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 chemistry. The ECW model is quantitative model that describes and predicts the strength of Lewis acid base interactions, −Δ"H". The model assigned "E" and "C" parameters to many Lewis acids and bases. Each acid is characterized by an "E" and a "C". Each base is likewise characterized by its own "E" and "C". The "E" and "C" parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. The equation is The "W" term represents a constant energy contribution for acid–base reaction such as the cleavage of a dimeric