Acid test (gold) mark is tested by applying aqua fortis (nitric acid), which dissolves the mark of any item that is not gold. If the mark remains, it is tested by applying aqua regia (nitric acid and hydrochloric acid). If the mark is removed, then this test dissolves the gold, proving the item to be genuine gold. More accurate testing of the item for its fineness or purity can be done through the use of differing strengths of aqua regia and comparative testing of gold items of known fineness. The figurative meaning of the expression, where it is applied to tests of character,
Acid–base reaction HA represents the acid, B represents the base, BH represents the conjugate acid of B, and A represents the conjugate base of HA. For example, a Brønsted-Lowry model for the dissociation of hydrochloric acid (HCl) in aqueous solution would be the following: The removal of H from the HCl produces the chloride ion, Cl, the conjugate base of the acid. The addition of H to the HO (acting as a base) forms the hydronium ion, HO, the conjugate acid of the base. Water is amphoteric—that is, it can act as both an acid and a base. The Brønsted-Lowry model explains