Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/537

Rh AND ALKALINE EARTHS.] CHEMISTRY 525 2Xa.OH. thrown on water, and The metals of this group all decompose water m the cold with evolution of hydrogen and production of a solution of the hydroxide : for example 2Na + 20H 2 = H 2 Lithium does not fuse when sodium fuses but does not inflame ; potassium and rubidium, however, not only fuse but also take fire and burn with a violet coloured flame. According to Thomsen s determina tions, more heat is developed in the decomposition of water ty lithium or sodium than by potassium ; thus (Li, Aq) =49,080 units of heat. (Na, Aq) = 48,100 (K, Aq) =43,450 ,, The difference in their behaviour when thrown on water is, perhaps, due to the circumstance that the action proceeds more rapidly with potassium than with sodium. All the metals of this group attract oxygen with avidity on exposure to air, but lithium is much less oxidizable than sodium or potassium, rubidium takes fire spontaneously in air, and caesium amalgam is much more readily oxidized than rubidium amalgam ; so that their affinity for oxygen, as for the halogens, is proportional to their atomic weights. Only one oxide of lithium, Li 2 O, has been obtained, but two oxides of sodium, Na. 2 0, and Na 2 O 2, and three oxides of potassium, K 2 0, K 2 O 2, and K 2 O 4 , are known ; the oxides of the remaining elements have not been investigated. The oxides of the form M 2 O are white deliquescent solids, which readily dissolve in water with the development cf much heat, forming solutions of the corresponding hydroxides, M .OH ; they are the most powerfully basic oxides known. The oxides of the form M 2 2 dissolve in water, but corresponding salts are unknown ; potassium tetroxide dissolves in water with evolution of oxygen. The hydroxides M .OH, or alkalies, are white fusible solids of remarkable stability, volatilizing at high temperatures apparently without undergoing decomposition; they are very soluble in water, furnishing strongly alkaline solutions, soapy to the touch. The metals of the alkalies appear to act uniformly as monads, and the salts derived from them are all formed on the types M Cl, M 2 SO 4, &c. Their salts are colourless excepting those derived from coloured acids ; and the cor responding salts of the several metals are isomorphous. Their chlorides, carbonates, sulphates, and phosphates are soluble in water, and the chlorides and sulphates furnish neutral solutions. Their sulphates form characteristic double salts called &quot; alums&quot; with sulphate of aluminium and the sulphates of allied elements. The alums all crystallize with the same number of molecules of water, and correspond in composition to ordinary alum, K 2 A1 2 (SC&amp;gt; 4 ) 4, 24H 2 0; they are also isomorphous. Although the general resemblance between the metals of this group is very great, it will be obvious that they differ in many important respects. The properties of potassium, rubidium, and caesium, which are very closely related, are especially different from those of lithium and sodium, which are also closely related. For instance, the normal lithium and sodium salts of chloroplatinic acid, H 2 PtCl 6 , the acid lithium and sodium salts of tartaric acid, H .C 4 H 4 O 6, and the lithium and sodium alums are readily soluble in water, whereas the corresponding salts of potas sium, rubidium, and caesium are difficultly soluble. Lithium, however, in many of its properties is more closely allied to magnesium and calcium than to sodium ; and it may be noticed that its chloride, like that of magnesium, dissolves in water with considerable development of heat. The reactions involved in the formation of a few com pounds of lithium, sodium, and potassium have been inves tigated by Thomsen, with the following results : Reaction. R=LL R=Na. R=K. i R,C1 93,810 97,690 105,610 1 ) Formation of the R, Br 85,730 95,310 | &amp;gt; crystalline salts R,I 69,080 80,130 ; ) from their elements. RC1, Aq 8,440 -1,180 -4,440 Heat developed on RBr, Aq -150 - 5,080 dissolving the crys RI, Aq 1,220 -5,110 talline salts. R, Cl , Aq K , Br , Aq 102J250 96,510 85,580 101,170 90,230 Formation in aqueous Soluti rm R, I , Aq 70,300 75,020 R,0, H 102,030 104,000 Solid hydroxide. ROM, Aq 9,780 12,460, Heat of dissolution. R,0,H,Aq 117,440 111,810 116,460 R,, , Aq 166,520 155,260 164,560 Formation in aqueous R,S,H,A q 60,450 65,100 solution. R a ,0,S0 3 Aq 197,810 186,640 195,850 METALS OF THE ALKALINE EARTHS. Electric Name. Symbol. At. TTt. Sp. gr. At. vol. conductivity 1 at 20 C. Calcium Ca 39-9 1-57 25-4 22-14 Strontium Sr 87-2 2-50 34-8 6-71 Barium Ba 136-8 ?4 34-2 The members of this group are strongly positive elements, which act uniformly as dyads ; the relations between them are similar to those which obtain in the preceding group, the element with the highest atomic weight being the most positive. Strontium, however, in some respects appears to be more closely related to barium than to calcium. They are best prepared by the voltaic decomposition of their fused chlorides ; their chlorides are not reduced by sodium, but calcium may be obtained by heating calcium iodide with sodium. Calcium is a yellow metal of the colour of gold largely alloyed with silver ; in hardness it is intermediate between lead and gold, and it is very ductile. It melts at a red heat. Strontium closely resembles calcium, but has a deeper colour ; little is known of barium, but it appears to resemble strontium and calcium. The behaviour of these elements with halogens is apparently similar to that of the metals of the preceding group with low atomic weights. They decompose cold water with evolution of hydrogen, but less readily than the alkali metals ; it is remarkable that concentrated nitric acid is almost without action on calcium and strontium even when heated to boiling, although the diluted acid dissolves them rapidly. They are less easily oxidized than the alkali metals, calcium being the least, and barium apparently the most oxidizable; in dry air calcium and strontium remain untarnished for a considerable period. They burn with great brilliancy when ignited in air. Each metal furnishes two oxides, a monoxide such as barium oxide, BaO, and a dioxide such as barium peroxide, BaO 2 ; these oxides are white, earthy, infusible substances ; the dioxides are decomposed into the monoxides and oxygen at a red heat. Their monoxides are powerfully basic, but apparently the dioxides are almost destitute of basic properties ; the action of acids on the two classes of oxides is illustrated by the following equations : BaO BaO 2 + 2HC1 = BaCl 2 + H 2 O 2. Barium monoxide is converted into the dioxide when heated to dull redness in an atmosphere of oxygen, but the dioxides of strontium and calcium cannot be prepared in this manner. By the action of water the monoxides are converted with developmentof much heat into the correspondinghydroxides, which are strongly alkaline. Barium hydroxide, Ba(OH) 2 , is dissolved by water, although to a less extent than the