Chromate-dichromate equilibrium: 2$CrO4^{2-}$(yellow) + 2$H^+$ ⇌ $Cr2O7^{2-}$(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: $Cr2O7^{2-}$ + 14$H^+$ + 6$e^-$ → 2$Cr^{3+}$(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.