9700_y16_sy

Cambridge International AS and A Level Biology 9700 syllabus Practical assessment
• Identify which key variables must be standardised in order to test a hypothesis effectively. Only
variables that are appropriate for the given scenario and likely to have some effect on the investigation
are expected, for example, time of investigation, but not those likely to have a very small effect, for
example, using the same test-tube.
Methods
From a given scenario, candidates will be assessed on their ability to describe a method that could be used
by another person without any further information to collect the necessary data without difficulty.
When describing the method, candidates should be able to:
• Use the information provided in the scenario to describe how to vary the independent variable, and the
ways in which they would make sure they had measured its values accurately
• Describe how to measure the dependent variable. This could include using either, as appropriate, the
apparatus or materials specified in the scenario or measuring instruments chosen to measure the
correct quantity to a suitable number of significant figures. This may include the use of monitoring
devices and computer technology to record changes
• Describe how to standardise each of the other key variables using appropriate methods for the
apparatus and nature of the investigation, e.g. using a dropping pipette is not appropriate to measure
small volumes of a solution
• Describe, where appropriate, suitable volumes and concentrations of reagents, and explain how
different concentrations would be prepared
Concentrations may be specified:
– in %(w/v), by adding a known mass of solute to a small volume of solvent, mixing until fully
dissolved and then making up to the final volume with solvent
– in mol dm –3 , by dissolving the molar mass of solute and then making up to 1 dm 3 with solvent
Dilutions may be made by:
– serial dilution, by making the same dilution step again and again, using the previous dilution as the
input to the next dilution in each step. Since the dilution-factor is the same in each step, the dilutions
are a geometric series
– proportional dilution, by adding a unit volume of a solution of a known concentration to a solvent to
obtain the required concentration, e.g. to dilute a stock solution by 5, 1 cm 3 of the stock solution is
added to 4 cm 3 of solvent. This gives a 1:5 dilution
• Describe, if appropriate, any control experiments to make sure that it is the independent variable that is
affecting the dependent variable and not some other factor
Control experiments can be of two types:
– where all factors are kept identical to the experimental set up except that the value of the
independent variable is zero, for example when water is used instead of a test solution
– where the control is to confirm that, for example, it is an organism that is causing a particular effect,
by leaving out or replacing the organism with non-living material, e.g. sterile glass beads instead of
an insect in a respirometer
• Describe, in a logical sequence, the steps involved in the procedure including how to use the apparatus
to collect results
• Describe how to ensure the quality of results by considering any anomalous results and the spread of
results by inspection and then by using standard deviation, standard error or 95% confidence interval
(CI). Results are precise if they are repeatable by the same candidate and reproducible by others
• Describe how to ensure the validity of the results by considering both the success at measuring the
intended dependent variable and the precision required
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