A substance which releases hydrogen ions in water and has a pH less than 7, e.g. hydrochloric acid in the human stomach is?

Pure water has a neutral pH. Pure water has a pH, of about 7, which is neither acidic nor basic. Water’s Physical Properties: Water is unique in that it is the only natural substance that is found in all three states — liquid, solid (ice), and gas (steam) — at the temperatures normally found on Earth.