_Phosphorous acid_, PH_{3}O_{3}, is generally obtained from phosphorus trichloride, PCl_{3}, by the action of water: PCl_{3} + 3H_{2}O = 3HCl + PH_{3}O_{3}. Both acids formed are soluble in water, but are easily separated, because hydrochloric acid is volatile whilst phosphorous acid volatilises with difficulty, and if a small amount of water be originally taken the hydrochloric acid nearly all passes off directly. Concentrated solutions of phosphorous acid give crystals of H_{3}PO_{3}, which fuse at 70°, attract moisture from the air, and deliquesce when ignited, giving phosphine and phosphoric acid,[22] and are oxidised into orthophosphoric acid by many oxidising agents. In its salts only two hydrogen atoms are replaced by metals (Würtz); the salts of the alkaline metals are soluble, and give precipitates with salts of the majority of other metals. [22] Phosphorous acid, when subjected to the action of nascent hydrogen (zinc and sulphuric acid), evolves phosphine, and when boiled with an excess of alkali it evolves hydrogen (PH_{3}O_{3} + 3KHO = PK_{3}O_{4} + 2H_{2}O + H_{2}); owing to its liability to oxidation, it is a reducing agent--for instance, it reduces cupric chloride to cuprous chloride, and precipitates silver from the nitrate and mercury from its salts. These reactions are perhaps connected with the fact that in this acid one atom of hydrogen should be considered as in the same condition as in phosphuretted hydrogen, which is expressed by the formula PHO(OH)_{2}, if we represent it as PH_{4}X, with the substitution of two of the hydrogen atoms by oxygen and of HX by two of hydroxyl. The direct passage of phosphorous chloride into phosphorous acid would, however, indicate that all the three atoms of hydrogen in it occur in the form of hydroxyl, because no difference is known between the three atoms of chlorine in PCl_{3}--they all react alike, as a rule. However, Menschutkin, by acting on alcohol, C_{2}H_{5}OH, with phosphorous chloride, obtained hydrochloric acid and a substance P(C_{2}H_{5}O)Cl_{2}, and from it by the action of bromine he obtained ethyl bromide, C_{2}H_{5}Br, and a compound PBrOCl_{2}, which proves, to a certain extent, the existence of a difference between the three atoms of chlorine in phosphorous chloride. If we turn our attention to the formation of phosphine by the ignition of phosphorous acid, we see that 4PH_{3}O_{3} only evolve 3H in the form of PH_{3}, and therefore the residue--that is, 3PH_{3}O_{4}--will still contain one hydrogen of the same nature as in phosphine, because in 4PH_{3}O_{3} we should recognise four such hydrogens as in phosphine. We arrive at the same conclusion by examining the decomposition of hypophosphorous acid, 2PH_{3}O_{2} = PH_{3} + PH_{3}O_{4}.