The hydroxides are, with some exceptions, generally spoken of as insoluble in water. The word " soluble" is a relative term; it is probable that very few, if any, substances are absolutely insoluble. Silver chloride is usually regarded as wholly insoluble in water, but pure water shaken up with that salt acquires increased conductivity, showing that some chloride must have gone into solution. In one of the equations given above, Al(OH)3 is followed by "Aq," implying that it is dissolved and ionised in solution. This method of writing is perfectly correct for the portion which is dissolved, but that constitutes only a very minute fraction of the whole. What is dissolved, however, is ionised and enters into reaction, and when it has been removed, as in the equation given, with formation of practically non-ionised water-molecules, its place is taken by more : equilibrium tends to establish itself between the dissolved portion and the portion remaining undissolved. We know well that if excess of common salt be placed at the bottom of a vessel of water it will not all dissolve, but, as the dissolved portion diffuses away into the upper layers of water, its place is taken by fresh salt, which dissolves, until, if sufficient time be given, the whole solution becomes saturated with salt. Similarly, the removal of the aluminium-^-as ions, it may be-and of the hydroxyl of the aluminium hydroxide, Al(OH)3, as water, on treatment with an acid, causes a fresh portion of the hydroxide to go into solution, and this continues to go on until all has undergone reaction.

The hydroxides of the elements may be classified like the halides; the analogy between the formulae is seen on comparing the tables on pp. 72, 73, with those on p. 50.

Oxygen compounds of fluorine are wanting.

rci(OH)7],

[Cl(OH)5],

[Cl(OH)3],

Cl(OH).

OCl(OH)5]f

[OCl(OH)3],

OCl(OH),

CLjO.

p2Cl(OH)3],

O2C1(0H),

[CLA].

O3Cl(OH),

[C12O5].

[ci2o7].

The formulas enclosed in brackets are of unknown substances. The whole scheme is given in order to show the gradual loss of water of an ideal heptoxide.

The compounds I(OH)6(ONa), OI(OH)5, O2I(OAg)3, and O3I(OAg) are known, corresponding to the theoretical perhalic acids. Those corresponding to the halic acids are O2Br(OH) bromic acid, and I(OH)5and O2I(OH), iodic acids. Br(ONa) and I(ONa), named respectively hypo-bromite and hypoiodite of sodium, are also known.

It may be noticed that the formulae of some of the compounds of chromium are analogous to those of sulphur and of molybdenum ; other compounds, on the contrary, show more resemblance to those of iron. While manganese, like chromium, also shows analogy with iron, it too forms O2Mn(OK)2, like O2S(OH)2 or O2Mo(OH)2, termed potassium manganate, as the others are hydrogen sulphate and hydrogen molybdate; and also MnO2, analogous to

LiOH

Be(OH)2

B(OH)3

Li2O

BeO

OB(OH)

OC(OH)2

ON(OH)

B2Os

co2

O2N(OH)

N2O3

oo2

N2Os

NaOH

Mg(OH)2

Al(OH)8

SI(OH)4

P(OH)3

NaoO

MgO

OAl(OH)

OSi(OH)2

OP(OH)3

OS(OH)2

Al,Os

SiO2

O2P(OH)

p2O3

O2s(oh)2

so2

p2o5

so3 ■ *

KOH

Ca(OH)2

Sc(OH)3

As(OH)3

K2O

CaO

OTi(OH)2

OAs(OH)3

OAs(OK)

OSe(OH)2

Sc2O3

TiO2

O2As(OH)

As2O3

O2Se(OH)2

SeO2

AsaOs

RbOH

Sr(OH)2

Y(OH)3

Sb(OH)3

Rb2O

SrO

OZr(OH)2

OSb(OH)3

OSb(ONa)

OTe(OH)2

Y2Os

ZrO2

O2Sb(OH)

