ACIDS

An acid 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,
where an acid of lower pH is typically stronger. Chemicals or substances having the property of an acid are
said to be acidic.
Common examples of acids include acetic acid (in vinegar), sulfuric acid (used in car batteries), and tartaric
acid (used in baking). As these three examples show, acids can be solutions, liquids, or solids. Gases such as
hydrogen chloride can be acids as well. Strong acids and some concentrated weak acids are corrosive, but
there are exceptions such as carboranes and boric acid.
There are three common definitions for acids: the Arrhenius definition, the Brønsted-Lowry definition, and
the Lewis definition. The Arrhenius definition states that acids are substances which increase the
concentration of hydronium ions (H3O+) in solution. The Brønsted-Lowry definition is an expansion: an acid
is a substance which can act as a proton donor. Most acids encountered in everyday life are aqueous
solutions, or can be dissolved in water, and these two definitions are most relevant. The reason why pHs of
acids are less than 7 is that the concentration of hydronium ions is greater than 10−7 moles per liter. Since
pH is defined as the negative logarithm of the concentration of hydronium ions, acids thus have pHs of less
than 7. By the Brønsted-Lowry definition, any compound which can easily be deprotonated can be
considered an acid. Examples include alcohols and amines which contain O-H or N-H fragments.