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(or this reaction may be thus expressed,
MnOSO2Aq + 2KOHAq + 2Cl = MnO ̧ + K ̧SO̟ ̧Âq + 2HClAq).
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 = NaClAq+Br; CaBr ̧Aq + 2C1 = CaCl ̧Aq + 2Br,
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.
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.
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. The chemical changes may be thus represented:
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+H,O= MOH + H (M = Li, Na, K, Rb, or Cs).
The compound MOH-called a hydroxide-dissolves in
the excess of water; the chemical change is therefore better represented by the equation
M+H2O+ Aq = MOHAq + H,
where Aq means a large, indeterminate, quantity of water.
That we may learn the exact meaning of this equation, let a weighed piece of sodium-say 1 gram-be thrown into a large quantity of water, weighing say x grams; the sodium moves about on the surface of the water with a hissing sound, and hydrogen is rapidly evolved; after a little time the sodium has entirely disappeared. The mass of hydrogen produced weighs ·044 gram; the mass of liquid remaining weighs 1+x- 044 grams; this liquid is evaporated so that the water which boils off may be collected and weighed, x 783 grams of water are obtained, and 1.74 grams of a white solid remain. This white solid is analysed; its composition is expressed by the formula NaOH (Na=23, O= 16). Hence 1 gram of sodium has interacted with 783 gram of water to produce 044 gram of hydrogen and 1.74 grams of sodium hydroxide (or caustic soda); the whole of the hydrogen produced has been evolved as gas, and the 1.74 grams of sodium hydroxide have dissolved in the excess of water, that is, in the water which did not chemically interact with the sodium. But as the combining weight of sodium is 23, and the reacting weight of water is 18 (0=16), the experimental results are expressed by the equation
Na + H2O + Aq = NaOHAq÷ H
(23: 181: 783, and 40: 1 1.74: 044).
The elements we are considering do not combine with hydrogen either directly or indirectly. They combine with oxygen, with oxygen and hydrogen, with the halogens, and with many other, chiefly non-metallic, elements. The compositions of the more important compounds are represented by the formulae: M2O, MOH, M,S, MSH, MX (X = F, Cl, Br, 1), M.SO, and MHSO,, M,CO, and MHCO, MNO, (M = Li, Na, K, Rb, or Cs).
Compounds with oxygen. Oxides M2O. The five elements 164 combine directly with oxygen at ordinary temperatures; but caesium oxide has not yet been obtained approximately pure. The oxides are white solids, which are unchanged by the action of heat; they dissolve very rapidly in water, and these solutions turn red litmus deep blue. The oxides are decidedly basic; they interact with aqueous solutions of acids to produce salts and water: thus
M2O + 2HClAq 2MCIAq + H2O,
MO+H2SO,Aq = M2SO,Aq
M2O + 2HNO2Aq = 2MNO2Aq + H2O,
These oxides interact with water to form hydroxides which dissolve in the excess of water: thus
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) = CaCO, + 2MOHAq.
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 + H2SO1Aq = M2SO,Aq + 2H2O ;
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
2 6 6
6NaOHAq + Fe, 3SO Aq = Fe,OH + 3Na2SO Aq,
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 MO 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.
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; ZnCl ̧.2 NaCl; 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.