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solutions to form normal salts. With the weak acids, especially with boric acid, these metals also form basic salts : e.g. 3Ca0.5B,0,, and 3SrO.BLOZ. A few double salts are known of these three metals; e.g. Caso, K_SO..H,O; and SrSO..K.SO The greater number of these double salts are derivatives of the weaker acids; e.g.

Ba (NH.) AsOn, and 2CaO.B.Oz. Na,B,0,.15H,0. The salts of beryllium are less definite compounds than those of the other three metals we are considering: many are basic salts, e.g.

3BeO.CO,; 3BeO. SOz; 2BeO. SOz; 7Be0.35e0g. 14H,O. The normal Be salts are more easily decomposed by heat, or by heating in presence of water, than the salts of Ca, Sr, and Ba. Beryllium also forms many double salts, e.g. Be (NH)PO,; BeSO,.K.SO,.2H0; 3BeCO,.2(NH2) CO;

3BeSig. Al,(SiO2) The carbonates MCO, are all decomposed by heat alone to MO + CO,; their stabilities towards heat increase as the atomic weight of M increases.

The existence of the gaseous molecules BeCl, and BeBr, indicates that the atom of beryllium is divalent; as the vapour densities of no compounds of the metals Ca, Sr, Ba, have as yet been determined, we cannot be certain as to the valency of the atoms of these elements, but judging from the analogies between these three, and the other, members of Group II., it is probable that the atoms of these elements are divalent.

The three elements, calcium, strontium, and barium, are evidently very closely related; they are much more like each other than any of them is like beryllium.

402

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403 We shall now consider the odd-series members of Group II.

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86

Odd-series MAGNESIUM.

ZINC.
CADMIUM.

MERCURY.
elements
Atomic weights

24
64.9
112

1998 Molecular weights

64.9
112

1998 Sp. grs. (approx.)

1.75
7.2

13.6 Sp. heats

.25
·095
056

*032

(for solid Hg) Melting points 500° -7000

420°
320°

-39o.5
(approx.)
Atom. weights
13.7

9
13

14.7
Sp. gravs.
Vapour-densities

64.9
112

199.8 (H=1) Colour, appear- White, lustrous, White with slight White, lustrous; Liquid above - 39o.5. ance, &c. fairly hard ; ductile; blue tinge, lustrous; 'malleable and duċ- White, lustrous;

crystalline; may be rather brittle, but tile; easily crystal- crystalline.
distilled at very high malleable at 100°— lised; vapourised at Boils at 350°.
temperature.

150°; crystalline; 750°-800°.
readily distilled ; va-
pourised at 900°–

1000°.
Occurrence and Carbonate, sul- Sulphide, carbonate, Usually

accom

The metal occurs in preparation. phate, chloride, and and oxide are fairly panies Zn in its ores. small quantities.

silicate, of Mg are widely distributed in Prepared by reduc- HgS is found in conwidely distributed in rocks.

ing Cdo by charcoal. siderable quantities, rocks and water, Prepared by deoxi

but not widely disPrepared by elec- dising Zno by char

tributed. trolysis of molten coal.

Prepared by heating MgCl2, or by reduc

HgS in air, or with ing MgCl, by Na.

Fe or Cao, and

condensing Hg. General chemical Oxidised by heating Oxidised by heating Closely resembles Oxidised by heating properties.) in air or 0. in air or O to redness. Zn in properties ;

in air or 0. Decomposes water Decomposes steam but oxide is not so- Oxide is decomposed at abt, 100°. at red heat.

luble in KOHAq. to Hg and o at full Combines directly Combines directly Combines directly red heat. with CI, Br, I. with Cl, Br, I, but with S.

Does not decompose Oxide is basic and not with S.

Metal has been gasi- water steam. slightly alkaline. Reacts with acids to fied, and molecule Readily combines Metal interacts with form salts and H. found to be mon- with CT, Br, I, and e. acids to form salts Oxide is basic but atomic.

