impure vs not pure

impure

adj
  • Unchaste; obscene (not according to or not abiding by some system of sexual morality) 

  • Containing undesired intermixtures 

  • Unhallowed; defiled by something unholy, either physically by an objectionable substance, or morally by guilt or sin 

not pure

How often have the words impure and not pure occurred in a corpus of books? (source: Google Ngram Viewer )