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.

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

OXIDES AND HYDROXIDES

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.

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

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 + Fe₂O_s = 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."