Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/80

70 METALS strongly basylous ; metals of group 2 are only slowly attacked, with formation of relatively feebly basylous and practically insoluble hydrates. Disregarding the rarer elements (as we propose to do in this section), the metals not named so far may be said to be proof against the action of pure water in the absence of free oxygen (air). By the conjoint action of water and air, thallium, lead, bismuth are oxidized, with formation of more or less sparingly soluble hydrates (ThHO, Pb0 2 H 2 , BiO 3 H 3 ), which, in the presence of carbonic acid, pass into still less soluble basic carbonates. Iron, as everybody knows, when exposed to moisture and air, " rusts," that is, undergoes gradual conversion into a brown ferric hydrate, Fe 2 O 3 arH 2 O ; but this process never takes place in the absence of air, and it is questionable whether it ever sets in in the absence of carbonic acid. What is known is that iron never rusts in solutions of caustic alkalies or lime (which reagents preclude the pre sence of free carbonic acid), while it does so readily in ordinary moist air containing CO 2 . When once started the process proceeds with increasing rapidity, the ferric hydrate produced acting as a carrier of oxygen; it gives up part of its oxygen to the adjoining metal, being itself reduced to (perhaps) Fe 3 O 4 , which latter again absorbs oxygen from the air to become ferric hydrate and so on (Kuhlmann). Copper, in the present connexion, is intermediate between iron and the following group of metals. Mercury, if pure, and all the " noble" metals (silver, gold, platinum, and platinum-metals), are absolutely proof against water even in the presence of oxygen and carbonic acid. The metals grouped together above under 1 and 2 act on steam pretty much as they do on liquid water. Of the rest, the following are readily oxidized by steam at a red heat, with formation of hydrogen gas, zinc, iron, cadmium, cobalt, nickel, tin. Bismuth is similarly attacked, but slowly, at a white heat. Aluminium is barely affected even at a white heat, if it is pure; the ordinary impure metal is liable to be very readily oxidized. Aqueous Sulphuric or Hydrochloric Acid, of course, readily dissolves groups 1 and 2, with evolution of hydro gen and formation of chlorides or sulphates. The same holds for the following group (A) : [manganese, zinc, magnesium] iron, aluminium, cobalt, nickel, cadmium. Tin dissolves readily in strong hot hydrochloric acid as SnCl 2 ; aqueous vitriol does not act on it appreciably in the cold ; at 1 50 it attacks it more or less quickly, accord ing to the strength of the acid, with evolution of sul phuretted hydrogen or, when the acid is stronger, of sulphurous acid gas and deposition of sulphur (Calvert and Johnson). A group (B), comprising copper, are, substantially, attacked only in the presence of oxygen or air. Lead, in sufficiently dilute acid, or in stronger acid if not too hot, remains unchanged. A group (C) may be formed of mercury, silver, gold, and platinum, which are not touched by either aqueous acid in any circumstances. Hot (concentrated) oil of vitriol does not attack gold, platinum, and platinum-metals generally ; all other metals (including even silver) are converted into sulphates, with evolution of sulphurous acid. In the case of iron, ferric sulphate, Fe 2 (SO 4 ) 3 , is produced ; tin yields a somewhat indefinite sulphate of its binoxide SnO.,. Nitric Acid (Aqueous). Gold, platinum, iridium, and rhodium only are proof against the action of this powerful oxidizer. Tin and antimony (also arsenic) are converted by it (ultimately) into hydrates of their highest oxides SnO 2 , ^2^5 (As 2 O 5 ), the oxides of tin and antimony being insoluble in water and in the acid itself. All other metals, including palladium, are dissolved as nitrates, the oxidiz ing part of the reagent being generally reduced to nitric oxide, NO, or sometimes to N 2 3 or N 2 O 4 . Iron, zinc, cadmium, also tin under certain conditions, reduce the dilute acid, partially at least, to nitrous oxide, N.,0, or nitrate of ammonia, NH 4 .N0 3 = N 2 O + 2H 2 O. Aqua Regia, a mixture of nitric and hydrochloric acids, converts all metals (even gold, the " king of metals," whence the name) into chlorides, except only rhodium, iridium, and ruthenium, which, when pure, are not attacked. Caustic Alkalies. Of metals not decomposing liquid pure water, only a few dissolve in aqueous caustic potash or soda, with evolution of hydrogen. The most important of these are aluminium and zinc, which are converted into aluminate, A1 2 O 3 3(K 2 or Na 2 )O, and zincate, ZnO.RHO, where R = K or Na respectively. But of the rest the majority, when treated with boiling sufficiently strong alkali, are attacked at least superficially; of ordinary metals only gold, platinum, and silver are perfectly proof against the reagents under consideration, and these accordingly are used preferably for the construc tion of vessels intended for analytical operations involving the use of aqueous caustic alkalies. For preparative purposes iron is universally employed and works well ; but it is not available analytically, because a superficial oxida tion of the empty part of the vessel (by the water and air) cannot be prevented. According to the writer s experience basins made of pure malleable nickel are free from this drawback ; they work as well as platinum, and rather better than silver ones do. There is hardly a single metal which holds out against the alkalies themselves when in the stato of fiery fusion ; even platinum is most violently attacked. In chemical laboratories fusions with caustic alkalies arc always effected in vessels made of gold or silver, these metals holding out fairly well even in the presence of air. Gold is the better of the two. Iron, which stands so well against aqueous alkalies, is most violently attacked by the fused reagents. Yet tons of caustic soda are fused daily in chemical works in iron pots without thereby suffering contamination, which seems to show that (clean) iron, like gold and silver, is attacked only by the conjoint action of fused alkali and air, the influence of the latter being of course minimized in large-scale operations. Oxygen or Air. The noble metals (from silver upwards) do not combine directly with oxygen given as oxygen gas (0 2 ), although, like silver, they may absorb this gas largely when in the fused condition, and may not be proof against ozone, O 3 . Mercury, within a certain range of tempera tures situated close to its boiling point, combines slowly with oxygen into the red oxide, which, however, breaks up again at higher temperatures. All other metals, when heated in oxygen or air, are converted, more or less readily, into stable oxides. Potassium, for example, yields peroxide, K 2 O 2 or K 2 O 4 ; sodium gives Na 2 O 2 ; the barium-group metals, as well as magnesium, cadmium, zinc, lead, copper, are converted into their monoxides MeO. Bismuth and antimony give (the latter very readily) sesquioxide (Bi.,O :i and Sb 2 O 3 , the latter being capable of passing into Sb 2 4 ). Aluminium, when pure and kept out of contact with siliceous matter, is only oxidized at a white heat, and then very slowly, into alumina, A1 2 O 3 . Tin, at high temperatures, passes slowly into binoxide, SnO 2 . Sulphur. Amongst the better known metals, gold and aluminium are the only ones which, when heated with sulphur or in sulphur vapour, remain unchanged. All the rest, under these circumstances, are converted into sulphides. The metals of the alkalies and alkaline earths, also magnesium, burn in sulphur- vapour as they do in oxygen. Of the heavy meals, copper is the one which exhibits by far the greatest avidity for sulphur, its subsulphide Cu 2 S being the stablest of all heavy metallic sulphides in

opposition to dry reactions. See METALLURGY.