The most important compounds of iron and their uses
Iron forms three oxides: FeO (black, basic), Fe₂O₃ (red-brown, mainly basic, very weakly amphoteric) and the mixed oxide Fe₃O₄ (FeO·Fe₂O₃, magnetite). Fe²⁺ salts are made by treating iron with acid: Fe + 2HCl → FeCl₂ + H₂↑; Fe³⁺ salts by oxidising iron with Cl₂ or concentrated acids: 2Fe + 3Cl₂ → 2FeCl₃. Hydroxides precipitate with alkali: FeCl₂ + 2NaOH → Fe(OH)₂↓ + 2NaCl (pale green/white, quickly turning brown in air: 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃) and FeCl₃ + 3NaOH → Fe(OH)₃↓ + 3NaCl (brown); on heating 2Fe(OH)₃ → Fe₂O₃ + 3H₂O. Fe(OH)₂ is basic and Fe(OH)₃ mainly basic (very weakly amphoteric); both dissolve in acids: Fe(OH)₃ + 3HCl → FeCl₃ + 3H₂O. Fe³⁺ can be reduced to Fe²⁺ with iron: 2FeCl₃ + Fe → 3FeCl₂. Qualitative reactions: Fe³⁺ + yellow prussiate K₄[Fe(CN)₆] → dark blue precipitate (Prussian blue); Fe³⁺ + thiocyanate (KNCS) → blood-red Fe(NCS)₃ solution; Fe²⁺ + red prussiate K₃[Fe(CN)₆] → blue precipitate (Turnbull's blue). Iron vitriol FeSO₄·7H₂O is used against agricultural pests and for making pigments; FeCl₃ and Fe₂(SO₄)₃ for water purification and in the textile industry. Iron is part of haemoglobin, which carries oxygen in the blood, so enough iron should come from food (meat, liver, pomegranate, vegetables); take iron preparations only on a doctor's advice.
Teacher demonstrations: FeCl₃ + KNCS (blood-red colour), FeCl₃ + NaOH (brown precipitate), FeSO₄ + NaOH (colour change of the precipitate). You make an observation table: “Ion”, “Reagent”, “Observed change”. Only the teacher works with cyanide complexes and thiocyanates; never mix them with acids.