Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Estimating the size of a molecule using an oil film

Demonstration

Finding the thickness of an oil film enables an estimate of the size of atoms to be made.

 

Apparatus and materials

Oil film kit

Olive oil in a bottle, and some small dishes

Lycopodium powder and dispenser

Paintbrush, soft, 5 cm

Vegetable black, 250 g

Paraffin wax, white, 3 kg

Tin can

Bucket

Metre rule

Camphor in a bottle

Retort stand

Hand lens

Paper towels, large pack

Health & Safety and Technical notes


Almost every class will contain several asthmatics. Lycopodium powder is dried pollen. See CLEAPSS guide L77 for advice on use of pollen.

To empty a tray, put a bucket under the hole and release the bung. The trays should be washed carefully in a detergent solution and then flushed with cold tap water for a considerable time before storing with, for example, some card separating tray from tray. 
 
Store the lycopodium away from the olive oil.

 

Procedure


Tray on bench

a Place the tray on the bench, with the corner that has the drain hole hanging over the bench edge. Close the hole with the rubber bung from below. Partially fill the tray with clean tap water and then level it by careful use of the wedges. Fill the tray to over-brimming with further levelling. Finally, clean the water surface by slowly moving the metal booms from the middle to the two ends of the tray. Leave them there. The advantage of this arrangement is that it makes it easy to clean the water surface. 

b Take the loop of very thin wire and dip it into the olive oil to catch a small drop. 
 
Wire loop apparatus 
Image courtesy of Mike Vetterlein 
 
Hold the loop in the special holders at eye level in a clamp stand. Adjust the position of the loop so that the drop can be clearly seen against the 0.5 mm graticule through the magnifying glass. Using a second loop of wire which has also been dipped in oil, tease the original drop or run several drops together until it is 0.5 mm in diameter. 
 
If there is an excess of oil on the loop, it can be wiped with filter paper. 
 Lightly dusk water with powder
 
c Lightly dust the clean water surface with the powder and touch the 0.5 mm drop of oil onto the water. 
 Measure the patch of oil
Image courtesy of Mike Vetterlein 
 
d Use a rule to measure the maximum diameter of the patch of oil. (With some water supplies, the patch contracts to a smaller size soon after it has formed. This is probably due to water-softening agents attacking the oil, though this is not certain. Whatever the cause of the contraction, the proper measurement to take is the initial maximum diameter.) 

e Place the booms, touching together, in a part of the water surface free from oil and then move them slowly apart to produce a fresh clean surface. Another student can then try the experiment.


Teaching notes


1 It is important that each student has an opportunity to do their own experiment. 

2 Students need to realize that if the oil has spread out to produce a patch that does not reach the edges of the tray, the film on the surface is likely to be one molecule deep. Furthermore, these chain molecules have one end in the water and the other in the air. They also need to realize that the volume of the oil film is the same as the volume of the initial drop. 
 
3 From the 0.5 mm width (d) of the drop, its volume can be found, even if it is taken as a cube by students not capable of dealing with the volume of a sphere. From the diameter (D), the area of the oil film and its thickness can then be calculated. If the drop was treated as a cube, the area should be taken as a square. (Approximating to a cube and a square only involves a factor of 2/3 less than the more accurate result.) 
 
4 Typical results: 
 
d3 = D2 x length of the molecule 
 
0.5 x 0.5 x 0.5 = 250 x 250 x length of molecule 
 
So the length of the molecule = 2 x 10-9 m 
 
5 There are approximately 12 atoms in the olive oil chain so the size of an atom is approximately 1.7 x 10-10 m. 
 
This experiment was safety-checked in August 2007

 

Related guidance


Estimating the size of a molecule using oil