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mercury chromate is an orange-red, water-soluble solid. It is a chemical compound of mercury (Hg) and chromate, Cr3+, which forms in the presence of acidic solutions. It is found in the mineral cinnabar and in corderoite and livingstonite ores. In a simple experiment a drop of mercury in a watch glass is covered with a solution of potassium chromate in sulfuric acid. This causes chromate to convert to dichromate, Cr2O72-, which oxidizes the mercury. Oxidation of mercury reduces the surface tension of the mercury and flattens the drop, but when it comes into contact with an iron nail the circuit is closed and electron transfer to the metal reverses the oxidation of mercury, and the drop becomes rounder. This is a classic demonstration of galvanic cell action.