The azeotropic mixture of hydrochloric acid and water contains about 20.2% HCl by mass and boils at 108.6°C. This mixture has a maximum boiling point and a constant composition, and it behaves like a single compound. The azeotropic mixture is formed due to the strong hydrogen bonding between HCl and water molecules, which results in a higher boiling point than either pure HCl or water.