155 156 (HIO) is known as a solid body; when heated it decomposes into iodine pentoxide (I,O,) and water. Compounds with non-metallic elements other than hydrogen or oxygen. Chlorine, bromine, and iodine, combine directly with many non-metallic elements. The compositions of these compounds can generally be expressed by formulae in which X represents one combining weight of any one of the three elements. The following table shews the compositions of some of the best known of these compounds. with arsenic; AsX, where X=Cl or Br only. antimony; Sbx: also SbCl5. boron; BX, where X-Cl or Br only. phosphorus; PX,: also PX, where X-Cl or Br only: also P2I4. selenion; SeX, also SeX: also SeX, where X = Cl or Br only. silicon; SiX, where X = Cl or I only. sulphur; SX2. tellurium; TeX2: also TeX. A few other compounds of chlorine, bromine, and iodine with non-metallic elements are formed indirectly; e.g. CX,, CX, CX, SiBr, Si,Cl, &c. 2 Compounds with metallic elements. The elements we are considering combine with most metals to form compounds with similar compositions. The compositions of some of these are represented in the following formulae NaCl, KBr, KI; CdX, CaX, BaX, ZnX; BiX,; Cr ̧X ̧; These compounds are usually produced by heating the metal in contact with chlorine, bromine, or iodine; in some cases however it is necessary to use indirect methods of preparation. The binary compounds of the three elements we are considering are usually called haloid compounds (i.e. compounds resembling common salt, NaCl); the elements themselves are often called halogens. Many of the haloid compounds of the non-metals other than hydrogen and oxygen, and some of those of the metals, can be gasified without decomposition. Most of the non-metallic haloid compounds interact with water to produce solutions of HX (where X = Cl, Br, or I), and generally either an oxide or an oxygen-containing acid of the non-metal formerly combined with halogen. The following equations represent some of these changes : 3 2 3 3 2AsCl ̧ + 3H ̧0 + Aq = As ̧О ̧Aq + 6HClAq. 22 2 2 3 3 In some cases the products of the interaction of a nonmetallic haloid compound and water are HX and a compound of the non-metal with oxygen and halogen; thus SbI ̧ + H2O + Aq = SbOI + 2HIAq. 2 Most of the haloid compounds of the metallic elements are chemically unchanged when brought into contact with water; several dissolve in water. In some cases however chemical change occurs; the usual products are haloid compounds of hydrogen (HX) and an oxychloride, oxybromide, or oxyiodide, of the metal :-—thus, BiCl ̧ + 2H ̧O + Aq = BiOCl + 2HClAq; 3 2SnCl ̧ + H2O + Aq = Sn ̧OCI ̧ + 2HClAq. Interactions with water. The three elements dissolve in 157 water, chlorine very freely, bromine less freely, and iodine only in small quantities. By cooling aqueous solutions of chlorine or bromine crystals separate having the composition Cl. 5H ̧O and Br. 5H O respectively: no hydrate of iodine-i.e. compound of iodine with water-has been obtained. Aqueous solutions of the three elements contain small quantities of hydrochloric, hydrobromic, and hydriodic acids, respectively; i.e. the water and chlorine &c. interact as shewn by the equation This reaction proceeds more rapidly when X = Cl than when X = Br. When XI but very little reaction occurs. These reactions are hastened by sunlight. If some easily oxidised substance is dissolved in water and chlorine is passed into the liquid the substance is usually oxidised; thus a solution of sulphur dioxide reacts with chlorine to produce sulphur trioxide, a solution of phosphorous oxide reacts with chlorine to produce phosphoric oxide:-or, in equations (1) SO2Aq+H2O + 2C1 = 2HC1Aq + SO„Aq. 2 2 (2) P2O,Aq+2H2O + 4C1 = 4HClAq + P2O2Aq. 3 The bleaching action of chlorine depends upon its interacting with water to produce oxygen. Dry chlorine does not bleach a piece of madder-dyed cloth; but if water is present the cloth is bleached. The colourless bodies produced are the results of the interaction of oxygen with the colouring matter of the cloth; this oxygen is produced from the water by interaction with chlorine as already described. An aqueous solution of bromine bleaches more slowly than a solution of chlorine, and a solution of iodine bleaches very slowly indeed : the bleaching action is more or less rapid according as the element decomposes water rapidly or slowly (v. supra). 