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Narrative
Two Petri dishes containing water are dusted with lycopodium powder. A drop of the nonpolar hydrocarbon octane is added to one dish. A drop of the polar compound octanoic acid is added to the second.

Discussion
Two Petri dishes containing water are dusted with lycopodium powder. A drop of nonpolar octane is added to one dish leaving the lycopodium powder undisturbed. When a drop of the polar compound octanoic acid is added to the second dish, the lycopodium is quickly pushed to the sides of the dish by the spreading film of octanoic acid.

When a drop of a nonvolatile, immiscible liquid is placed on water, the liquid may either spread out uniformly over the surface, or it may resist spreading and remain as a compact droplet. For the substance to spread, its molecules must attract water more than each other. This condition is met for molecules containing ionic or polar groups such as carboxylic acids.