(a) A black precipitate forms when H₂S is added to an aqueous solution of a Cu(II) salt. The precipitate redissolves when Na₂S is added to the solution. Suggest a reason for this observation.

(b) In the presence of small amounts of water, the reaction of SO₂ with CsN₃ leads to Cs₂S₂O₅ as a by-product in the formation of Cs[SO₂N₃]. Suggest how the formation of Cs₂S₂O₅ arises.

(c) The complex ion [Cr(Te₄)₃]³⁻ possesses a Δλλλ-conformation. 

(i) To what the symbols Δ and λ refer, 

(ii) How the Δλλλ-conformation arises.