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,
Hydrazoic acid lithium) or alkaline earth metals are not explosive, but decompose in a more controlled way upon heating, releasing spectroscopically-pure gas. Solutions of hydrazoic acid dissolve many metals (e.g. zinc, iron) with liberation of hydrogen and formation of salts, which are called azides (formerly also called azoimides or hydrazoates). Dissolution in the strongest acids produces explosive salts containing the ion, for example: The ion is isoelectronic to diazomethane. The decomposition of hydrazoic acid, triggered by shock, friction, spark, etc. goes as follows: Hydrazoic acid is volatile and highly toxic. It has a pungent smell and its vapor can cause violent headaches.