adsorption
/ ad-SORP-shun /
Open a box of baking soda in the fridge and after a week the bad smells are gone. The soda did not swallow the odour molecules into itself — it caught them on its surface, like flies on flypaper. That sticking of molecules onto a surface, where they pile up in a thin layer instead of mixing all the way in, is adsorption.
More precisely, adsorption is the accumulation of a substance (the adsorbate) at the surface of another material (the adsorbent), so the adsorbate becomes more concentrated at the interface than in the bulk gas or liquid nearby. It is driven by the leftover bonding energy of surface atoms, which attract and hold passing molecules. Because only the surface is involved, materials with enormous surface area — charcoal, silica gel, clays — are powerful adsorbers.
This matters wherever you need to grab molecules out of a gas or liquid: gas masks, water filters, the little silica packets in shoe boxes, and the first step of nearly every surface-catalysed reaction. The key caveat is the spelling-trap: adsorption (molecules cling to the surface) is not absorption (a substance soaks all the way through, the way a sponge takes up water). One letter, two very different pictures.
Activated charcoal in a water filter has a hidden surface area larger than a football field per gram. Impurity molecules in the water stick onto that vast surface and stay there, leaving the water cleaner.
Adsorption: molecules cling to a surface rather than soaking inside.
The reverse process — molecules leaving the surface again — is called desorption. Adsorption and desorption usually go on at the same time; when their rates balance, the surface coverage settles to a steady value described by an adsorption isotherm.