An acid (from the Latin acidus/acēre meaning sour) is a
substance which reacts with a base. Commonly, acids can be
identified as tasting sour, reacting with metals such as
calcium, and bases like sodium carbonate. Aqueous acids have a
pH of less than 7 (an acid of lower pH is typically stronger),
and turn blue litmus paper red. Chemicals or substances having
the property of an acid are said to be acidic. Acid is the
opposite of alkaline. Articles About Acid in Saltwater
Aquariums: