chromium chemistry
Chromium gets its name from the Greek for color, and it earns it: chromium compounds are the green of emeralds and bottle glass, the red of rubies, the brilliant yellow of old road-marking paint, and the orange of dichromate. The shiny mirror-finish on car trim and taps is chromium plating. Behind this rainbow lies a metal that lives mainly in two oxidation states, +3 and +6, and the contrast between them tells you a lot about how the early-middle transition metals behave.
Chromium's stable everyday state in water is chromium(III). The hydrated ion [Cr(H2O)6]3+ is violet, its chloride salts often green, and chromium(III) forms vast numbers of stable, slow-to-react (kinetically inert) octahedral complexes — its d3 configuration is especially stable in an octahedral field. The high state, chromium(VI), exists only bound to oxygen, as the yellow chromate ion (CrO4 2-) in alkali and the orange dichromate ion (Cr2O7 2-) in acid, the two interconverting as you change the pH. Chromium(VI) is a powerful oxidizing agent, eagerly dropping to green chromium(III) while it oxidizes something else — which is exactly why it was a classic laboratory oxidant. Chromium(II), the blue 'chromous' ion, also exists but is a strong reducing agent, quickly air-oxidized back up to chromium(III).
Chromium matters industrially and biologically. Adding chromium to steel makes stainless steel, because a thin invisible layer of chromium(III) oxide passivates the surface and stops rust — the same self-healing oxide film that makes chromium plating protective. But chromium(VI) compounds are toxic and carcinogenic, a serious hazard in tanning, plating and pigments, so its use is now tightly regulated. The clean lesson of chromium is the sharp difference between a stable, benign low state (+3) and an aggressive, oxidizing, hazardous high state (+6) of the very same element.
Dichromate's pH-driven equilibrium is a textbook sight: in acid the solution is orange (Cr2O7 2-), but add alkali and it turns yellow (2 CrO4 2-) as the equilibrium Cr2O7 2- + H2O is in balance with 2 CrO4 2- + 2 H+ shifts. Add a reducing agent in acid and the orange collapses to the green of chromium(III), as Cr(VI) is reduced to Cr(III).
Orange dichromate (acid) and yellow chromate (alkali) interconvert with pH; reducing Cr(VI) gives green Cr(III).
Chromium(III) and chromium(VI) are dramatically different in toxicity: trace chromium(III) is relatively benign, while chromium(VI) compounds are toxic and carcinogenic. The element alone does not tell you the hazard — the oxidation state does.