It's up to the DM, but there are examples for the DM to consider
There is not an explicit definition of "item" in the core source books, but we still know some things about what can be an "item." First of all, we know that all the items you can buy in the section on "Adventuring Gear" are items, because that section in the PHB begins with:
This section describes items that have special rules or require further explanation.
So, that means that:
- a book is an item
- a vial of acid is an item
- a fishing tackle kit is an item
- a climber's kit (with all the tools in it) is an item
- and so on...
Additionally, we can look at ordinary language rules. "Item" is what is called a "countable noun" in English as opposed to a "mass noun" (like water, dirt, or wine).
This distinction suggests that mass nouns are not items, but they can be a part of items. Water is not an item, but a bottle of water is an item.
So, a puddle of purple worm poison is not an item, but a vial of purple worm poison is.
And that means, technically, a vat of purple worm poison is an item, in some sense.
But it is up to the DM to clarify this. It may be a case where the DM would distinguish between RAW (rules as written) vs RAI (rules as intended).
I would rule that "item" here is not intended to include containers with masses of material far beyond the ordinary unit of exchange (a bag of ball bearings, a vial of poison).