Biology 11–12 · Year 12

Tracing the spread of an infection: the fluid-exchange epidemic with an indicator test

Module 7: Infectious Disease (Causes of Infectious Disease: transmission during an epidemic)

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

A hidden infection passed on by direct contact spreads through a group faster with every round of contact, and a record of who met whom lets the source be traced back, though never beyond the first pair.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

0.1 mol/L sodium hydroxide irritates eyes and skin
Phenolphthalein indicator is made up in ethanol and is flammable; phenolphthalein is a hazardous substance

Controls

Safety glasses throughout; wash splashes off with water; no pipetting by mouth
Only the teacher handles the indicator bottle, away from flames
Tubes are capped before inverting; wipe spills at once

Note

Sodium hydroxide and phenolphthalein: NSW Department of Education Chemical Safety in Schools package (https://education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf) and RiskAssess (https://www.riskassess.com.au/).

What you need

One test tube with a lid per participant, marked at 5 mL and at 10 mL, each holding 5 mL of distilled water, except one tube (chosen at random by the teacher and not revealed) holding 5 mL of 0.1 mol/L sodium hydroxide; the tubes look identical
Phenolphthalein indicator solution in a dropper bottle, held by the teacher
A contact record sheet with a column for each round; safety glasses for everyone

How to do it

Each participant takes a tube and records its number; nobody knows which tube is 'infected'.
Round 1: pair with anyone, record your partner, pour your whole 5 mL into your partner's tube, cap it, invert it twice to mix, and pour back to your own tube until each holds 5 mL again.
Rounds 2 and 3: repeat with a new partner each time, recording every partner.
The teacher adds one drop of phenolphthalein to every tube; a tube that turns pink holds sodium hydroxide and counts as infected.
Pool the contact records on the board and work backwards round by round to find the smallest set of people who could have been the source.
Run the live model with the class size and number of rounds you used, compare the class count with the expected count, and repeat the model run many times to see the spread.
Rerun the practical or the model with some participants sitting out rounds 2 and 3 (isolation) and compare the outcome.

What you should see

In a class of 24 with one source, exactly 2 people are infected after round 1, the expected count after round 2 is 3.91 and after round 3 is 7.32 (computed exactly from the random-pairing probabilities, and confirmed by 200,000 simulated classes, which gave 7.33). Every infected tube turns pink: each exchange with a clean tube at most halves the concentration, so after three rounds the weakest infected tube holds at least 0.0125 mol/L sodium hydroxide, pH 12.1 at 25 degrees Celsius (computed), well above the range in which phenolphthalein turns pink. Tracing narrows the source to the two people who exchanged in round 1 but cannot separate them, because after round 1 their tubes are identical. Isolating participants lowers the final count. The learner knows it worked when the pink tubes all trace back through the records to one round-1 pair.

What changes

What you change: number of contact rounds (and, in the rerun, the number of participants isolated)
What you measure: number of infected (pink) tubes
What you keep the same: volume exchanged
sodium hydroxide concentration
mixing method
indicator volume

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

An epidemic spreads by a fixed number of new cases each day; each round can roughly double the number infected while most people are still susceptible.
Contact tracing always finds the single source; after the first contact the source and their contact look identical.
People who test negative were never exposed; some met only uninfected partners, and exposure timing matters.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO12-14 BIO11/12-3 BIO11/12-4
Biology 11–12 Syllabus (2025), Year 12 focus area Diseases; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-12-02
Biology 11–12 Syllabus (2025), Year 12 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-12WS-03 BI-12WS-04 BI-12WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities