Gold (Au) only dissolves in an very strong acid mixture called aqua regia (a mixture of nitric acid and hydrochloric acid). Usually platinum ad gold are not soluble in acids, but metals like copper, zinc are soluble in acids. When a necklace exposed to acid bubbles, it infers that the metal reacted with acid producing bubbles of hydrogen gas. So, the necklace is not made of pure gold as gold does not displace hydrogen from an acid. Therefore, we can say that the gold is not pure.
