Reactions of aqua ions
Metal ions in water exist as hexaaqua ions [M(H₂O)₆]ⁿ⁺. 3+ ions are more acidic than 2+: higher charge density polarises the O–H bonds in water ligands, releasing H⁺. With OH⁻ or a little NH₃, ions are deprotonated to neutral hydroxide precipitates: Cu²⁺ blue; Fe²⁺ green (turns brown in air); Fe³⁺ orange-brown; Cr³⁺ grey-green; Al³⁺ white; Mn²⁺ off-white (browns). Excess NaOH dissolves amphoteric Al(OH)₃ and Cr(OH)₃. Excess NH₃ causes ligand exchange: Cu²⁺ gives deep blue [Cu(NH3)4(H2O)2]2+; Cr³⁺ gives purple [Cr(NH3)6]3+. With carbonate, 2+ ions give MCO₃ precipitates; 3+ ions are acidic enough to give the hydroxide + CO₂.
≈ 45 min