CARBIDES are, strictly, compounds of carbon with one other element. The term, however, is generally used to designate only those binary compounds of carbon which are comparatively infusible and non-volatile. Carbides are generally products of the electric furnace, and it may be said that Henri Moissan, who made many brilliant researches with this furnace, founded the chem istry of carbides. The volatile carbon compounds of oxygen, sulphur and the halogens are not included in the category of carbides.
The two most important carbides are calcium carbide and sil icon carbide (carborundum, q.v.). Calcium carbide, which is of industrial value as the source of acetylene (q.v.), is manufactured by heating together lime and coke or powdered coal in an electric furnace, the molten product being tapped off at 1,800° C. The lime may be replaced by limestone (calcium carbonate). The electric furnace is employed merely as a convenient source of heat ; there is no specific electrolytic action in the process, Ca0+3C = Commercial calcium carbide is a brown ish-grey crystalline mass decomposed by water with evolution of acetylene, + = Ca (OH) 2 + The carbides of stron tium and barium, which resemble the calcium compound, are pre pared in a similar manner. Moissan prepared carbides from the alkali metals as greyish-white powders by heating them in a stream of acetylene, when, in the case of sodium, an intermediate product, (or was formed; this on further heating evolved acetylene and left a residue of sodium carbide, Aluminium and beryllium carbides, yellow and yellowish-brown crystalline products respectively, are of scientific interest since they evolve pure methane when decomposed by water. The former is made by heating together aluminium and carbon, the reaction proceeding rapidly at 1,400° C. The latter is produced by the reduction of beryllia with carbon at 1,q00° C. Carbides or acety lides of doubtful composition are precipitated by passing acetylene into ammoniacal solutions of cuprous and argentous salts, the products being reddish-brown and yellowish-white respectively. These compounds are explosive, as are also the acetylides of mer cury and of gold.
The carbides may be roughly classified according to their inter actions with water or dilute acids : (I) The acetylides, which evolve acetylene, are those of the alkali metals, the alkaline-earth metals, the currency group, and the carbides of magnesium and mercury; (2) the methanides are the carbides of aluminium, beryllium and manganese, since these substances evolve methane. Manganese carbide, obtained by the interaction of carbon and manganese oxide, at 1,500° C, is decomposed by water with evolution of methane and hydrogen; (3) mixed car bides, which on treatment with water or acids evolve both acety lene and methane, as for example, the carbides of the rare-earth metals (lanthanum, yttrium, etc.). Uranium carbide, or is decomposed by water to give hydrogen, acetylene, ethylene and methane. Iron carbide (cementite), is decomposed by hot acids with evolution of hydrogen and a mixture of hydrocarbons; (4) carbides not decomposed by water or dilute acids. The last group includes silicon carbide, SiC, obtained in pale green to steel blue crystals; this is a typical furnace product which due to its extreme hardness is widely employed as an abrasive (q.v.). Boron carbide, titanium carbide, TiC, and zirconium carbide, are hard refractory substances all prepared in the electric furnace, the first by combination of its constituent elements and the other two by reduction of the corresponding oxides by carbon.