Methylene blue (CI 52015) is a heterocyclic aromatic chemical compound with the chemical formula C₁₆H₁₈N₃SCl. It has many uses in biology and chemistry; for example, it can be used as a stain and as a pharmaceutical drug. At room temperature it appears as a solid, odorless, dark green powder, that yields a blue solution when dissolved in water. The hydrated form has 3 molecules of water per unit of methylene blue. Methylene blue should not be confused with methyl blue, another histology stain, new methylene blue, nor with the methyl violets often used as pH indicators.