Certain chemicals, such as oxalic acid, attack copper but do not affect either silver or gold. Using such a chemical, it is possible to remove only the copper in an alloy, leaving both silver and the gold behind.
Certain chemicals, such as oxalic acid, specifically target copper while leaving silver and gold unaffected. As a result, it is possible to isolate only the copper in an alloy by using a chemical like this, preserving both silver and gold.