|MadSci Network: Chemistry|
This question is familiar and important to me because I have taught surface chemistry to second year University students for many years, and I am always needing to stress the difference so that they get the terms right. In terms of Latin origins of prefixes, my dictionary tells me that ab- means "from" while ad- means "to". That is where the two different words come from, but it does not help explain the differences in the meaning that scientists attach to the words. "Absorb" refers to a situation where something is taken into a medium, and disappears as a consequence (from?). "Adsorb" refers to a situation where something gets stuck onto (to?) the surface of a medium. We would use "absorb" for when light is absorbed by a coloured or opaque object, or for when water is soaked into a sponge, or even for when my students manage to "take in" some of what I am telling them. It is an ordinary English word. "Adsorb" is used for a very specific situation where molecules get stuck onto a surface. It does not occur as a word in ordinary language, outside its scientific meaning. Adsorption is very important, because many chemical reactions can go a lot faster when the reacting molecules are adsorbed at a surface (reactions of hydrogen gas on the surface of nickel, for example), or chemical reactions can be prevented by the presence of a barricading adsorbed layer on the surface (aluminium failing to react with air, for example), or impurities can be removed from a solution by adsorbing them onto a finely divided solid (activated charcoal for decolorizing solutions, or for medicinal use, for example).
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