acerbate vs qualm

acerbate

verb
  • To make bitter or sour. 

  • To exasperate; to irritate. 

adj
  • Embittered; having a sour disposition or nature. 

qualm

verb
  • To have a sickly feeling. 

noun
  • A prick of the conscience; a moral scruple, a pang of guilt. 

  • A feeling of apprehension, doubt, fear etc. 

  • A sudden sickly feeling; queasiness. 

How often have the words acerbate and qualm occurred in a corpus of books? (source: Google Ngram Viewer )