'Is it not' as a question.
can you say 'Is it not heavy?' - because it is a question? (like saying 'it is heavy' as a question)

Question

Answer

Ni hau ma, Hisun :)

It would be more common to ask "isn't it heavy?", especially in North America. I wouldn't be surprised to hear more "is it not heavy?" in England or Australia. Asking "is it not heavy?" in North America might make you sound like a guy from China learning English... :D
Either way, both are correct.