Chemistry 11–12 · Year 12

Modelling dynamic equilibrium by transferring water between two cylinders with tubes of different diameter

Module 5: Equilibrium and Acid Reactions

Practical, model not builtLow risk

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The idea

When a forward and a reverse process run at the same time, the amounts stop changing once the two rates are equal, not when the amounts are equal, and the final ratio is set by the ratio of the rate constants.

What you need

Two identical 50 mL measuring cylinders labelled R (reactant) and P (product)
Two open glass tubes (or rigid straws) longer than the cylinders, internal diameters about 6 mm (forward, used in R) and about 3 mm (reverse, used in P)
Water tinted with food colouring, 40.0 mL
Results table or spreadsheet

How to do it

Fill cylinder R with 40.0 mL of water; leave P empty.
Round 1: one learner lowers the wide tube to the bottom of R, closes the top with a finger and moves the water it holds into P; at the same time a partner does the same with the narrow tube from P into R. Read both volumes.
Repeat for at least 30 rounds, recording both volumes after each round.
Plot the volume in each cylinder against round number; mark when the volumes stop changing.
At the end, compare the volume carried by each tube in one round and note that the two transfers are equal though the volumes differ.
Enter the tube and cylinder diameters in the simulation, compare its curves with the class data, then change the starting condition (all water in P, or 20 mL in each) and predict the final volumes before testing.

What you should see

With a cylinder of internal diameter 22 mm, a 6 mm forward tube and a 3 mm reverse tube, each round the forward tube carries 7.4 percent of the water in R and the reverse tube 1.9 percent of the water in P (the water in each tube stands at the level in its cylinder). Round 1 moves 2.98 mL, leaving 37.02 mL in R. The volumes settle at 8.0 mL in R and 32.0 mL in P, a ratio of 4.0, equal to the ratio of the tube cross-sections (4.0); the system is 90 percent of the way there after 24 rounds. Starting with all the water in P ends at the same volumes. Class data scatter around the model curve because of drips and uneven tube placement.

What changes

What you change: round number (and, in extensions, starting volumes or tube diameters)
What you measure: volume in each cylinder (mL)
What you keep the same: same tubes used for each direction
tube pushed to the bottom each time
simultaneous transfers
total volume

Common misconceptions

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

At equilibrium the amounts of reactant and product are equal (the rates are equal; here the volumes settle in a ratio of 4 to 1).
At equilibrium the transfers stop (both tubes keep carrying water every round; they carry equal volumes).
The final volumes depend on which cylinder started full (the same tubes give the same final volumes from either start).

Safety card

Low riskLearners carry it out

Hazards

glass tubes can break
water spills

Controls

tubes with fire-polished ends, or rigid plastic straws
work in a tray and wipe spills

Note

Water and food colouring only; no hazardous chemicals, so the NSW Department of Education Chemical Safety in Schools package does not apply beyond ordinary laboratory rules.

Curriculum references

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

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-12 CH11/12-4 CH11/12-5 CH11/12-7
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-01 CH-12WS-04 CH-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