One of a class of compounds, generally but not always
   distinguished by their sour taste, solubility in water, and reddening
   of vegetable blue or violet colors. They are also characterized by the
   power of destroying the distinctive properties of alkalies or bases,
   combining with them to form salts, at the same time losing their own
   peculiar properties. They all contain hydrogen, united with a more
   negative element or radical, either alone, or more generally with
   oxygen, and take their names from this negative element or radical.
   Those which contain no oxygen are sometimes called hydracids in
   distinction from the others which are called oxygen acids or oxacids.