If you add a single grain of salt to a glass of that water, it's no longer pure. Drinking it you probably wouldn't notice, and some people might colloquially call it "pure", but we know it isn't because we added some salt to it.

If you add a teaspoon of salt to to a different glass of pure water, it's also no longer pure, and now most people would probably notice the salt and recognise it's not pure.

If you add a tablespoon of salt to to a different glass of pure water, it's definitely not pure and you probably wouldn't want to drink it either.

You could say the teaspoon of salt glass is purer than the tablespoon of salt glass, the grain of salt glass is purer than both of them and so the purest of the three. And yet, we know that it isn't pure water, because we added something else to it.

So pure > purest > purer > less pure. Also note that I was required to use "less pure" for the last one, because all of them except pure are "impure" or "not pure", even though were what I originally thought of writing.