What could you determine gold necklace that bubbles when it is exposed to an acid?

If a gold necklace bubbles when exposed to acid, it likely means that the necklace is not pure gold, as pure gold does not react with most acids. The bubbling could be a result of a chemical reaction between the acid and a metal alloy used in the necklace. It is advisable to have the necklace tested by a professional to determine its composition accurately.