Acids and Bases are complementary terms that are often used carelessly in common discourse.
Chemists require precise definitions. Two systems, the Brønsted and the Lewis systems, are widely used and are described in the text. The range of acid (and base) strengths is enormous, encompassing over fifty powers of ten! We are accustomed to calling strong acids, such as sulfuric acid (H₂SO₄), acids. However, water is also an acid, albeit far weaker; and the acidity of methane (CH₄) is so weak that it is normally not considered to be an acid.
Some compounds may function as both acids and bases, a characteristic called amphoterism. Water is an example of an amphoteric compound.