Oxalic acid is a relatively strong, dibasic carboxylic acid and the simplest dicarboxylic acid molecule. It occurs naturally in many plants, including spinach, rhubarb, and sorrel, often in the form of its salts, the oxalates. In the human body, excessive intake can promote the formation of kidney stones, as oxalic acid forms sparingly soluble salts with calcium. Therefore, in high concentration, it must be classified as harmful to health or even toxic. The acid typically exists as the dihydrate, a white, crystalline solid. Its strong reducing properties make it a useful reagent in chemistry and industry.