Oxalic acid is an organic compound with the formula H₂C₂O₄. It is a colorless crystalline solid that forms a colorless solution in water. Its condensed formula is HOOCCOOH, reflecting its classification as the simplest dicarboxylic acid. Its acid strength is much greater than that of acetic acid. Oxalic acid is a reducing agent  and its conjugate base, known as oxalate (C₂O₄^2-), is a chelating agent for metal cations. Typically, oxalic acid occurs as the dihydrate with the formula H₂C₂O₄·2H₂O. Excessive ingestion of oxalic acid or prolonged skin contact can be dangerous.