How does an acid test work?

SIMPLE ACID TEST FOR GOLD JEWELRY

1. Make a scratch in an inconspicuous place to remove any protective coating, such as lacquer. A scratch will not be needed if there is no protective coating

2. Using rubber gloves and a non-metal item, apply one drop of nitric acid to the scratch

3. There will be no reactions if the object being tested is gold. A bright green reaction means the object is common metal a milky reaction in the  scratch means it is gold over silver