Reaction of Copper, Cu2+
Cu2+(aq) (pale blue) 1ons can be obtained b
Dissolving a solub1e copper (II) salt in water, for example solid copper (II) sulphate (pale blue).
Dissolving black CuO in an acid such as sulphuric acid.
| Test | Observation | Notes |
|---|---|---|
| (a) Add dil HCl and warm | Solution turns form light blue to yellow-green | warming drives out some water and thus increases the concentration of Cl ligands, resulting in the formation of yellow complex. Ggreenish-yellow is caused by mixing of blue of Cu2+ and yellow of [CuCl4]2 |
| Add conc HCl | Solution turns from light blue to yellowish-green | Chlorine ligands replace water ligands from the aqua complex |
| Add NaOH(aq) | Pale blue ppt formed, insoluble in exess NaOH | Precipitation of Cu2+ by OH– ions |
|
Add KI(aq) followed by;
(i) Na2S2O3
(ii) Starch solution |
Solution turns pale blue to brown. white ppt is formed. Brown colour disappears, leaving a white solid
blue black colour appears |
2Cu2+(aq) + 4I– → 2CuI(s) + I2(aq). CuI(white ppt) Brown iodine is reduced to colourless iodide by sodium thiosulphate
This is a test to confirm the presence of iodine |
| Add NH3(aq) | A blue ppt is formed soluble in xs ammonia to give a deep blue solution | Cu(OH)2(s) is first precipitated. The ppt dissolves in xs ammonia to form a deep blue complex |