Although I know the basic distinction between "one" and "it", I need a better explanation for the students. Look at the two sentences (1)and (2).
(1) Have you ever seen a panda? Yes, I've seen one.
(2) Have you ever seen snow? Yes I've seen it.
In (1) a panda is not a particular panda but any one of the pandas, so the pronoun is "one" not " it". So far so good. But in (2) snow is any snow, not any particular snow. And yet, the pronoun is "it". "one" is not possible because snow is non count. Is this the reason we use "it" instead of "one"? When the noun is non count, we use "it" instead of "one" even when the object in question is not something specific?