"Do you want some?"
Translation:Tu en veux ?
That would mean "do you want a bit?" which is a little bit different - "some" is looking for a less specified quantity.
Not wrong, but it's a contraction that is not well accepted in written sentences. Maybe while speaking to someone, but you have to know him/her pretty well.
Quelque means "some" as a vague sense, not necessarily a quantity sense. So "quelque chose" means something or "quelqu'un" means someone, but in the sense of it not being anyone specific, rather than meaning some amount of a thing.