International A Level Biology / Practical Skills in Biology II
Controlling variables: saying exactly how revision notes
Edexcel International A Level Biology (YBI11), chapter 26: Practical Skills in Biology II, specification point U6.1, U6.2. Free lesson.
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A strong control answer names the variable, then states exactly how it is controlled, including the method or apparatus and a value. In field studies, you measure and monitor variables instead, and explain how they could affect the results.
How to make a control precise
Naming a variable is only half the answer. To show how it is controlled, name the apparatus or method and give a value. This makes clear that the conditions are genuinely kept alike. Temperature: use a thermostatically controlled water bath at 30 °C. pH: add 2 cm³ of pH 7 buffer to every tube. A buffer resists changes in pH. Volume: measure 5 cm³ of yeast suspension with a syringe. Organisms: use the same species and batch, with all seeds germinated for 3 days. Acclimatisation, which means time to adjust: leave woodlice in the chamber for 10 minutes before timing. Storage: keep all…
How to answer the three-part question
For a control question, identify a variable, say exactly how to control it, then give the effect on the results if it is not controlled. This final part should describe what happens to your data. For brine shrimp eggs hatching in copper ion solutions, salinity is an abiotic variable, meaning it is a non-living condition. Keep salinity the same by using 35 g of sea salt per dm³ in every solution. If salinity is not controlled, it could change the percentage of eggs that hatch, so the results would not be valid. Do not name a variable the question has already fixed, such as the species…
How to distinguish abiotic and biotic variables
Abiotic variables are non-living physical or chemical conditions. Biotic variables relate to living organisms. The distinction matters because a question may specifically ask for one type. For brine shrimp, temperature, salinity and light are abiotic. The batch of eggs and the number of eggs per tube are biotic. In the field, soil pH, light and water content are abiotic. Grazing and competition from other species are biotic. If asked for an abiotic variable, do not give a biotic one instead.
How to account for variables in field work
You usually cannot control conditions in the field, so measure and monitor them instead. Choose similar sites where possible, and collect measurements in a consistent way. Soil pH: measure it with a probe and choose sites where it is about the same. Soil water content: measure it and sample every site on the same day, so rain affects them alike. Temperature: record it at each site and sample at the same time of day. Light: measure it with a light meter at every quadrat. For mayfly nymphs in two streams, measure water temperature with a thermometer at every sampling point, at the same time…
How to calibrate an instrument
Calibration checks an instrument against standards of known value, so you can trust its readings. State which standards are used, not just that the instrument is calibrated. Calibrate a pH probe in buffers of pH 4 and pH 7. Zero a colorimeter with a blank, then read it using known concentrations.