Scientific Skills: Diagrams, Experiments and Data Interpretation
This domain tests the practical skills the written paper can still examine: drawing and labelling biological diagrams correctly, describing controlled experiments with their observations and conclusions, and reading tables and graphs accurately.
Drawing and labelling
- Use a sharp pencil, continuous clean lines, no shading, and keep the drawing large and proportional.
- Draw label lines straight and horizontal, never crossing each other, with the label written to the right and not inside the drawing.
- Give a title and, where relevant, state magnification. Plant cells show a cell wall; animal cells do not.
Standard experiments
- Starch test: iodine turns blue-black. Carbon dioxide: limewater turns milky. Oxygen: a glowing splint relights.
- Photosynthesis starch test: destarch the plant in the dark, partly cover a leaf, expose to light, boil in alcohol using a water bath to decolourise, then add iodine - only the exposed part turns blue-black.
- Transpiration: cobalt chloride paper turns from blue to pink in moist air; a potometer measures water uptake.
- Germinating seeds respire: limewater turns milky and a thermos flask shows a rise in temperature.
Interpreting data
- Identify the independent variable (what you change), the dependent variable (what you measure) and the control.
- Read units from the axis, quote figures exactly, and describe trends before explaining them.
Worked example
A student covers half a destarched leaf with black paper and exposes the plant to light for six hours. After the alcohol and iodine steps, the exposed half is blue-black and the covered half stays brown. The exposed half made starch by photosynthesis in light; the covered half could not, because light was the limiting factor. The covered half is the control.
Traps that cost marks
- Never write a conclusion that the data do not support, such as 'the plant grew faster' when only starch presence was tested.
- Do not confuse observation with inference.
- Label lines with arrowheads and shading lose marks.