Commercially available nitric acid is an azeotrope with water at a concentration of 68% . This solution has a boiling temperature of 120.5 °C (249 °F) at 1 atm. It is known as "concentrated nitric acid". The azeotrope of nitric acid and water is a colourless liquid at room temperature.
Two solid hydrates are known: the monohydrate  or oxonium nitrate  and the trihydrate .
An older density scale is occasionally seen, with concentrated nitric acid specified as 42 Baumé.