in D20 there were rules to cover such an eventuality ( if you fail a save, you start to make saving throws vs worn items, there is a chart in the dmg iirc)
in ARPG there isn't such a thing, the system depends on the person running the game to govern the ridiculousness/feasibility of such things and deal with it in effective story manner.
I don't worry about blast powder because its usually kept in a powder horn, or similar device, alchemical items on the other hand.
and if rules existed for attacking items or breaking worn items, id worry about alchemical items, but as it stands that's left to the purview of the GM for the table (just don't be a dink about it I guess)
As for asking if Alchemical Items are considered weapons:
Quote:
This sphere may be thrown at an enemy as a common thrown weapon.
I think the text of the item really does explain itself, if there is a alchemical item that's a weapon that doesn't have that text then its a different thing entirely