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,
Solid acid Solid acid Solid acids are acids that do not dissolve in the reaction medium. They are often used in heterogeneous catalysts. Most of the acids solid in state are generally organic acids that includes oxalic acid,tartaric acid,citric acid,maleic acid,etc Examples include oxides, which function as Lewis acids including silico-aluminates (zeolites, alumina, silico-alumino-phosphate), and sulfated zirconia. Many transition metal oxides are acidic, including titania, zirconia, and niobia. Such acids are used in cracking. Many solid Brønsted acids are also employed industrially, including sulfonated polystyrene, solid phosphoric acid, niobic acid, and heteropolyoxometallates. Solid acids are used in catalysis in many industrial chemical