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M+H2O + Aq = MOHAq + H (all taken in grams),
M Li; 49,084 gram-units of heat produced,
Interactions with acids. The alkali metals interact with acids to produce salts, hydrogen is generally evolved; thus M + HClAq = MClAq + H; 2M + H2SO,Aq = M2SO,Aq + 2H ; 3M + H PO Aq= M PO Aq + 3H ; M + HCO,Åq = MCO, Ảq+H; M + HỌC,O,Aq= MH,C,O,Aq +H; M+ HClOAq= MCIOAq + H.
Very many of the salts of the alkali metals are soluble in water; several of the lithium salts are less soluble than those of the other metals of the group. The sulphates (M,SO), with the exception of lithium sulphate, combine with sulphate of aluminium to form double salts called alums, the composition of which is M.SO,. A1,3SO,.24H,O.
169 The alkali metals are evidently very similar in their properties. They are all light, soft, very easily oxidised, very positive, elements; all combine with oxygen to form oxides, which dissolve in water with production of alkaline hydroxides MOH. Their oxides and hydroxides are strongly basic; and are unchanged by the action of heat. None of these elements combines directly or indirectly with hydrogen. They all interact with acids to form salts having similar compositions and similar properties. As the combining weights of the alkali metals increase the metals become heavier (sodium and potassium are exceptions), more positive, more easily oxidised, and more easily melted.
The halogen elements, which have been already considered, differ in the most marked way from the alkali metals; some of the more prominent differences are presented in the following table.
Halogens. Chlorine, Bromine,
One gaseous; one liquid; one
Form compounds with hydrogen, HX, which dissolve easily in water producing strongly acid liquids.
Alkali metals. Lithium, Sodium,
More electro-positive than any other elements.
Do not combine with hydrogen.
The halogens may be taken as typical electro-negative or non-metallic elements; and the alkali metals, so far as chemical properties are concerned, as typical electro-positive or metallic elements.
We shall now consider a group of elements which on the 170 whole are non-metallic and negative but in some respects shew analogies with the more positive or metallic elements. This group comprises the three elements sulphur, selenion, and tellurium.
ductor of elec-
solid; one form
The three elements are found uncombined 171 with others in nature; sulphur in large quantities in volcanic districts, the others only in small quantities. Very many compounds of sulphur, chiefly with metals, are found as minerals; certain compounds of selenion and tellurium with metals (copper, bismuth, &c.) also occur, but only in small quantities.
Preparation. Sulphur is obtained by purifying native sulphur, or by roasting sulphide of iron or copper out of contact with air. Selenion and tellurium are prepared by tedious and indirect methods which cannot advantageously be considered here.
Chemical properties. The three elements combine directly with hydrogen, oxygen, the halogens, and many other elements. The hydrides are feebly acidic; the oxides, as a class, are acidic; many oxyhaloid compounds are known; the elements do not interact with acids to form salts. The compositions of the more important compounds are expressed by the following formulae, where M = S, Se, or Te:-MH,, MO, MO, MOH, MOH„, M ̧X, MX, (X = Cl, Br, I), MOX, MOX,, M ̧0,X, Sulphur and selenion exhibit differences in physical, and to some extent also in chemical, properties, according to the conditions under which they are prepared.
Ordinary native sulphur crystallises in rhombic octahedral forms. If a quantity of sulphur is melted in a crucible, and allowed to cool slowly until a crust forms on the surface, and if holes are then pierced in this crust and the still molten sulphur is poured out, it is found that the sulphur remaining in the crucible has crystallised in monoclinic prisms. These however soon change to octahedral forms. If sulphur is melted, heated to 400° or so, and then suddenly cooled, by pouring into cold water, a semi-pasty, soft, plastic, solid, having properties somewhat like caoutchouc, is formed: this soft solid is sulphur. Plastic sulphur soon becomes brittle and crystalline. If an acid is added to an aqueous solution of potassium tetra- or penta-sulphide (K,S, or KS)—obtained by fusing solid potash with sulphur-sulphur is precipitated in the form of a white amorphous solid. If an acid is added to a warm aqueous solution of sodium thiosulphate (Na,S,O,) a yellow solid is produced which is also amorphous sulphur. These five sulphurs differ in specific gravity, solubility in carbon disulphide, &c.; the more prominent differences are presented in the following table.
