Copper sulphate and alum crystals. Copper (II) sulphate (blue,CuSO4) is a chemical compound of copper and sulphate ions. The copper is in its +2 (II) oxidation state. The blue colour is due to the formation of a hydrated complex of the copper ion. Potash alum,or potassium alum (white,KAl[SO4]2) is a chemical compound of potassium,aluminium and sulphate ions. Chrome alum,or chromium alum (dark purple,KCr[SO4]2) is a chemical compound of potassium,chromium and sulphate ions. Chromium and copper are transition metal elements,and form coloured compounds,whereas potassium and aluminium usually do not,so potash alum is colourless |