11. What is potassium dichromate and sodium dichromate?

Potassium dichromate (K₂Cr₂O₇), commonly known as potassium red alum, is an orange-red monoclinic or needle-like crystal. It has a melting point of 398℃ and a boiling point of 500℃ (decomposes). The substance is water-soluble, forming an acidic solution with corrosive, toxic, and strong oxidizing properties.
Sodium dichromate (Na₂Cr₂O₇·2H₂O), commonly known as sodium red alum, exists as red monoclinic or needle-like crystals. It has a melting point of 320℃ and a boiling point of 600℃ (decomposes). At 100℃, it loses its water of crystallization to form anhydrous sodium dichromate, which melts at 356.7℃ and decomposes at 400℃, releasing oxygen. This highly hygroscopic compound is extremely soluble in water, forming a corrosive and toxic solution. Its aqueous solution exhibits acidic properties and strong oxidizing capacity.