Franklin’s teaspoon of oil

Use this ancient account to discover more about bond lengths and the effects of a simple teaspoon of oil.

Commentary

The students are invited to interpret a historical experiment based on a full and interesting account of Franklin’s contribution to surface chemistry written by Professor Charles Giles, whose researches show that the site of the experiment was the Mount Pond on Clapham Common. Franklin’s communications were published in the Philosophical Transactions of the Royal Society.

Calculation

If it is assumed that the oil forms a monomolecular layer, then the thickness of the film should correspond to the height of a molecule of triolein lying on the surface of the water. Franklin estimated that one teaspoonful of olive oil spread to cover half an acre of the pond’s surface.

Half an acre = 2420 yd² = 2420 x 0.9144² m²

One teaspoonful, say 2 cm³ = 2 x 10–6 m²

Hence, the thickness of the film when one teaspoonful covers one half acre is:

(2 x 10^–6)/(2420 x 0.9144²) = 9.9 x 10^–10 m

Extension

This experiment is the forerunner of the classical ‘oil drop experiment’. Although teachers will probably have met this, it may be unfamiliar to students who could be asked to repeat Franklin’s experiment on a small scale. Designing an experiment in which the pond is replaced by water contained by the sides of a flat tin tray would provide several opportunities for problem-solving.

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.