caustic soda vs sodium carbonate

caustic soda

noun
  • sodium hydroxide (NaOH), a strong alkaline substance used in the manufacture of paint, soap and detergents, or as a drain cleaner. 

sodium carbonate

noun
  • A salt, Na₂CO₃ formally derived from sodium hydroxide and carbonic acid; it is a white powder, soluble in water, with domestic, commercial and industrial uses; it occurs naturally as natrite 

How often have the words caustic soda and sodium carbonate occurred in a corpus of books? (source: Google Ngram Viewer )