Nitric acid AsO, SbO, and TiO respectively. Some precious metals, such as pure gold and platinum-group metals do not react with nitric acid, though pure gold does react with "aqua regia", a mixture of concentrated nitric acid and hydrochloric acid. However, some less noble metals (Ag, Cu, ...) present in some gold alloys relatively poor in gold such as colored gold can be easily oxidized and dissolved by nitric acid, leading to colour changes of the gold-alloy surface. Nitric acid is used as a cheap means in jewelry shops to quickly spot low-gold alloys (< 14 carats) and to rapidly assess the
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