Taking this out of a comment and into a thread of its own.
Is my heavy water still heavy? I have a bottle of 99% D2O, and it's hygroscopic, I don't know the history of it before I got it. It's about half full. It might have sucked in a lot of normal water from the air.
@Dtl I was going to say that you're venturing dangerously close to philosophy with those kinds of questions.