Forms two series of and H. not alkaline; it is

salts. soluble in KOHAq.

Oxide HgO

is soluble Metal has been gasi

in molten KOH. fied, and molecule

Metal has been gasifound to be mon

fied, and molecule atomic.

found to be monatomic.

404 General formulae and chemical characters of compounds. Oxides and hydroxides. MO, MO H,, when M=Mg, Zn, Cd.

M,0 and Mo, when M = Hg; no

hydroxides of Hg known.
Sulphides. MS; and also MS when M =Hg.
Haloid compounds. MX,; and also MX when M=Hg.
Salts.
MSO,, M2CO, M2NO,,

M2NO, &c.; and
also M SO, MNO, &c. when
M = Hg.

No compound of Mg has been gasified; several haloid compounds, MX, of the other metals have been gasified.

The oxides MO are obtained by heating the metals in 405 oxygen, or by precipitating solutions of their salts by an alkali, and heating the hydroxides MO, H, thus formed; the oxide HgO is obtained when an alkali is added to the solution of a mercuric salt, e.g. to HgCl Aq or Hg(NOx), Aq. Mercuric oxide (HgO) is a red or yellow solid; Cdo is brown; Zno is pale yellowish-white; and MgO is white; they can all be obtained in crystals. The specific gravities of these oxides are approximately 3.1 for MgO, 5:6 for ZnO, 6.96 for Cdo, and 11:1 for Hgo. MgO is slightly soluble in water, 1 part of Mgo dissolving in about 60,000 parts of cold water; the other oxides

are almost insoluble in water. MgO combines directly with · water to form Mgo, H.; this hydroxide is decomposed to

MgO+H,O at a red heat. The other oxides MO do not directly combine with water; HgO appears not to combine with water under

any

conditions. The oxide Hgo dissolves in molten potash forming the compound K,0.2Hgo. The oxide exists in two forms; red Hgo, obtained by heating Hg in O, and yellow Hgo, obtained by precipitating the solution of a mercuric salt by KOHAq: these oxides shew considerable differences in their reactions with acids &c., e.g. red HgO scarcely interacts with chlorine, whereas yellow HgO readily interacts to produce an oxychloride of mercury and CI 0.

The oxides of Zn and Cd are reduced to metal by heating with charcoal, or in H or CO; HgO‘is reduced to metal by heat alone ; Mgo is not deoxidised by ordinary reducers.

The hydroxides MgO, H, ZnO,H,, and Cdo H,, are obtained by precipitating solutions of salts of the metals by an alkali, and drying; they are all decomposed by heat alone to MO+H,0; their stabilities towards heat are inversely as the atomic weights of the metals. These hydroxides are all basic, MgO,H, has a slightly alkaline reaction towards litmus; ZnO,H, is soluble in KOHAq, but is precipitated again when a concentrated solution is placed over sulphuric acid in vacuo, the other hydroxides are insoluble in alkali solutions.

Mercurous oxide Hg,O is a black solid, specific gravity 10-7, obtained by adding KOHAq to the solution of a mercurous salt, e.g. to HgNO,Aq, or to solid HgCl. This oxide is decomposed by heat to mercury and mercuric oxide, and at a higher temperature to mercury and oxygen.

406

The sulphides, MS, are solids. MgS resembles the sulphides of Ca, Sr, and Ba, it is an unstable compound which interacts with water to form MgS H, and MgO,H, ; the MgS, H, is quickly changed to Mgo H, and H S. The compounds Mgs and Mgs have also been prepared. The other sulphides, MS, where Ñ = Zn, Cd, or Hg, are more definite and stable compounds. They are produced by passing HS into solutions of the salts of the metals; ZnS is soluble in and decomposed by most acids, the two others are insoluble in dilute acids. CdS and HgS are also obtained by directly combining the metals with sulphur, the reaction proceeds slowly and to a limited extent with Cd.