Insoluble in carbon disulphide.
2. Plastic. Sp. grav. 1·95.
The insoluble varieties of sulphur have not been obtained wholly free from the soluble. If ordinary sulphur, which is wholly soluble in carbon disulphide, is heated to 130° or 140° and quickly cooled no insoluble sulphur is produced; at 170° a considerable quantity of the insoluble variety is formed, but not much more at 230°. According to Berthelot, the soluble varieties of sulphur are electro-negative to the insoluble varieties.
Selenion may be obtained by passing sulphur dioxide (SO2) into an aqueous solution of selenious acid (H,SeO). If the reddish black solid thus produced is melted and allowed to cool, a black, amorphous, lustrous, solid, with a fracture like glass, is obtained: if this is heated to 95°-100° the temperature suddenly rises to 200° or 230°, and a reddish-grey, metal-like, crystalline, somewhat malleable, solid is obtained. The crystalline variety of selenion is about 4.8 times, and the amorphous glassy variety about 4.3 times, heavier than water. Other varieties of selenion appear to exist. Amorphous selenion is a non-conductor of electricity: crystalline selenion conducts electricity; the conductivity increases as the temperature rises, and also increases very markedly when the selenion is exposed to sunlight. The crystalline variety is usually called metallic selenion.
Tellurium does not exhibit any changes of properties analogous to the change of one variety of sulphur, or selenion, into the other varieties. Tellurium is a very bad conductor of electricity, but the conductivity is slightly increased by raising the temperature considerably and also by exposure to sunlight.
The specific gravities of sulphur and selenion as gases vary with the temperature; the specific gravity of tellurium gas is constant. Thus ;
Specific gravities of gases; (air = 1).
Temp. 450°-500° 700° 850° 1100° 1160° 1420° 1390°-1440° Sulphur. 6.6-6.9 2.8 2.4 2.1 2.1
The proof that each of the five varieties of sulphur, and each of the two varieties of selenion, is composed of
M. E. C.
only sulphur, or only selenion, consists in the facts-(1) that a given mass of one variety is changeable into exactly the same mass of another variety; (2) that equal masses of the different varieties when oxidised produce each exactly the same mass of the same oxide. Thus when 1 gram of any variety of sulphur is wholly burnt in oxygen, 2 grams of sulphur dioxide (SO) are produced. The change of a specified mass of any variety of sulphur other than the octahedral into the same mass of the octahedral variety is accompanied by the production of heat; the change of a specified mass of amorphous selenion into the same mass of crystalline selenion is accompanied by the production of heat; therefore there is less energy in a specified mass of octahedral sulphur, or crystalline selenion, than in the same mass of any other variety of sulphur, or selenion.*
Sulphur and selenion are said to exhibit allotropy; the change from one variety of sulphur, or selenion, to another is called an allotropic change. Tellurium does not exhibit allotropy.
We shall consider allotropic changes more fully hereafter; meanwhile the student should compare what has been said regarding the existence of more than one variety of the same element with what he learned about the composition of elements in chap. II. par. 32.
Compounds of sulphur, selenion, and tellurium, with hydrogen. MH2. These compounds are gases under ordinary conditions; they have all an extremely powerful and disagreeable smell, and are very poisonous. They are obtained (1) by passing hydrogen over the molten elements, (2) by the interaction of acids in aqueous solution on various metallic sulphides, selenides, or tellurides; thus
ZnM + H2SO2Aq= ZnSO ̧Aq + MH ̧.
The hydrides MH, dissolve in water; the solutions of the sulphur and selenion compounds redden litmus and react with alkalis as acids do; thus
H,SAq + KOHAq = KHSAq + H2O;
KHSAq + KOHAq = K ̧SAq + H2O.
An aqueous solution of tellurium hydride does not exhibit acidic functions.
The aqueous solutions absorb oxygen from the air and are changed to water and the element M; in the case of H,S,
s. further Chap. XIV.