4e isn't expected to work like that...
As I set out in this answer, D&D 4e is a firmly mechanical system that isn't particularly interested in "what makes sense" or "how it would work in the real world." It's sacrificed that for balance, regularity, and other meta-issues that plagued earlier editions of the game. (I render no absolute judgement on whether this is a good thing; it depends entirely on what kind of game experience your group values).
So there aren't any rules about mundane water sterilization (because it's Dungeons & Dragons, not Dungeons & Utilities Services, and the rules reflect that priority), and damage types work normally against anything that isn't explicitly resistant or immune to them (which is good, balance-wise, because there are entire classes dedicated to dealing psychic damage and they'd be hosed against undead in the scenario your player proposes).
...but your group doesn't have to follow the system's expectations.
Just recognize there will be systemic complications which rise from that kind of precedent.
It's up to your group whether you bring real-world considerations (physics, logic, etc) into the game's rules... but if you do it's important to remember that the system isn't expecting such considerations. You'll probably find increasing contradictions and weird cul-de-sacs of logic if you go down this road, and you'll need to be prepared to deal with them.
This isn't solely a 4e issue
D&D 3.5 has similar problems, just disguised under a massive pile of increasingly complicated subsystems designed to try handling these issues. No RPG system can accurately reflect reality; we don't know enough about reality, nor do we have the time to model it. If 4e hasn't chosen to model the elements of reality that your party wants to explore, either tack on the house-rules you need or find system more compatible with the group's desired experience.