Chemistry
Testing for gold with acid focuses around the fact that gold is a noble metal which is resistant to change by corrosion, oxidation or acid. The acid test for gold is to rub the gold-colored item on black stone, which will leave an easily visible mark. The 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 dissolved 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 differencing strengths of aqua regia and comparative testing of gold items of known fineness.
Read more about this topic: Acid Test (gold)
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