Sb2Os

O2Te(OH)2

TeQ2

Sb2O5

TeOa

CsOH

Ba(OH)2

La(OH)3

Er(OH)3

Cs2O

BaO

OCe(OH)2

La2O3

CeO2

Er2O3

Yb(OH)3

Yb2O3'

O2Bi(6*H)

Bi(OH)3

Bi2Os

Bi2Os

OTh(OH)2

ThQ2

t

OXIDES AND HYDROXIDES

Zn(OH)2

Ga(OH)3

i

t

Cu2O

ZnO

OGe(OH)2

OV(OH)3

GeO2

O2V(OH) V2O3

OXr(OK)2

CrO2.nH2O

v,o5

CrO3

CrO2

AgOH

Cd(OH)2

In(OH)8

Nb2O5.nH2O

O2Mo(OH)2

MoO2.nH2O

AgsO

CdO

In2Os

SnO2.nH2O

Nb2Os

MoO3

MoO2

GdO

TbOs?

OPb(OH)2

Ta2O5.nH2O

OW(OH)4

AuaO

HgO

OTl(OH)

PbO2

Ta2O5

O2W(OH)2

.

TLA

WQj

i

OU(OH)4

UO2.nH2O

t

t

O2U(OH)2

uo2

01

uo3

SO2 and MoO2 ; but manganese also forms O3Mn(OK), termed potassium permanganate, which is analogous in formula as well as in crystalline form with potassium perchlorate, O3Cl(OK). It is convenient, however, also to include chromium and manganese in the iron group of elements.

Cr(OH)3

Mn(OH)3

Fe(OH)3

Ni(OH)3

Co(OH)3

OCr(OH)

OMn(OH)

OFe(OH)

Cr2O3

Mn2O3

Fe2O3

Ni263

Co2'63

Cr(OH)2

Mn(OH)0

Fe(OH)2

Ni(OH)2

Co(OH)a

CrO

MnO

FeO

NiO

CoO

Elements of the palladium group have a very wide range of valency ; hence they form a large group of compounds.

OsO4

IrO3

RhO3

O2Ru(OK)2

O2Os(OH)2

O2Ir(OK)2

Rh(OH)4

Ru(OH)4

Pd(OH)4

Os(OH)4

Ir(OH)4

Pt{OH)4

RhO2

RuO2

PdOa

OsO2

IrO2

PtO2

Rh(OH)3

Ru(OH)3

...

Rh2Os

Ru2Os

»»

Os263

Ir263

Pd(OH)2

Pt(OH)2

RhO

RuO

PdO

OsO

IrO

PtO

The hydroxides of lithium, sodium, potassium, rubidium, and caesium are all soluble white compounds, melting to colourless liquids at a red heat. They do not lose water, even at the highest temperatures, hence the oxides cannot be prepared from them ; indeed, the oxides are produced only by the action of the metal on the hydroxide, at a high temperature; for instance, 2NaOH + 2Na = Na2O -f H2. They are white, non-crystalline substances, combining at once with water to form the hydroxides Na2O + H2O = 2NaOH. The hydroxides are prepared from the carbonates by boiling a solution with slaked lime (calcium hydroxide) : Na2CO3.Aq + Ca(OH)2.Aq = 2NaOH.Aq + CaCO3; or by heating to redness a mixture of the carbonate with ferric oxide, when the ferrite is formed : K2CO3 + Fe2Os = 2KFeO2 + CO2. On treatment with water, potassium ferrite is decomposed, thus: KFeO2 + 2H2O.Aq = KOH.Aq + Fe(OH)3.

In either case, the solution of hydroxide is evaporated to dryness in an iron vessel and fused.

These hydroxides are said to be basic, for they are neutralised by acids, forming salts. Thus, with hydrochloric acid, KOH.Aq + HCl.Aq = KCl.Aq + H2O, the point of neutralisation-that is, when the acid and base are present in theoretical quantity to form the salt and water -is determined by the addition of an " indicator."