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9700/31 — Paper 3 Advanced Practical Skills 1

Cambridge Assessment International Education · October/November 2025

DOCUMENTS
CI · QP · MS
TOTAL PAGES
34
QUESTIONS
17
MARKS
40
Sit this paper →

WHAT THIS PAPER ASKS YOU TO DO

Question 1PRACTICAL

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.

[22]
  1. 1(a)(i)Complete Fig. 1.1 to show how you will prepare your serial dilution of the 2.0% hydrogen peroxide solution, H, by half at each step. Each beaker needs a labelled arrow for the volume of H2O2 transferred, a labelled arrow for the volume of distilled water W added, and a label underneath for the resulting concentration.[3]
  2. 1(a)(ii)Record, in an appropriate table, the time taken for a blue-black colour to appear for each of the five hydrogen peroxide concentrations (2.0%, 1%, 0.5%, 0.25%, 0.125%). If the colour hasn't appeared after 180 seconds, stop timing and record 'more than 180'.[5]
  3. 1(a)(iii)Record the time taken for a blue-black colour to appear for U1, U2 and U3 (three repeats of sample U).[1]
  4. 1(a)(iv)Calculate the mean time taken for the blue-black colour to appear for sample U. Show your working.[1]
  5. 1(a)(v)Use your results in (a)(ii) and (a)(iv) to estimate the concentration of hydrogen peroxide in sample U.[1]
  6. 1(a)(vi)Explain why repeating the measurement for sample U allows you to have more confidence in your estimate.[1]
  7. 1(a)(vii)With reference to your estimate for sample U, describe one other modification to the procedure that would allow a more accurate estimate of the concentration of hydrogen peroxide in sample U.[1]
  8. 1(b)(i)Scientists investigated the effect of temperature on hydrogen peroxide production by Streptococcus pyogenes at 20°C and 37°C over 168 hours (Table 1.2). Plot a graph of this data on the grid in Fig. 1.2.[4]
  9. 1(b)(ii)State two conclusions from the results of the investigation at the two temperatures.[2]
  10. 1(b)(iii)A sample taken at 20°C showed 14.5% of the bacteria were able to produce hydrogen peroxide. Use your graph in Fig. 1.2 to estimate when the sample was taken. Show on your graph how you obtained your estimate, and give your answer to the nearest hour.[2]
  11. 1(b)(iv)State one variable that the scientists would need to keep constant so that the results at the two temperatures could be compared.[1]
Sit this question →Run it on the guided bench →
Question 2PRACTICAL

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.

[18]
  1. 2(a)(i)J1 is a slide of a stained transverse section through a plant stem. Draw a large plan diagram of the whole section on J1. Use one ruled label line and label to identify the xylem.[5]
  2. 2(a)(ii)Observe the epidermis of the stem on J1. Select a group of four adjacent epidermal cells, each touching at least one other. Make a large drawing of this group of four cells and the waxy cuticle. Use one ruled label line and label to identify the waxy cuticle.[5]
  3. 2(a)(iii)Fig. 2.1 is a photomicrograph of a stained transverse section of a stem from a different type of plant from J1. Identify three observable differences, other than colour, between the stem section on J1 and the stem section in Fig. 2.1, and record them in Table 2.1.[3]
  4. 2(b)(i)Fig. 2.2 is a photomicrograph of a stained transverse section of a stem from a different type of plant. The circle represents the area of the stem. To calculate the density of vascular bundles you will first need to count the number of whole vascular bundles in sector P.[1]
  5. 2(b)(ii)Use your answer to (b)(i) to estimate the total number of vascular bundles in the stem section shown in Fig. 2.2. The angle θ = 15°. Show your working.[2]
  6. 2(b)(iii)The stem section shown in Fig. 2.2 has an actual area of 44 mm². Use your answer in (b)(ii) to calculate the density of vascular bundles in the stem section. Give your answer to two significant figures. Show your working.[2]
Sit this question →Run it on the guided bench →

What the lab technician is told: exact solutions, concentrations and apparatus to set out. This is where the bench comes from.

QUESTION 1 — SOLUTIONS

R1–R4 are the reagents for an iodine-clock (iodine–thiosulfate–starch) timing reaction. H is the 2.0% stock hydrogen peroxide the candidate serially dilutes by half each step; U is the unknown 'patient sample' whose concentration is estimated against the dilution series.

  • solution R1MH— dilute sulfuric acid, 0.05 mol/dm³100 cm³
  • solution R2— starch solution, 0.4%10 cm³
  • solution R3— potassium iodide solution, 0.5 mol/dm³10 cm³
  • solution R4— sodium thiosulfate solution, 0.01 mol/dm³10 cm³
  • solution HMH— 2.0% hydrogen peroxide solution — serially diluted by the candidate25 cm³
  • solution UMH— solution representing a patient sample — unknown H₂O₂ concentration10 cm³
  • solution W— distilled water, for the serial dilution100 cm³

QUESTION 1 — APPARATUS

  • 10 cm³ syringes2
  • 1 cm³ syringes5
  • beakers— capacity 50–100 cm³5
  • test-tubes, large— capacity 40–50 cm³8
  • test-tube rack— to hold 8 large test-tubes1
  • glass rod1
  • container labelled 'For washing'— approximately 200 cm³ tap water1
  • container labelled 'For waste'— capacity approximately 300 cm³1
  • paper towels8
  • glass marker pen, permanent1
  • stop-clock or timer showing seconds1
  • suitable eye protection1

QUESTION 2 — MICROSCOPE AND SLIDE

Shared on a rota, one microscope and slide between two candidates. Slide J1 is a confidential, Cambridge-supplied stained transverse section of a plant stem — candidates aren't told what it is in advance, and it must already be off the stage, scope set to low power, when a candidate's hour begins.

  • microscope— eyepiece lens ×10; low-power objective ×10; high-power objective ×40 — no other lenses fitted1 between 2
  • slide J1— stained transverse section through a plant stem (confidential specimen)1 between 2

⚠ Half the candidates start on Q2 and get the microscope + slide J1 first, for a maximum of one hour; the rest start on Q1 and swap in after an hour. Two candidates never share a microscope and slide at the same time. Q1's materials must be available to every candidate for the whole exam.

THE CURATED BENCHES BUILT FROM THIS

Same skills, taught rather than examined — guided step by step, with the apparatus mistakes explained as you make them.

Iodine clock: estimating hydrogen peroxide concentration by serial dilution →Microscopy and stereology: a plant stem cross-section (slide J1) →