$ 175. If the ferrocyanide of potassium be fused with carbonate of potassium, it is wholly converted into cyanide of potassium and carbonate of iron, according to the equation The fused cyanide of potassium is poured off from the subsided carbonate of iron upon a cold surface, whereupon it solidifies into a marble-like mass. It may be purified by solution in alcohol. Cyanide of potassium is a powerful poison; it is extensively used in photography. It is employed largely in the laboratory as a "reducing agent," having at high temperatures a great affinity for oxygen, which converts it into the cyanate of potassium, KCyO. § 176. When cyanide of potassium is treated with sulphuric acid, hydrocyanic acid, also called prussic acid, is obtained. 2KCy + H2SO1 = K2SO1 + 2HCy. 4 4 Hydrocyanic acid, HCy, is a transparent gas, which may be condensed by cold or pressure into a transparent liquid. Hydrocyanic acid dissolves very abundantly in water. The acid itself, and all soluble cyanides, are violent poisons, and are used in small doses in medicine. § 177. Cyanogen itself is obtained from the cyanide of mercury by heat. The salt is decomposed into mercury and cyanogen. HgCy, Hg + 20y. = In this decomposition a part of the cyanogen undergoes a change, being converted into "paracyanogen," a body which has the same chemical composition as cyanogen, but different physical properties, being a fixed solid. Cyanogen is a colourless gas of peculiar suffocating smell, and is very poisonous. 100 volumes of water dissolve about 400 volumes of cyanogen. Cyanogen consists of 1 vol. of carbon weighing 12, united to 1 vol. of nitrogen weighing 14, the whole condensed to 1 volume. Its density is therefore 12+14 or 26 (compared with air 1.79). Cyanogen may be burnt in the air, and gives a purple flame; the carbon only burns. CHAPTER IX. CHLORINE AND ITS COMPOUNDS WITH THE PRECEDING ELEMENTS. Symbol, Cl. Equivalent, 35.5. Density, 35.5 (compared with air 2.448). § 178. Chlorine exists abundantly in the sea as chloride of sodium, NaCl (common salt), chloride of magnesium, MgCl, &c., and in a few minerals as chloride of silver, AgCl, &c. Masses of mineral salt, rock-salt, are also found in the earth, having been probably deposited from sea-water. Chlorine is never found in the free state in nature. This is owing to its great affinity for hydrogen and the metals. Its affinity for hydrogen is so great that it will expel oxygen from water in order to combine with its hydrogen. Hence, owing to the universal distribution of water, chlorine is never found in the elementary state. § 179. Chlorine unites with all the known elements (excepting perhaps fluorine, see Chapter XI.), forming chlorides; and it is from some of these chlorides that chlorine itself may be procured. Some metallic chlorides when heated give up the whole or part of their chlorine. Others will bear a red heat without decomposition; other metallic chlorides are volatile without decomposition. $180. The readiest way of preparing chlorine is by removing the hydrogen from the chloride of hydrogen, HCl, or hydrochloric acid. Hydrochloric acid is formed, as we shall see (§ 192), when chloride of sodium, NaCl, and sulphuric acid act upon one another, the products being sulphate of sodium and hydrochloric acid. The HCl dissolves in water and forms what is known as "spirit of salt," "muriatic acid," or hydrochloric acid. If hydrochloric acid be brought into contact with certain bodies containing abundance of oxygen, it loses hydrogen, and the chlorine is liberated. Binoxide of manganese, MnO,, as we have already learnt (§ 76), is such a body ready to give up a part of its oxygen. On warming hydrochloric acid and binoxide of manganese together, the following reaction takes place : 4HCl + MnO2 =2H,O + MnCl, + 2C1. There are therefore formed water, chloride of manganese, and chlorine. § 181. Instead of hydrochloric acid and binoxide of manganese, we may employ a mixture of chloride of sodium, sulphuric acid, and binoxide of manganese, because the first two of these give rise to sulphate of sodium and hydrochloric acid (according to the first equation in §180), which latter acts upon the binoxide of manganese (according to the second equation in § 180). The whole reaction in this case may be represented thus: 2NaCl + MnO2 + 2(H2SO ̧) = MnSO1 + Na2SO1 + 2H ̧O + 2C1. acid. 4 Sulphate of Sulphate of Water. Chlo rine. Chloride of Binoxide of Sulphuric sodium. manganese. In order to remove traces of hydrochloric acid which accompany the gas, the