158 Interactions with solutions of alkalis. Chlorine, bromine, and iodine interact with cold aqueous solutions of caustic potash, soda, &c. to produce potassium or sodium (&c.) chloride, bromide, or iodide, and also potassium (&c.) hypochlorite, hypobromite, or (probably) hypoiodite. Thus in equations (X= Cl, Br, or I) 6KOHAq + 6X = 3KXAq + 3KXOAq + 3H2O. The interaction which occurs between one of the halogens and a hot solution of caustic potash, soda, &c. is expressed thus: : 6KOHAq+6X=5KXAq + KX0 ̧Aq + 3H ̧0. The products are potassium (&c.) chloride (bromide or iodide), potassium (&c.) chlorate (bromate or iodate), and water. Solutions in water of potassium, sodium, (&c.) hypochlorite or hypobromite are changed by heat into potassium (&c.) chloride or bromide, and chlorate or bromate, thus 3KCIOAq (heated) = 2KClAq + KClO2Aq. If an easily oxidised substance is present it is oxidised and only potassium chloride or bromide is produced. From these facts it follows that if an easily oxidised substance is dissolved in an aqueous solution of caustic potash, the solution is heated and chlorine is passed in, oxidation ought to occur. Experiment shews that this conclusion is correct; experiment further shews that an element or compound which is not soluble in aqueous caustic potash may often be oxidised by suspending it in hot potash solution and passing in chlorine. Examples of such interactions are these : (1) SeO2+2KOHAq + 2C1 = SeO2Aq + 2KClAq + H ̧O, (2) Bi20, +4KOHAq + 4Cl = Bi2O ̧ + 4KClAq + 2H2O, (3) MnSO,Aq + 2KOHAq + 2C1 3 = 5 · MnO ̧ + K2SO ̧Aq + 2HClAq, 4 (or this reaction may be thus expressed, MnOSO,Aq+2KOHAq + 2Cl = MnO ̧ + K ̧SO,Aq + 2HClAq). 2 Interactions between one of the halogens and binary com- 159 pounds of the others. Chlorine reacts with most bromides to form a chloride and bromine; bromine generally reacts with iodides to form iodine and a bromide. These changes occur most readily when the aqueous solutions are employed; thus NaBrAq+Cl = NaCl Aq + Br; CaBr, Aq + 2C1 = CaCl ̧Aq + 2Br, 2 Hence it follows that an aqueous solution of an iodide will be decomposed by chlorine; e.g. NaIAq+Cl = NaClAq + I. The chemical changes described in the foregoing paragraphs shew that the three elements chlorine, bromine, and iodine, are chemically similar. They are produced from similar compounds under similar conditions. They combine with the same elements to form compounds similar in composition and in properties. The reactions described also shew that chlorine bromine and iodine are markedly negative or non-metallic elements; their oxides are acidic; they decompose water to produce oxygen and, in each case, a hydride; they form numerous compounds with oxygen and another element; none of them interacts with acids to produce salts. The facts we have learned concerning the three elements also shew a gradation of properties from chlorine to iodine, and exhibit a connexion between this gradation and the combining weights of the three elements. As the combining weight increases the elements become heavier, darker in colour, and more solid; the oxides and oxygen compounds generally become more stable, and the hydrides become less stable, as regards the action of heat; the rate at which water is decomposed decreases. The binary compounds of the element with largest combining weight are generally decomposed by the other elements of the group. The elements lithium, sodium, potassium, rubidium, and 160 caesium, form a group or family. Let us briefly consider their properties. Lithium. Sodium. Potassium. Rubidium. Caesium. 161 162 163 and Cs. Occurrence. None of these elements is found in nature uncombined with others. Nitrates, chlorides, silicates, and some other compounds of sodium and potassium, occur in large quantities in rocks and mineral waters. Silicates and phosphates &c. of lithium and rubidium are very widely distributed but occur only in very small quantities; caesium compounds are found in very minute quantities in several rocks and mineral waters. Preparation. Sodium, potassium, and rubidium, are prepared by heating a mixture of their carbonates (M.CO; M = Na, K, or Rb) and carbon to a high temperature. chemical changes may be thus represented : M ̧CO2+2C = 2M + 3CO. The Lithium is prepared by passing an electric current through fused lithium chloride (LiCl) mixed with ammonium chloride; and caesium by electrolysing fused caesium-barium cyanide [CsCN.Ba(CN). Chemical properties. These five elements are very easily oxidised; when exposed to air at ordinary temperatures the surface of the element at once becomes covered with a film of oxide. They decompose cold water rapidly with formation of hydrogen and a compound of oxygen, hydrogen, and the element; thus M+H2O = MOH + H (M = Li, Na, K, Rb, or Cs). The compound MOH-called a hydroxide-dissolves in |