Chromate-dichromate equilibrium: 2CrO4^2-(yellow) + 2H^+ ⇌ Cr2O7^2-(orange) + H2O. Acid favours dichromate; base favours chromate. Preparation: Chrome iron ore (FeCr2O4) fused with Na2CO3 + air → Na2CrO4, then acidified → Na2Cr2O7, then KCl → K2Cr2O7 (less soluble, crystallises out). Oxidising reactions in acid: Cr2O7^2- + 14H^+ + 6e^- → 2Cr^3+(green) + 7H2O. Oxidises: Fe^2+ → Fe^3+, I^- → I2, H2S → S, SO3^2- → SO4^2-, Sn^2+ → Sn^4+. Breathalyser test: Orange K2Cr2O7 turns green with ethanol (CH3CHO produced). Used in leather tanning and as primary standard in volumetric analysis.