a correlation: see table below.
Correlation Correlation is a relationship between the two variables (independent and dependent) in a
given experiment. This is often obtained from a graph.
 If the gradient (slope) of a graph is positive (i.e. the graph slopes upwards) we can say
there is a positive correlation.
 If the gradient is negative, we can say there is a negative correlation between the
variables.
 If a straight line goes through the origin of a graph and the gradient is positive, we can
say that the variables are directly proportional to each other.
Just because two factors correlate well and there is a good line of best fit, does not
necessarily mean that one variable is causing the other variable to increase or decrease.
6. Evaluate data and methods
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Comment on the reliability/repeatability and accuracy of the data. This basically means: how can accuracy
and reliability/repeatability be improved.
Accuracy
Precision
An accurate measurement is a measurement that is close to the true or accepted value.
An 'accepted value’ comes from the work of many scientists who have measured that
value, agreed with it and published the value. We can refer to a textbook, data tables, or
the internet to find these values.
Accuracy can be improved by:
 Using measuring devices with smaller scale divisions.
 Improve design to eliminate errors such as heat loss.
 Allow a precipitate to settle before measuring its height
 Measure the highest temperature not just a higher temperature
 Allowing a salt to dry before you measure its mass
 Ensuring reaction is complete
If the measurement gives the same result each time, it would be precise. The greater the
number of decimals, the greater the precision e.g.  0.01g is more precise than .1 g.
Precision can be improved by:
Using an instrument with a smaller scale division e.g. a ruler with a millimeter scale is more
precise than a ruler with a centimeter scale.
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Validity
Reliability
Validity is about how well an experiment measures what it says it is going to measure.
Validity is usually improved by:
 Making the investigation a fair(er) test which means controlling variables you are not
investigating.
 Improving precision.
Valid investigations are accurate and precise.
Reliability or repeatability is about getting the same results if the experiment is repeated
under the same conditions, if it does than the method is reliable. Reliability is about the
method; precision is about results.
Reliability can be improved by:
 Repeating anomalous results until they become concordant
 Effective control of variables.
 Increase precision.
Anomalous Anomalous readings are readings that fall outside the normal, or expected, range of
results
measurements. If we take a large number of readings, we can be more certain about
saying which readings are anomalous (do not fit the pattern) and which are not anomalous.
Anomalous readings will show on a graph as a point, or points, standing clearly away from
a line of best fit.
Concordant Concordant readings are obtained when any reading is repeated and all the readings are
results
close, or identical.
Example: If you obtained four measurements such as 6.20 cm, 6.10 cm, 6.30 cm and 6.20
cm, you would say they were concordant – they are within 0.2.
If however, you obtained 6.20, 6.10, 7.10 and 5.30 for example, then these readings are
not concordant. The 7.10 cm and 5.30 cm readings would probably be anomalous readings
and these two readings should be repeated again.
Anomalous results should not be included when averaging results. The more concordant
your results are the more reliable they are likely to be.
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Draw conclusions and offer explanations. Conclusions should relate to the data collected and
processed.
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Suggest improvements to the procedures. Control of variables.
Suggest further experimental work that may be required
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o
how good was the experimental technique and how could this be improved?
extending work e.g. a greater range of variables, other variables which affect the experiment.
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