The chemical compound nitric acid (HNO3), also known as aqua fortis and spirit of nitre, is an aqueous solution of hydrogen nitrate (anhydrous nitric acid). It is a highly corrosive and toxic acid that can cause severe burns. Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming nitric acid. Fuming nitric acid is characterized as white fuming nitric acid and red fuming nitric acid, depending on the amount of nitrogen dioxide present.