chlorine is made to bubble through a wash-bottle containing water. If it is required dry, it passes through a second similar wash-bottle containing sulphuric acid. Chlorine dissolves in, and gradually decomposes water; and it immediately combines with mercury. It cannot, therefore, be collected over either of these liquids; but being more than twice as heavy as air, it may be collected by downward displacement. § 182. Chlorine may be obtained in the liquid form by subjecting it to great pressure or great cold. At 16° C it becomes liquid under a pressure of 4 atmospheres, and then appears as a yellow liquid of the density 1.3. § 183. The properties of chlorine are very well marked. A greenish-yellow gas of peculiar penetrating smell, it acts injuriously upon the respiratory organs even when largely diluted with air, in a pure state producing instant suffocation. It dissolves readily in water, which at ordinary temperatures takes up between two and three volumes of the gas, producing a solution called chlorinewater. It combines directly and eagerly with most metals, giving rise to chlorides, the formation of which is often attended by light. § 184. When in the free state it is detected by the power it has of expelling iodine from iodides (see §§ 207-208), and by the blue colour which the so liberated iodine imparts to starch. Bromine and some of the oxides of nitrogen, however, enjoy the same property as chlorine in this respect. If chlorine be shaken up with water containing zinc in a finely divided state, the zine unites with the chlorine and forms chloride of zinc, ZnCl ̧, which is soluble in water. If this chloride of zinc be treated with nitrate of silver, AgNO,, nitrate of zinc and chloride of silver are formed. ZnCl2 + 2AgNO, = Zn2NO, + 2AgCl. The latter body, chloride of silver, is a white curdy body, insoluble in water, insoluble in nitric acid, but easily soluble in ammonia. $ 185. Chlorine is largely employed as a bleaching agent. Flowers and most vegetable colours, exposed to the action of chlorine, become white. As a bleaching agent, free chlorine is usually employed as a solution in water. Chlorine is also employed in the manufacture of "bleaching powder" and "disinfecting powder." These important substances are obtained by passing chlorine over lime, magnesia, &c.; and that got when lime is used is called chloride of lime. 2CaO 4C1 = CaCl,, CaCl,0,. Lime. Chlorine. Bleaching powder. The composition of bleaching powder is therefore that of a feebly united double salt, chloride of calcium, CaCl,, united with CaCl2, which, as we shall see immediately (§ 188), is hypochlorite of calcium. The empirical formula of bleaching powder may be, and often is, written CaOCl,, which expresses the same ratio between the elements as the above rational formula. When bleaching powder is exposed to the air, the carbonic acid of the air gradually decomposes the hypochlorite of lime, forming carbonate of calcium, and setting free hypochlorous acid. CaC1,02+ CO2 = CaCO, + C1,0. 2 2 The latter product, hypochlorous acid, Cl2O, destroys many kinds of organic matter (colouring-matter, miasma)-its oxygen forming carbonic acid with the carbon thereof, and the chlorine forming hydrochloric acid with the hydrogen. § 186. All chlorides are soluble in water, excepting AgCl, PbCl, and HgCl. Compounds of Chlorine with Oxygen. § 187. There are three anhydrous oxides of chlorine:— C10, anhydrous hypochlorous acid; C10,, anhydrous chlorous acid; 39 The first two of these form hydrogen acids, as do also two higher oxides; thus:— HCIO, hypochlorous acid; HCIO,, chlorous acid; HCIO,, chloric acid; HCIO,, perchloric acid. Of these five oxides and acids we need only consider (1) C10 and HClO, (2) HClO,, (3) HC1O § 188. Hypochlorous acid, C1,O, is prepared by passing dry chlorine over dry oxide of mercury which is kept cold; thus:Hg04C1 HgCl + C1,0. = Hypochlorous acid is a deep-red liquid, easily decomposed by heat, and then violently exploding. It boils at 24° C and mixes in all proportions with water. A solution of it in water may be easily made by passing chlorine into cold caustic potash (hydrate of potassium), carbonate of potassium, phosphate of potassium, &c. ; thus: KHO+2C1 = KCl + HClO. On gently warming, the HClO passes over, and may be collected in water. 3 § 189. Chloric acid, HC1O,.—If chlorine is passed into a hot concentrated solution of hydrate of potassium, the chlorine combines with the potassium, partly as chloride, partly as chlorate. 6KHO+6C1 = 5KCl + KC10, +3H,0. 3 |