There are a lot of ways to think about the chemical reagents that we have stirring around in our flasks. But one of the basic ones, and one of the most useful, divides them into classes according to whether they’re in solution or not.
When things are in solution, they may act funny, but at least everything’s starting out on the same footing. If all the components are dissolved (and if everything’s stirring the way it should), then they all have the chance to find each other and do their respective things. But if some reagent is still a solid in there (powder, chips, what have you), that takes you into the nonintuitive world of surface chemistry.
This actually happens quite a bit. Plenty of standard organic reactions involve insoluble things where the chemistry takes place on the surface. There’s formation of a Grignard reagent from magnesium turnings, deprotonation with powdered sodium hydride, hydrogenation over palladium-on-charcoal – these are all classics. And I'm not even mentioning the surface-driven industrial scale catalyst systems today, which is unfair of me, since the economies of the entire industrialized world depend on them. But in all cases, the real details at the molecular level of these reactions are not easy to work out.
People are still arguing, for example, over just how catalytic hydrogenation works on the metal surface, although the general details of the mechanism are known. That one’s complicated by not just being the plain metal, but a weirdo solution of hydrogen in the metal lattice. There’s no dispute, though, that the reaction is taking place on the surface of the metal, and that the higher the surface area the better off you are.
That’s one big variable right there: surface area. Finely divided substances are very different players in these systems, and many chemists find (early in their lab careers) that they’ve unwittingly bought front-row seats for a demonstration of just how different they can be. Finely divided powders have a lot of surface area in them, and if that’s a rate-limiting factor, you can find yourself with something that’s easily a hundred times more reactive just by picking up a different bottle of what appears (at first glance) to be the same substance. I once saw someone substitute lithium powder for lithium sand in a prep without thinking about this issue, and not so much later, I got to see the same guy clean the inside of his fume hood out with a scrub brush.
But there’s more than just surface area affecting some of these reactions. Grignard formation, for example, seems to take place (at least initially) in fresh breaks or cracks on the magnesium surface. That exposes metal that hasn’t had a chance to become coated with anything (like a layer of magnesium hydroxide), and (zooming in) it also may reveal individual reactive magnesium atoms, left out on the edge and insufficiently surrounded by their teammates. Once these react and fly off into solution, the ones around them become exposed, and so on, and the oxidized layers become undermined and flake off. The standard Grignard-initiation tricks are all designed to speed this process along. A drop of iodine will react quickly with any magnesium points or edges, exposing still more fresh rough surface, as will reaching down under the solution and breaking the turnings with a spatula (or, alternately, grinding them with a heavy stir bar).
These days, what’s really complicating things is the ability to generate (and characterize) nano-sized particles. At some point, these things can stop behaving like tiny bits of the bulk substance (which can be enough of a difference in itself, as mentioned above), and start acting like completely new beasts. And the really nano-sized stuff has a better chance of actually being in solution – but that brings on various headache-inducing thoughts about what “being in solution” means on this scale. If you have clumps of (say) palladium a few dozen atoms wide, which manage to be solvated enough to float around, is that a heterogeneous reaction or a homogeneous one? At that size, is that a "surface", or not (and is the reaction really taking place on it?) What if the nanoparticles are immobilized on a solid support - do they stay and react there, or is the reaction driven by the few that escape? (That effect has been noted in the Heck reaction, among others).
We need to understand these things better than we do - there are surely a lot of very useful things that could be done if we had better control over catalysis and surface chemistry. It's going to keep a lot of people occupied for a very long time.