Question

Cu⁺ is colourless while Cu²⁺ is coloured in aqueous solution.

1. Is it true? 

2. What are the reasons for this?

Answer 1

1. Yes.

2. In Cu⁺ it is 3d¹⁰,4s⁰ and there is no d-d transition. Hence no colour. But in Cu²⁺ it is 3d⁹,4s⁰ it has d-d transition and hence it has colour.