EXAM CONDITIONS
Your bench has been set out for 9700/31.
Everything on this paper is what the confidential instructions told the technician to put in front of you — the same solutions, the same glassware, labelled exactly as the candidate sees them. You are not told what B, C, D, E or F are. That's the point.
2 tasks · 17 questions · 40 marks
Estimating hydrogen peroxide concentration by iodine-clock timing
Serially dilute 2.0% hydrogen peroxide by half four times, then time how long each dilution takes to turn blue-black with the iodine–starch–thiosulfate mixture. Use the same timing method on an unknown 'patient sample' U to estimate its hydrogen peroxide concentration, then interpret Cambridge-supplied data on bacterial hydrogen peroxide production at two temperatures.
Microscopy and stereology of a plant stem section (slide J1)
Draw a large low-power plan of the whole stem section on J1 (labelling the xylem), then a high-power drawing of four adjacent epidermal cells (labelling the waxy cuticle). Compare J1 with a printed photomicrograph of a different stem, then use a sector count and an angle from a second photomicrograph to estimate its vascular-bundle density.