Transition elements form a large number of compounds due to their variable oxidation states and ability to form complexes. Some important compounds are:
1. Ferrous Sulphate (Green Vitriol), FeSO4.7H2O
- Anhydrous FeSO₄ → White, Hydrated FeSO₄ (FeSO₄·7H₂O) → Green
- Shows isomorphism with Magnesium sulfate heptahydrate (Epsom salt) and Zinc sulfate heptahydrate
- Gets oxidized in air (turns brown due to Fe³⁺ formation)
- Forms brown complex nitroso ferrous sulphate (Fe(NO)SO₄)
- Forms double salts with alkali sulfates , with ammonium sulfate forms Mohr’s salt: FeSO₄·(NH₄)₂SO₄·6H₂O
2. Ferric Oxide, Fe2O3
The anhydrous salt is a yellow, deliquescent substance that is very soluble in water. When heated, it produces FeCl2 and Cl2. Because of hydrolysis, its aqueous solution is acidic.
3. Silver Nitrate, AgNO3
With various salt solutions, silver nitrate precipitates, which aids in the detection of acid radicals. It decomposes when heated.
4. Mercury(I) Chloride (Hg2Cl2)
It is a white powder that is soluble in chlorine water but not in water. It turns black when treated with ammonia due to the creation of finely split mercury.
5. Mercury (II) Chloride (HgCl2)
It is a white crystalline substance that is only sparingly soluble in cold water but completely soluble in hot water. By adding Cl, its solubility can be increased. It is converted to mercury when handled with SnCl2.
6. Mercury-II Iodide
Mercuric iodide comes in two colors: red and yellow. Above 400 K, the yellow form is stable, while the red form is stable below this temperature. Nessler's reagent is an alkaline solution of K2HgI4 that is used to detect the presence of NH4+ by producing a brown precipitate due to the creation of iodide of Million's base.
7. Copper(II) Sulphate Pentahydrate or Blue Vitriol (CuSO4.5H2O)
It has 5 water molecules of crystallization, all of which can be eliminated by heating to yield colorless CuSO4. When heated at a high temperature, it produces cupric oxide. Iodine is liberated from soluble iodides. It is used as an electrolyte in copper electroplating, electrotyping, and refining. It is used to keep weeds at bay in reservoirs and swimming pools.
8. Potassium Dichromate (K2Cr2O7)
Because sodium and potassium dichromates are strong oxidizers, acidified K2Cr2O7 will oxidize iodides to iodine, sulfides to sulfur, tin (II) salts to tin (IV), and iron (II) salts to iron (III). In volumetric analysis, it is employed as an oxidizing agent. It is also used in mordant dyes, the leather industry, photography (for film hardening), and chromyl chloride tests. It's used to clean glassware.
