Which halogen is known to cause the reaction given below?

Step-by-step solution

All Cu 2 + halides are known except the iodide. In this case, Cu 2 + oxidises I – to I 2 . 2 Cu 2 + + 4 I - → Cu 2 I 2 + I 2 However, many copper (I) compounds are unstable in aqueous solution and undergo disproportionation. 2 Cu +   →   Cu 2 +   +   Cu