An acid (from the Latin acidus/acēre meaning sour) is a chemical substance whose aqueous solutions are characterized by a sour taste, the ability to turn blue litmus red, and the ability to react with bases and certain metals (like calcium) to form salts. 

Aqueous solutions of acids have a pH of less than 7. A lower pH means a higher acidity, and thus a higher concentration of hydrogen ions in the solution. Chemicals or substances having the property of an acid are said to be acidic.


Lemons contain citric acid. image: wikipedia

Common examples of acids include:

  •  hydrochloric acid (a solution of hydrogen chloride which is found in gastric acid in the stomach and activates digestive enzymes), 
  • acetic acid (vinegar is a dilute solution of this liquid), 
  • sulfuric acid (used in car batteries), and 
  • tartaric acid (a solid used in baking). 

As these examples show, acids can be solutions or pure substances, and can be derived from solids, liquids, or gases. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid.