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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. 3CaО. 5B,,, and 3SrO. BO,. A few double salts are known of these three metals; e.g. CaSO. K2SO. H2O; and SrSO. K2SO. The greater number of these double salts are derivatives of the weaker acids; e.g.

Ba (NH) ASO, and 2CaO. B,O,. Na B ̧0,. 15H ̧0.

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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.SO,; 2BeO.SO,; 7BeO. 3SeO,.14H,0. 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.

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Be (NH) PO; BeSO,. K.SO.2H2O; 3BeCO,. 2(NH1),CO ̧ ; 3 BeSiO,. Al(SiO3)3

The carbonates MCO, are all decomposed by heat alone to MO+CO2; their stabilities towards heat increase as the atomic weight of M increases.

The existence of the gaseous molecules BeCl, and BeBr, 402 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.

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

[blocks in formation]

Occurrence and preparation.

General chemical properties.

404

White, lustrous,
fairly hard; ductile;
crystalline; may be
distilled at very high
temperature.

Carbonate, sul-
phate, chloride, and
silicate, of Mg are
widely distributed in
rocks and water.
Prepared by elec-
trolysis of molten
MgCl2, or by reduc-
ing MgCl2 by Na.

Oxidised by heating
in air or O.
Decomposes water
at abt. 100°.
Combines directly
with Cl, Br, I.
Oxide is basic and
slightly alkaline.
Metal interacts with
acids to form salts
and H.

64.9

White with slight
blue tinge, lustrous;
rather brittle, but
malleable at 100°-
150°; crystalline;
readily distilled; va-
pourised at 900°-
1000°.

Sulphide, carbonate,
and oxide are fairly
widely distributed in
rocks.
Prepared by deoxi-
dising ZnO by char-
coal.

Oxidised by heating
in air or O to redness.
Decomposes steam
at red heat.
Combines directly
with Cl, Br, I, but
not with S.
Reacts with acids to
form salts and H.
Oxide is basic but
not alkaline; it is
soluble in KOHAq.
Metal has been gasi-
fied, and molecule
found to be mon-
atomic.

112

White, lustrous;
malleable and duc-
tile; easily crystal-
lised; vapourised at
750°-800°.

Usually accom-
panies Zn in its ores.
Prepared by reduc-
ing CdO by charcoal.

Closely resembles
Zn in properties;
but oxide is not so-
luble in KOHAq.
Combines directly
with S.

Metal has been gasi-
fied, and molecule
found to be mon-
atomic.

199-8

Liquid above - 39°•5.
White, lustrous;
crystalline.
Boils at 350°.

The metal occurs in small quantities. HgS is found in considerable quantities, but not widely distributed. Prepared by heating HgS in air, or with Fe or CaO, and condensing Hg. Oxidised by heating in air or O. Oxide is decomposed to Hg and O at full red heat. Does not decompose water or steam. Readily combines with Cl, Br, I, and S. Forms two series of salts.

Oxide HgO is soluble in molten KOH. Metal has been gasified, and molecule found to be monatomic.

General formulae and chemical characters of compounds.

Oxides and hydroxides.

Sulphides.
Haloid compounds.
Salts.

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MO, MO,H,, when M = Mg, Zn, Cd.
M.O and MO, when M=Hg; no
hydroxides of Hg known.

MS; and also MS when M = Hg.
MX,; and also MX when M = Hg.
MSO,, M2CO, M2NO,, &c.; and
also M2SO,, 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 MOH, 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(NO,),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 + H2O 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,O. 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 CIO.

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.

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The hydroxides MgO,H,, ZnO,H,, and CdO.H„, obtained by precipitating solutions of salts of the metals by an alkali, and drying; they are all decomposed by heat alone to MO + H2O; 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.

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Mercurous oxide Hg,O is a black solid, specific gravity 10.7, obtained by adding KÖHAq 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

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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 MgSH, is quickly changed to MgO,H, and H,S. The compounds MgS, and MgS, have also been prepared. The other sulphides, MS, where M = Zn, Cd, or Hg, are more definite and stable compounds. They are produced by passing H,S 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 H.S; 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 K,S. 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.

407 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 xMCI,.yMO; a great many mercury oxychlorides have been obtained. MgCl, cannot be obtained by evaporating a solution in HClAq 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°, ZnCl, at approximately 500°-600o, CdCl, at about 550°, and HgCl, at about

300°. They all combine with ammonia, and also with various other haloid compounds, to form double compounds; e.g. ZnCl,. 2NH Cl; 2CdCl,. 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(CH ̧O2),, &c.

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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: ZnCl2, 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.

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The salts of the metals we are considering are very 408 numerous; each metal forms salts of the form M2NO„, MSO, MCO, &c. and besides these Hg forms a series of mercurous salts, M2SO,, 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. 4MgCO. MgO,H,; 4ZnO.SO ̧; Cd2NO. CdOH; 3HgO.SO; 3Hg0. Ñ,0; a very great many basic mercury salts are 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

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