A compound containing copper ions in a +2 oxidation state and sulfur ions is likely to be copper II sulfide CuS . When exposed to air for several weeks, copper II sulfide can react with oxygen and moisture, leading to the formation of copper II sulfate CuSO4 and other copper oxide compounds. Copper II sulfate is a blue crystalline solid, while copper oxides can be black or green. Therefore, the color of the compound after being exposed to air for several weeks could be a mixture of blue, black, and green, depending on the extent of the reactions that have occurred.