The oil drop experiment
We have considered the forces that exist 2r
in a solid between two adjacent
molecules and we have therefore
Figure 1
assumed the existence of these
molecules.
The following simple experiment can be h
2R
used to give a rough idea of the size of a
molecule, and hence an atom.
The radius r of a small drop of oil is found and the volume of the drop calculated. The drop is now
placed on the surface of some dust-covered water and the drop spreads out into a roughly circular
patch of height h and radius R. The diameter of the patch is measured and hence the radius R is
found.
Now the volume of the original drop is the same as that of the film; therefore, if h is the thickness of
the film and the original volume of the drop is known the thickness of the film can be found.
Now the thickness of the oil film cannot be less than the size of a molecule and so the size of a
molecule of oil must be equal to or less than the thickness of the film.
If the thickness of the film (h) can be found this is the upper limit of the size of an oil molecule.
Example problem
A small drop of oil with a diameter of 0.15 mm is dropped onto a dust covered water surface. The
resulting circular patch of oil has a diameter of 20 cm.
Calculate the maximum size of an oil molecule.
Volume of drop = 4/3 r3 = 4/3 x x (7.5x10 ) = 1.77x10 m
-5 3 -12 3
Volume of circular patch = = R h = x (0.1) x h
2 2
-10
Therefore thickness oil film (h) = maximum size of an oil molecule = 0.6x10 m
Results from this experiment suggest that the diameter of a molecule of oil is about 10 -10m, and
this has been confirmed by X-ray diffraction.
The sizes of important particles may also be found from the following experiments:
(a) Size of nucleus – alpha particle scattering
(b) Size of atom – X ray diffraction
(c) Size of molecule – oil film