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.
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