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Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Additional information

Appearance and physical state

Anhydrous hydrogen bromide (HBr) is an irritant, colorless gas. Solutions of HBr in water form hydrobromic acid which dissociates to ions (H+ and Br-) and are clear liquids developing fumes that become brown on exposure to air and light..


Melting point/freezing point

Garlanda, T., and Basilico, S. (1993) reported an experimental melting point for anhydrous HBr of -88.5 ºC. The melting point for a solution of hydrobromic acid in water (69.2%) is -11.3 ºC. This value is supported by O’Neil, M.J. (2001) which records the melting point of hydrogen bromide as -86.9 ºC.

Boiling point

Garlanda, T., and Basilico, S. (1993) reported a boiling point in range of -67.0 to -66.5 ºC for anhydrous HBr. The boiling point for aqueous mixtures at 101.323 hPa (760 mm Hg) pressure is 126 ºC. At 53 hPa and 13 hPa (respectively 400 and 100 mm Hg), the boiling point of aqueous solutions of hydrobromic acid is given as 107 and 74.1 ºC, respectively. O’Neil, M.J. (2001) reports an experimental value of -66.5 ºC at 101.323 hPa (760 mm Hg).



Garlanda, T., and Basilico, S. (1993) report the experimental vapour density for anhydrous HBr (gas) as 2.82 relative to that of air (= 1). The specific gravity for liquid HBr was reported as 2.77 at -67 ºC. The specific gravity of aqueous solutions of hydrobromic acid ranges from 1.08 for a 10 % solution at 20 ºC to 1.70 for a 60 % solution at 20 ºC. O’Neil, M.J. (2001) reports the density of HBr as 3.307 g/L.


Vapour pressure

Garlanda, T., and Basilico, S. (1993) report a vapour pressure range of 133 Pa at -138.8 ºC to 2026500 Pa at 16.8 ºC.


Surface tension

The substance is not expected to have surface tension properties.


Water solubility

Garlanda, T., and Basilico, S. (1993) report HBr as very soluble in water, forming an azeotropic mixture in water of 47.63 % with boiling point of 124.3 ºC at 101323 Pa (760 mm Hg). A saturated solution occurs at 66.5 % at 25 ºC.


Partition coefficient

Hydrobromic acid dissociates into the hydrogen and bromide ions in contact with water. As an inorganic substance HBr is expected to partition to water. A calculated value Log Kow of 0.6287 can be made; however, the KOWWIN model was trained with organic compounds, so the value should be treated with caution.


Flash point/flammability/self-ignition temperature

Hydrogen bromide is an inorganic non-flammable gas. Hydrogen bromide is not flammable in aqueous solution (hydrobromic acid).



Hydrogen bromide is an inorganic substance that does not contain a chemical group associated with explosivity



Hydrogen bromide gas can react violently when mixed with some chemicals, metals or water, forming a flammable, explosive gas. Hydrogen bromide is a reducing agent donating hydrogen. In redox reactions with other inorganic acids it can form bromine which is a strong oxidizing agent.


Stability in organic solvents

Garlanda, T., and Basilico, S., (1993) report anhydrous hydrogen bromide as soluble in alcohol, acetic acid, hexane, benzene, and toluene. Hydrobromic acid is soluble in common solvents also. Stability in organic solvents is not required for inorganic substances.


Dissociation constant

Chou, I.-M., and Eugster, H.P. (1976) report an experimental value of log K = 20.56 at 2.0 x 10-8 Pa (2 kb), 25 ºC with a generally accepted pKa of approximately -9.



Hydrogen bromide (HBr) is a gaseous substance. HBr is very soluble in water, with solutions of HBr in water forming hydrobromic acid which dissociates to ions (H+and Br-).


Critical temperature and pressure

O’Neil, M.J. (2001) reports the critical temperature of HBr as 89.8 ºC and the critical pressure as 8.56 x 106 Pa (84.5 atm).


Henry’s Law Constant

O’Neil, M.J. (2001) reports a Henry's law constant for hydrobromic acid as 1.32 x 109mol2 • kg-2 • atm-1at 25 ºC