physisorption and chemisorption
There are two ways something can stay stuck to a surface, just as a magnet can hold a note on a fridge two ways. A fridge magnet clings loosely — bump it and it slides or falls, and you can move it anywhere. Glue, by contrast, forms a strong bond that you cannot undo without effort. Adsorption comes in these same two strengths: a gentle, reversible kind (physisorption) and a strong, bonded kind (chemisorption).
More precisely, physisorption (physical adsorption) holds molecules to a surface by weak van der Waals forces, the same faint attractions that act between all molecules. It is feeble, easily reversed by warming, builds up in multiple layers, and barely changes the molecule. Chemisorption (chemical adsorption) instead forms real chemical bonds between the molecule and the surface — strong, often hard to reverse, limited to a single layer, and capable of pulling the molecule apart.
The distinction matters most in catalysis: physisorption can gather molecules near a surface, but it is usually chemisorption that activates them, weakening their bonds so they can react. Telling the two apart is a key diagnostic. The caveat is that they are ends of a spectrum, not a strict either-or — and as temperature rises, weak physisorption can give way to true chemisorption on the very same surface.
On a catalyst, hydrogen gas first physisorbs loosely onto the metal, then chemisorbs — the H–H bond breaks and each atom forms a real bond to the surface. Only those split, surface-bonded atoms are reactive enough to take part in the next step.
Physisorption: weak and reversible. Chemisorption: a real chemical bond.
Quick tells: physisorption releases little heat (roughly the size of a condensation enthalpy), works best when cold, and piles up in many layers; chemisorption releases much more heat (bond-forming amounts), often needs some activation energy, and stops at one layer because each site bonds just once.