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 oil

It would be wonderful if you could measure the size of a single atom, but atoms are so small that it cannot be done directly. Yet you can find the size of a molecule in a roundabout way and from that calculate the size of an atom. 
Lord Rayleigh (English physicist, 1842 to 1919. Nobel prize winner, 1904) made a guess, one of the earliest good ones, by doing an experiment in which he put a little oil on clean water and watched it spread. He bought a big tub nearly a metre across, cleaned it carefully, filled it with water and then put a tiny droplet of olive oil on the surface. He tried it again and again until he found the amount of oil that would just cover the whole surface, by using crumbs of camphor. Where the water was oily, the camphor did not move, but where it was clean, the camphor rushed around. 
Lord Rayleigh knew that the oil molecule consisted of long chains of atoms with one end clinging to the water. He expected the oil to spread until it did not spread any more; until it was one molecule thick. It was a risky guess, but this has since been verified with alternative measurements. 
Langmuir introduced the idea of cleaning the surface between experiments by moving the waxed booms along the surface, gathering up the oil film. Thin films in the electronics industry are made in Langmuir troughs today. 
Measurements of the diameter of the oil drop and the diameter of the oil patch on the water are obviously very rough but they are worth having. Students may indeed not accept that the patch of oil comes from such a tiny drop in the first place!


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