[24] Fluorine and bromine give PX_{3} and PX_{5}, like chlorine. With respect to iodine PI_{5} is, in a chemical sense, a very unstable substance, and generally _phosphorus tri-iodide_ only is formed (from yellow or red phosphorus and iodine in the requisite proportions). It is a red crystalline substance, fuses at 55°, is easily decomposed by water, forming phosphorous and hydriodic acids, and when heated it evolves iodine vapours and forms phosphorus di-iodide, PI_{2}. This substance may be obtained in the same manner as the preceding by taking a smaller proportion of iodine (8 parts of iodine to 1 part of phosphorus, whilst the tri-iodide requires 12·3); it also forms red crystals, which melt at 110°. When decomposed by water it not only gives phosphorous and hydriodic acids, but also phosphine and a yellow substance (a lower oxide of phosphorus). In its composition di-iodide of phosphorus corresponds with liquid phosphuretted hydrogen, PH_{2}, and probably its molecular weight is much higher: P_{2}I_{4} or P_{3}I_{6}, &c. As the iodine compounds of phosphorus give hydriodic and phosphorous acids with water, and as both these substances are reducing agents in the presence of water (and hydrates), iodide of phosphorus also acts as a reducing agent. Phosphorus burns in chlorine, forming phosphorous chloride, PCl_{3}, and with an excess of chlorine, phosphoric chloride, PCl_{5}. The oxychloride, POCl_{3}, as the simplest chloranhydride according to the type PX_{5}, and also phosphoric chloride, correspond with orthophosphoric acid, PO(OH)_{3}, while phosphorous chloride, PCl_{3}, corresponds with phosphorous acid and the type PX_{3}. Phosphoric oxychloride, POCl_{3}, is a colourless liquid, boiling at 110°. Phosphorus trichloride is also a colourless liquid, boiling at 76°,[25] whilst phosphoric chloride is a solid yellowish substance, which volatilises without melting at about 168°. They are all heavier than water, and form types of the _chloranhydrides_ or chlorine compounds of the non-metallic elements whose hydrates are acids, just as NaCl or BaCl_{2} are types of halogen metallic salts. [25] In a liquid state the density of phosphorous chloride at 10° = 1·597, and therefore its molecular volume = 137·5/1·597 = 86·0, and that of phosphorus oxychloride is equal to 153·5/1·693 = 90·7; hence the addition of oxygen has produced considerable increase in volume, just as in the conversion of sulphur dichloride, SCl_{2}, into sulphuryl chloride, SOCl_{2}, the volume changes from 64 to 71. It is the same with the boiling-points; phosphorus trichloride boils at 70°, the oxychloride at 100°, sulphur dichloride at 64°, and sulphuryl chloride at 78°--that is, the addition of oxygen raises the boiling points.