That Sucks

Slate.com's newest "Explainer" column asks the interesting question How Does Activated Carbon Work? and then answers it thusly:

Through adsorption. Carbon has a natural affinity for organic pollutants like benzene, which bind to its surface. If you "activate" carbon—by steaming it at 1,800 degrees, for example—it forms little pores and pockets that increase its surface area. (It's said that a teaspoon of activated carbon has the area of a football field.) Pesticides, chloroform, and other contaminants slide into the holes of this honeycomb and hold fast.

That's all well and good, but what exactly is "adsorption", and how does it differ from regular absorption?

In order to answer this question, we're going to have head back to that old mainstay of English, Latin. The root of both words is "sorbere", Latin for "to suck". (If you think that word looks like "sorbet", you're right, it does. But if you think they have anything to do with one another, you're wrong; "sorbet" is the French version of "sherbet", a word extracted entire from Turkish with no link to Latin.) "Adsorb" and "absorb" have different prefixes: "ad-" is Latin for "towards", while "ab-" means "away from", which doesn't clear up the matter as much as we might hope; doesn't every instance of suction pull something away from something and towards something else?

In this case, unfortunately, we can't go by mere appearances. What it boils down to is that by common usage, "adsorb" means "to pull (a liquid, gas or substance in solution) to the surface of" while "absorb" means "to pull (a liquid or gas) into the centre of": absorption is a quality of something which is, well, absorbent, like a sponge, while adsorption isn't, which has a certain unsatisfying "because I said so, that's why" feel to it, but that's English for you.