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We have now gained some fairly clear notions of the methods adopted in chemistry for classifying elements and compounds. Similar elements are put into the same class. Similar elements are those which interact with other elements and with compounds under similar conditions to produce similar compounds.
Compounds again are said to be chemically similar or analogous when their compositions and their properties are similar.
The questions to be answered with regard to any element before its position in a scheme of chemical classification can be determined are such as these ;-Does the element combine with oxygen
n? Under what conditions are its oxides formed ? What is the composition of each of these oxides? Are its oxides basic or acidic ? Does it combine with hydrogen ? What is the composition of its hydride or hydrides? Does it form any acids ? Under what conditions are these acids produced, and what are their compositions and properties? Does it interact with acids to form salts? What are the products of its interaction with water? Or is it unchanged when brought into contact with water or steam? Does it form chlorides, bromides, oxychlorides, &c.? 'Under what conditions are these compounds produced? What are their compositions, and how do they interact with other bodies ?
As chemistry is the study of the connexion between changes of composition and changes of properties, the subject of classification must be all important in chemistry. We must therefore proceed to examine a few classes of elements that we may learn how answers are gained to such questions as these
just proposed, and what kind of answers they are which are gained.
The three elements chlorine, bromine, iodine, are placed in 150 the same class.
Chlorine. Bromine. Iodine. Combining Weights.
127 Specific gravities as solids or liquids ; water=1.
3.18 (liquid) 4.94 (solid) Melting points (approxi
1150 Boiling mate). points
above 2000 Appearance and pro- Yellow-green gas, Dark-reddish. Grey, lustrous minent physical pro- liquified at có black liquid; solid; vapour is perties.
under pressure of solidifying at deep violet
than that of
the two other odour.
strong; rapidly elements.
corrodes ani. Poisonous.
Occurrence. None of these elements is found uncombined 151 with others. The commoner compounds are those with sodium, potassium, calcium, and magnesium. Chloride, bromide, and iodide of potassium, sodium, calcium, or magnesium occur in sea water, in mineral waters, and in some rocks. Chlorides of some or all the metals named are widely distributed ; bromides occur in smaller quantities; and iodides, with iodates (salts of the acid HIO), are found only in very small quan
Preparation. Sodium (or potassium chloride, bromide, or 152 iodide, is mixed with manganese dioxide and sulphuric acid, and the mixture is heated; manganese sulphate, sodium sulphate, water, and chlorine, bromine, or iodine, are formed. Putting X as = Cl, Br, or I, the change may be thus expressed ;2NaX + 2H_SO, + MnO, = MnSO, + Na SO, +2H,0 + 2X.
If NaX is heated with sulphuric acid only, hydrogen chloride, bromide, or iodide is formed ; if one of these compounds is heated with manganese dioxide, chlorine, bromine, or iodine is formed along with manganese chloride, bromide, or iodide : hence the change represented in the above equation probably takes place in two parts occurring simultaneously.
Chemical properties. Compounds with hydrogen. The elements chlorine, bromine, and iodine, combine directly with hydrogen, to form the compounds HX, where X = Cl, Br, or I. Hydrogen chloride, HCl, is formed by exposing a mixture of equal volumes of hydrogen and chlorine to diffused daylight; hydrogen bromide is formed by passing a mixture of hydrogen and bromine vapour through a hot tube; hydrogen iodide is formed, but only in small quantities, by passing a mixture of hydrogen and iodine vapour through a tube containing powdered glass or finely divided platinum, heated to 300° or 400°. These compounds, HX, are also produced by decomposing NaX or KX by an acid; e.g.
2NaCl + H SO, = Na SO, + 2HCl. They are also formed, along with phosphorous acid (H PO), when water interacts with chloride, bromide, or iodide, of phosphorus; thus PX, +3H 0=3HX+ POH
The compounds HX are gaseous at ordinary temperatures and pressures; they are very soluble in water and the solution in each case is markedly acid (s. chap. ix.). Hydrogen iodide is decomposed by heat into hydrogen and iodine ; the change begins at about 180°; hydrogen bromide is also decomposed by heat but the change does not begin until a temperature higher than 180° is reached; hydrogen chloride is not decomposed by heat.
Compounds with oxygen. None of the elements, chlorine, bronine, iodine, combines directly with oxygen. No oxide of bromine has been isolated; one oxide of iodine, and two (or perhaps three) oxides of chlorine are known. The compositions of these oxides are as follows ;–21,0, C10, (? C1,0z), 1.0, The oxides C1,0 and 10, are prepared by carrying outochemical changes in which oxygen is produced in presence of chlorine and iodine, respectively. To prepare CI,O, mer
curic oxide (HgO) is heated in a stream of dry chlorine. When mercuric oxide is heated it is decomposed into mercury and oxygen; therefore by passing chlorine over heated mercuric oxide we carry out a reaction in which oxygen is produced in presence of chlorine. When a concentrated solution of nitric acid (HNO3) is heated, oxygen, water, and oxides of nitrogen are produced. If iodine is heated with concentrated nitric acid, a part of the oxygen coming from the nitric acid combines with the iodine to form iodine pentoxide (1,03). The two reactions are represented in equations thus ;
(1) 2HgO + 4C1 = HgCl,. HgO + CI,O.
In the interaction of water with chlorine oxygen is evolved; if iodine is suspended in water and chlorine is passed into the water hydrated iodine pentoxide is produced ; thus
21 +6H 0 + 10Cl + Aq=1,0 H 0+10HCIA. When the compound 1,05. H,0 thus produced is heated, water is removed and 1,0, remains.
The other oxide of chlorine, CIO, is produced by decomposing chloric acid, HCIO, by heat. Chloric acid is very easily decomposed; when the potassium salt of the acid interacts with sulphuric acid, chloric acid and potassium sulphate are formed, but the heat produced in the reaction suffices to decompose the chloric acid into chlorine dioxide (C10,) and other products. These reactions may be represented thus in equations;
(1) 3KCIO, + 3H SO, = 3HC10, +3KHSO, ;
(2) 3HCIO, = 2C10, + HCIO, + H,O. It is still undecided whether the body described as chlorine trioxide, C1,0.3, is a definite compound or a mixture of chlorine dioxide and chlorine.
The oxides of chlorine and iodine are acidic. Each interacts with water to form an acid or acids; thus
(1) CI,O+H,0 + Aq = 2HCIO Aq;
The three acids hypochlorous (HCIO), chlorous (HCIO), chloric (HC103), are known only in aqueous solutions; when these solutions are concentrated to remove water the acids are decomposed into oxides of chlorine and water. Iodic acid
(HIO2) is known as a solid body; when heated it decomposes into iodine pentoxide (1,06) and water.
Compounds with non-metallic elements other than hydro-
antimony; SbXz: also SbClg.
A few other compounds of chlorine, bromine, and iodine with non-metallic elements are formed indirectly ; e.g. CX, CX, CX, SiBr, Sici, &c.
c 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,; BiXy; Cr, X.; Cu X, and CuX,; Fe, X, and Fe, X.; Hg, X, and HgX, ; PtX, and PtX,; SnX, and SnX.
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 :