165 These oxides interact with water to form hydroxides which dissolve in the excess of water: thus M2O+H2O+ Aq = 2MOHAq. Compounds with oxygen and hydrogen. Hydroxides MOH. Prepared by evaporating aqueous solutions of the oxides to dryness and heating the residual solids; also by the interactions between solutions of the carbonates of sodium &c. and lime, thus- M.CO,Aq+CaO + H2O (boiled) = CaCO12+ 2MOHAq. 3 2 The hydroxides are white solids which melt at high temperatures without undergoing any chemical change. They cannot be decomposed into oxides and water by the action of heat alone. When molten each hydroxide interacts with its own metal to produce hydrogen and metallic oxide; thus MOH (molten) + M = M2O + H. The hydroxides (MOH) dissolve rapidly in water, forming alkaline solutions. These solutions interact with acids to produce salts and water, thus 2MOHAq + H2SO2Aq = M2SO,Aq + 2H ̧O ; they readily combine with carbon dioxide (gas) to produce carbonates and water, thus 2MOHAq + CO2 = 2M,CO ̧Aq + H2O; they interact with solutions of salts of iron, manganese, chromium, zinc, mercury, and many other metals, to produce salts of sodium, potassium, &c. and compounds of iron, manganese, &c. with hydrogen and oxygen, thus (M = Li, Na, &c.; N = Fe, Cr, Zn, &c.). Hydroxide; Salt of Hydroxide Salt of of N. 6 6 2 M. 6NaOHAq + Fe ̧3SO Aq = Fe,OH + 3Na,SO,Aq, 3 In some cases an oxide, not a hydroxide, of the metal N is produced; thus with mercury salts and potash we have 2KOHAq+Hg2NO,Aq=2KNO,Aq+HgO + H2O. Aqueous solutions of the hydroxides MOH have a soaplike, but corrosive, action on the skin, and a burning, but not sour, taste; they interact with fats to form soaps and glycerine. Solutions having these properties are said to be alkaline. The oxides M2O are said to be alkali-forming because they interact with water to produce the alkalis MOH. The elements lithium, sodium, potassium, rubidium, and caesium, are called the alkali-metals. The word alkali is of Arabic origin; it was originally applied to the ashes of sea-plants, and was afterwards extended to include all substances which more or less resembled these ashes in being very soluble in water, and feeling somewhat soapy to the touch, and reacting with acids to produce substances which exhibited neither the properties of the alkalis nor of the acids. 2 Compounds with the halogens. MX. These compounds are 166 produced (1) by the direct union of the elements; thus sodium heated in chlorine forms sodium chloride (NaCl): (2) by reactions between aqueous solutions of HX (X=Cl, Br, I) and the oxides MO or hydroxides MOH; thus sodium hydroxide dissolves in aqueous hydrochloric acid to produce sodium chloride and water (NaOHAq + HClAq = NaClAq + H2O). The haloid compounds MX (M = Na &c., X = Cl &c.) are white solids, soluble in water, unchanged by heat; they combine with many other haloid compounds to form double compounds, thus HgBr,. KBr; ZnCl2. 2NaCl; CuCl ̧.2KC1; CdBr ̧.KBr; CdI.. 2KI, &c. Interactions with water. The alkali metals interact with 167 water at ordinary temperatures to produce hydroxides (MOH) and hydrogen (s. par. 163). This chemical change is accompanied by a considerable running down of energy*. The system M+H2O+ Aq (M = Na, &c.) is able to do much more work than the system MOHAq+ H. The following numbers shew the gram-units of heat produced when 7 grams of lithium, 23 grams of sodium, and 39 grams of potassium, interact with water, to produce a solution of 24 grams of lithium hydroxide, 40 grams of sodium hydroxide, and 56 grams of potassium hydroxide, respectively, and, in each case, I gram of hydrogen; * A fuller treatment of this subject will be found in chap. XIV. 2 M+ H2O + Aq = MOHAq + H (all taken in grams), 168 169 Interactions with acids. The alkali metals interact with acids to produce salts, hydrogen is generally evolved; thus M+ HClAq= MClAq + 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,. A13SO,.24H ̧0. 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, Markedly electro-negative. 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. Halogens. Oxides formed only indirectly: generally unstable; interact with water to produce acids; are markedly acidic. Unite readily with many other elements, especially with positive elements. Do not interact with acids to produce salts. Interact with steam to produce acids (HX) and evolve oxygen. Alkali metals. Oxides formed directly at ordinary temperatures: very stable; interact with water to produce alkalis; are markedly basic. Unite readily with negative elements. Interact with acids to produce salts. Interact with cold water to produce alkalis (MOH) and evolve 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. Occurrence. 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. 172 173 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 MS, Se, or Te:-MH, MO, MO, MOH, MOH, MX, MX, (X = Cl, Br, I), MOX, MOX, м0,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 K,S)-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 ̧SO) 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. & |