An example of a simile used in popular science words:
"To make such motion visible one must use two different kinds of molecules distinguishable for example by their different colours.Thus we can fill one half of a chemical test tube with a water solution of potassium permanganate, which will give to the water a beautiful purple tint. If we now pour on the top of it some clear fresh water, being careful not to mix up the two layers, we shall notice that the colour gradually penetrates the clear water. If you wait sufficiently long you will find that all the water from the bottom to the surface becomes uniformly coloured. This phenomena, familiar to everybody, is known as diffusion and is die to the irregular thermal motion of the molecules of dye among the water molecules. We must imagine each molecule of potassium permanganate as a little drunkard who is driven to and fro by the incessant impacts received from other molecules."
George Gamow (1961) One, Two, Three…Infinity. Facts and speculations of science, Revised Edition, Dover Publications, Inc., New York.
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Gamow refers to 'molecules' of potassium permanganate. Potassium permanganate is a salt, so is composed of ions, and will be dissociated in aqueous solution. The colour is due to the permanagnate ion – but hydrated potassium ions also diffuse, but as they do not give rise to any colour this cannot be detected. Gamow, as a research physicist, was likely using 'molecule' generically without distinguishing between actual molecules and ions, but many learners assume all materials, including salts, will be molecular so this is a common alternative conception (misconception).
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