Pentasulphides of zinc and cadmium, MS, seem to exist; they are unstable compounds. Mercuric sulphide exists in two forms; black and amorphous, by precipitating solutions of mercuric salts by HS; red and crystalline, by directly combining Hg and S, or by subliming the amorphous form.

When HgS is precipitated from solutions of mercuric salts by excess of ammonium sulphide, and potash is added, the HgS dissolves and the solutions probably contain a compound of KS and HgS; the compound KS. 2HgS is said to have been obtained. None of the other sulphides MS exhibit any tendency to form compounds with the sulphides of the strongly positive metals. HgS also forms many double compounds, chiefly with mercuric salts; e.g. 2HgS.HgCl,; HgS.HgI, ; Hgs. 2HgSo. The sulphides of Zn and Cd form a few oxysulphides, e.g. Zno. ZnS.

Mercurous sulphide, Hg,S, if it exists is extremely easily decomposed to HgS and Hg.

The haloid compounds, MX,, are obtained by the direct combination of metal with halogen, or by dissolving the metals, oxides, or carbonates in aqueous solutions of the haloid acids HX. They are white solids, generally soluble in water and alcohol; the solubility decreases as the atomic weights of the metals increase. They all form oxyhaloid compounds of the general form xMČI, YMO; a great many mercury oxychlorides have been obtained. MgCl, cannot be obtained by evaporating a solution in HCIAq as when most of the water has been removed this solution is decomposed to MgO and HCl.

The haloid compounds all melt at temperatures below 1000°; e.g. MgCl, melts at about 700", Znci, at approximately 500°—600°, CdCi, at about 550°, and HgCl, at about

407

300°. They all combine with ammonia, and also with various other haloid compounds, to form double compounds; e.g. ZnCl,. 2NH,Cl; 2CdCi,. SrCl,; 3HgCl,. MgCl,. The haloid mercury compounds form a very great number of such double compounds; they also combine with various salts, e.g. with K,Cr,O,, Cu(C,H,02),, &c.

The mercurous haloid compounds, HgCl, HgI, and HgBr, are obtained by heating the corresponding mercuric compounds with mercury; they are nearly insoluble in water, and are partially decomposed by heat into mercury and the corresponding mercuric compounds, e.g. 2HgCl = HgCl, + Hg.

The following haloid compounds have been gasified : ZnCl,, CdBr,, HgCl, HgI,; these formulae represent the compositions of molecules of the compounds, hence the atoms of Zn, Cd, and Hg are divalent in these molecules. It is probable, but not quite certain, that the formula HgCl represents the gaseous molecule of mercurous chloride ; if this formula is molecular the atom of Hg is monovalent as well as divalent.

The salts of the metals we are considering are very 408 numerous; each metal forms salts of the form M2NO3, MSOQ, MCO,, &c. and besides these Hg forms a series of mercurous salts, M.SO, MNO,, &c. Many of the salts are isomorphous; some salts of mercury are isomorphous with corresponding salts of copper.

The metals all form basic salts, e.g. 4MgCO3. MgO,H,; 4Zn0.S03; Cd2NO3. CdO,H,; 3H30.SO,; 3HgO.NO ; a very great many basic

mercury known; Mg seems to form fewer of these salts than

any

other of the four metals. Double salts of all these metals are numerous, especially in the case of mercury. The salts of the four metals, as a class, are stable and well defined; those of Hg, on the whole, are less soluble in water than the others. The mercurous salts are considerably less stable than the mercuric salts; they are easily changed into the latter. Mercury salts form a very large number of compounds with ammonia ; the composition of many of these is complex; in this respect mercury resembles copper, gold, platinum, and chromium.

The elements placed in Group II. are evidently closely 409 related in their chemical properties. The four elements Ca, Sr, Ba, and Mg, more nearly resemble one another than they resemble any of the other members of the group. This resemblance is well shewn in the alkalinity of their

salts are

5

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