Chromium(II), (III) and (VI) compounds
As the oxidation state of chromium rises, the oxides change from basic to acidic: CrO is basic, Cr₂O₃ is amphoteric (a green powder), CrO₃ is acidic (dark red crystals). Cr(OH)₃ is amphoteric: CrCl₃ + 3NaOH → Cr(OH)₃↓ + 3NaCl; Cr(OH)₃ + 3HCl → CrCl₃ + 3H₂O; Cr(OH)₃ + 3NaOH → Na₃[Cr(OH)₆]. CrO₃ with alkali gives a chromate: CrO₃ + 2NaOH → Na₂CrO₄ + H₂O; chromates are yellow, dichromates (e.g. K₂Cr₂O₇) are orange. Cr²⁺ compounds are reducing agents, Cr⁶⁺ compounds strong oxidising agents: in acidic medium dichromate is reduced to Cr³⁺, e.g. 6FeSO₄ + K₂Cr₂O₇ + 7H₂SO₄ → 3Fe₂(SO₄)₃ + Cr₂(SO₄)₃ + K₂SO₄ + 7H₂O (Fe²⁺ → Fe³⁺ gives an electron, Cr⁶⁺ → Cr³⁺ takes electrons). The Ba²⁺ ion gives a yellow precipitate with chromate: Ba²⁺ + CrO₄²⁻ → BaCrO₄↓. Chrome alums of Cr³⁺ are used to tan leather. Chromium(VI) compounds are poisonous and affect the skin, so only the teacher works with them.
Teacher demonstration: NaOH is added drop by drop to CrCl₃ solution; the grey-green Cr(OH)₃ precipitate and its dissolving in excess alkali are observed. In your notebook write the equations for the “acidic and basic properties of Cr(OH)₃”. Experiments with dichromate are done by the teacher in a fume cupboard and gloves.