Introducing the oil film experiment

Class practical

Experiments to help a class understand this experiment:

Estimating the size of a molecule using an oil film

Apparatus and Materials

For each student pair
• Crystallizing dish
• Eye dropper
• Beaker, 400 ml
• Splint
• Iron wire, 20 cm of 16 swg
• Bunsen burner
• Heat resistant mat
For the class
• Lycopodium powder dispensers, several (or talcum powder)
• Olive oil in a bottle
• Alcohol in a bottle
• Crumbs of camphor
• Detergent for cleaning dishes (for technician use)
• Paper towels, large pack

Health & Safety and Technical Notes

Remember that lycopodium powder is dried pollen. See CLEAPSS guide L77 for advice on use of pollen. Any students who suffer badly from hay fever might try talcum powder instead. It is not quite as good but would do. Indeed, any student wanting to repeat these experiments at home could use talcum powder. Check school records for asthma sufferers.

When ethanol is in use, remember to have no naked flames in the laboratory.

Read our standard health & safety guidance

Cleanliness is essential. As the dishes become dirty quickly, it is advisable to have spares. Before the lesson begins, the dishes must be cleaned carefully. They should be cleaned with a non-foaming and soda-free detergent and then carefully washed to remove all detergent.

In storing apparatus, it is essential to keep the lycopodium far away from any oil or camphor.

Procedure

1. Fill each dish to overflowing with clean water. Hold it with fingers outside, well away from the rim and tip it sharply to pour out half the water. (Students must be very careful not to let their fingers touch the water or the inside of the dishes.)
2. Dust the water surface with lycopodium powder – very lightly.
3. Allow a drop of alcohol to fall on the surface.
4. With a new lot of water and another light dusting of powder, try a very small quantity of olive oil. A matchstick dipped in oil and then wiped clean should provide enough.
5. With a very clean dish, dip what you think is a clean finger in a dusted surface and see the ‘grease ring’ that forms.
6. With a fresh, clean dish, try placing crumbs of camphor on a clean water surface.
7. With a fresh, clean dish filled with water and dusted with lycopodium powder, bring a red-hot iron wire near to the surface.

Teaching Notes

• The secret of these experiments is cleanliness. If the expected behaviour is not observed, it is almost certainly due to the presence of unwanted oil. The dish must be cleaned again or replaced. A monomolecular layer of oil (about 25 micrograms) can spoil the experiment. A finger touched to a hair on one’s head can spoil it. It is also undesirable to put too much lycopodium powder on the water surface – a common student failing.
• After steps 1 to 3, the students should observe a clean patch appearing as the powder moves away towards the edges. The powder returns as the alcohol dissolves in the water or evaporates.
• After procedure 4, the powder is again seen to be pushed aside, this time by the oil. Now the powder does not return as it did with the alcohol.
• After procedure 6, if the camphor seems lazy in its movement it is a sure sign that the water surface is oily. A full recleaning is necessary.
• Here is a test for cleanliness before the dish is released for use. Fill the dish with water. Then put a drop of alcohol on the water surface with a very little powder on it. If the powder rushes to the edge and back again quickly, it is clean enough.
• Step 7 could be done as a demonstration by the teacher. The powdered surface rushes away from the iron wire. The rise in temperature weakens the surface skin.

This experiment was safety-tested in July 2006