MadSci Network: Chemistry

Re: What is the difference between adsorb and absorb?

Date: Tue Feb 17 08:25:57 2004
Posted By: John Christie, Faculty, Dept. of Chemistry,
Area of science: Chemistry
ID: 1076303078.Ch

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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