6.1.3Inquiry Process
Controlling Variables in Biology — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 5 of 10
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Calibrating Apparatus
Calibration checks an instrument against a known standard so that systematic error is reduced.
pH meter
- pH meter Calibrate with at least two standard buffer solutions that bracket the pH values you plan to measure.
Balance
- Balance Zero or tare it before each use so that only the mass of the sample is displayed.
Thermometer or probe
- Thermometer or probe Check readings in melting ice at 0 °C and in boiling water near 100 °C, allowing for atmospheric pressure.
Eyepiece graticule
- Eyepiece graticule Calibrate it against a stage micrometer at each magnification, because the size of one unit changes with the lens.
Gas sensors
- Gas sensors Calibrate against the known composition of air before using them to follow respiration or photosynthesis.
Section 2 of 5Constant Environmental Conditions
Constant Environmental Conditions
Temperature
- Temperature Use a thermostatically controlled water bath or incubator and let all samples reach that temperature before they are mixed.
Light and humidity
- Light and humidity Use a lamp at a fixed distance for light, and keep samples away from windows, draughts and heat sources.
Uncontrollable factors
- Uncontrollable factors If a factor cannot be held constant, such as room temperature, monitor and record it and discuss its effect in the evaluation.
Timing
- Timing Keep exposure times identical, because the effect of many treatments depends on how long the tissue is left in them.
Section 3 of 5Samples and Controls
Living material varies, so sampling and controls are needed to show that the independent variable caused the effect.
1
Select samples that are as alike as possible, such as cylinders from one potato or leaves of the same age.
2
Use random sampling, for example random quadrat positions, so that the sample represents the whole area and avoids bias.
3
Use many replicates so that one unusual individual has little influence on the mean.
4
Set up a control that is treated in the same way but without the independent variable applied.
5
A negative control, such as boiled enzyme or water in place of enzyme, shows the effect is not due to something else.
Section 4 of 5Worked Example
Question
- A graticule has 40 units that line up exactly with 25 divisions on a stage micrometer.
- Each stage division is 0.01 mm.
- Find the length of one graticule unit, then the real length of a cell that spans 8 units.
- Length covered by 25 stage divisions = 25 × 0.01 mm = 0.25 mm.
- This equals 40 graticule units, so one unit = 0.25 mm ÷ 40 = 0.00625 mm.
- Convert0.00625 mm × 1000 = 6.25 µm per unit.
- Cell length = 8 × 6.25 µm = 50 µm.
Answer One graticule unit is 6.25 µm and the cell is 50 µm long.
Section 5 of 5Exam Checklist & Tips
6.1.3 – Key Points for the Exam
- Calibration Checking and adjusting an instrument against a known standard.
- Systematic error An error that shifts all readings in the same direction.
- Negative control A set-up without the independent variable, showing what happens when the factor is absent.
- Random sampling Choosing samples by chance so they represent the whole population and are not biased.
- Stage micrometer A slide with a precise scale used to calibrate the eyepiece graticule.
Common Mistakes
- Stating that temperature will be controlled without naming a water bath or incubator and a set value.
- Calibrating the graticule once and then using it at a different magnification.
- Choosing samples by hand because they look good, which introduces bias.
- Forgetting a control run, so there is no proof that the independent variable caused the change.
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