Bromine at ordinary temperatures is a mobile liquid of fine red colour, which appears almost black in thick layers. It boils at 59° C. According to Sir W. Ramsay and S. Young, bromine, when dried over sulphuric acid, boils at 57.65° C., and when dried over phosphorus pentoxide, boils at 58.85° C. (under a pressure of 755.8 mm.), forming a deep red vapour, which exerts an irritating and directly poisonous action on the respiratory organs. It solidifies at -21° C. (Quincke) to a dark brown solid. Its specific gravity is 3.18828 (0/4°), latent heat of fusion 16.185 calories, latent heat of vaporization 45.6 calories, specific heat 0.1071. The specific heat of bromine vapour, at constant pressure, is 0.05504 and at constant volume is 0.04251 (K. Strecker). Bromine is soluble in water, to the extent of 3.226 grammes of bromine per 100 grammes of solution at 15° C., the solubility being slightly increased by the presence of potassium bromide. The solution is of an orange-red colour, and is quite permanent in the dark, but on exposure to light, gradually becomes colourless, owing to decomposition into hydrobromic acid and oxygen.
By cooling the aqueous solution, hyacinth-red octahedra of a crystalline hydrate of composition Br·4HO or Br·8HO are obtained (Bakhuis Roozeboom, Zeits. phys. Chem., 1888, 2. p. 449). Bromine is readily soluble in chloroform, alcohol and ether.
Its chemical properties are in general intermediate between those of chlorine and iodine; thus it requires the presence of a catalytic agent, or a fairly high temperature, to bring about its union with hydrogen. It does not combine directly with oxygen, nitrogen or carbon. With the other elements it unites to form bromides, often with explosive violence; phosphorus detonates in liquid bromine and inflames in the vapour; iron is occasionally used to absorb bromine vapour, potassium reacts energetically, but sodium requires to be heated to 200° C. The chief use of bromine in analytical chemistry is based upon the oxidizing action of bromine water. Bromine and bromine water both bleach organic colouring matters. The use of bromine in the extraction of gold (q.v.) was proposed by R. Wagner (Dingler's Journal, 218, p. 253) and others, but its cost has restricted its general application. Bromine is used extensively in organic chemistry as a substituting and oxidizing agent and also for the preparation of addition compounds. Reactions in which it is used in the liquid form, in vapour, in solution, and in the presence of the so-called "bromine carriers," have been studied.
Sunlight affects the action of bromine vapour on organic compounds in various ways, sometimes retarding or accelerating the reaction, while in some cases the products are different (J. Schramm, Monatshefte fur Chemie, 1887, 8, p. 101). Some reactions, which are only possible by the aid of nascent bromine, are carried out by using solutions of sodium bromide and bromate, with the amount of sulphuric acid calculated according to the equation 5NaBr + NaBrO + 6HSO = 6NaHSO + 3HO + 6Br. (German Patent, 26642.) The diluents in which bromine is employed are usually ether, chloroform, acetic acid, hydrochloric acid, carbon bisulphide and water, and, less commonly, alcohol, potassium bromide and hydrobromic acid; the excess of bromine being removed by heating, by sulphurous acid or by shaking with mercury. The choice of solvent is important, for the velocity of the reaction and the nature of the product may vary according to the solvent used, thus A. Baeyer and F. Blom found that on brominating orthoacetamido-acetophenone in presence of water or acetic acid, the bromine goes into the benzene nucleus, whilst in chloroform or sulphuric acid or by use of bromine vapour it goes into the side chain as well.
The action of bromine is sometimes accelerated by the use of compounds which behave catalytically, the more important of these substances being iodine, iron, ferric chloride, ferric bromide, aluminium bromide and phosphorus. For oxidizing purposes bromine is generally employed in aqueous and in alkaline solutions, one of its most important applications being by Emil Fischer (Berichte, 1889, 22, p. 362) in his researches on the sugars. The atomic weight of bromine has been determined by J.S. Stas and C. Marignac from the analysis of potassium bromide, and of silver bromide. G.P. Baxter (Zeit. anorg. Chem. 1906, 50, p. 389) determined the ratios Ag: AgBr, and AgCl: Ag Br.