1902 Encyclopedia > Lime

Lime




LIME is the name of the strongly basic monoxide Ca0 of the metal calcium. This base is widely diffused throughout the three kingdoms of nature in the form of salts, of which the carbonate CaCO3 and the hydrated sulphate CaSO4.2H20 are by far the most abundant. Both are found in the mineral kingdom in a variety of forms. Of native carbonates of lime., calc-spar (Iceland spar), though comparatively rare, may be mentioned first as representing the purest native form of the compound. It generally presents itself in the form of well-developed transparent colourless rhornbohedra, which possess to a remarkable degree the property of producing double refraction of light, whereupon is founded its application in the construction of certain optical instruments. Of the varieties of massive or crystalline carbonate of lime, those which, through the fineness of their grain and other qualities, lend themselves for the purposes of the sculptor go by the name of marble, while the remainder are embraced under the generic term of limestone. This name, however, is understood to exclude chalk, a soft, amorphous variety which, according to Ehrenberg, consists mainly of Formula ifera shells. All limestones contain at least traces of magnesia. When this foreign base is present in considerable proportion the rock is termed " dolomite " (see MauxEsrum). Among the native forms of (hydrated) sulphate of lime the mineral " selenite " (glacier Maria-) corresponds to Iceland spar among the carbonates. It forms colourless transparent clino-rhombic prisms, generally united into "twins," and flattened down into plates readily cleavable along planes parallel to the surface. Hardness ranges from 1.5 to 3; the specific gravity is 2.3. Far more common than selenite are the massive varieties known as ALABASTER (see vol. i. p. 439) and ordinary GYPSUM (vol. xi. p. 337).

Both sulphate and carbonate of lime, apart from their occurrence as independent minerals, are almost universally diffused throughout the earth's crust, and in the waters of the ocean. Now the sulphate is appreciably soluble in even pure water, while the carbonate, though practically insoluble in pure, is quite decidedly soluble in carbonic acid water. As all atmospheric water must necessarily hold carbonic acid gas in absorption, most natural waters, and certainly all deep-well waters, are contaminated with more or less of bicarbonate or sulphate of lime, or with both. When such a water is being boiled, there is an escape of the free and the loosely combined carbonic acid, and the carbonate of lime comes clown as a loose precipitate or as a "crust"; and, when the water is sufficiently concentrated by evaporation, the sulphate likewise is partly deposited. The decomposition of the "bicarbonate" in fact takes place, though slowly, even at ordinary temperatures, when the water in which it is held in solution is exposed to the atmosphere. It is in this manner that stalagmites and stalactites frequently seen within rock-caverns are produced, and there is no difficulty in accounting for the grotesque and fantastic forms which the latter often exhibit.

Q aicklime. - The native carbonate always serves as the starting-point in the preparation of calcium compounds. From it the oxide CaO, known as quicklime or caustic lime, is produced industrially by heating limestone or marble in kilns, between layers of fuel, which in the United Kingdom is generally coal. The carbonic acid goes away with the gaseous products of combustion, and the oxide remains in unfused lumps of the form of the original stones. Lime, when pure, is an amorphous white solid, which is absolutely infusible and nonvolatile ; and on this account, when raised to high temperatures, it emits a brilliant white light (" lime-light "). The commercial article is generally grey or otherwise discoloured by the presence of foreign metallic oxides.

The decomposition that goes on in a limekiln is not brought about by the effect of heat alone. Gay-Lussac found long ago that carbonate of lime, when heated to intense redness in a closely covered crucible, loses its carbonic acid only very slowly, while the acid goes off readily even at somewhat lower temperatures when a current of steam is passed over the heated limestone. This may be accounted for by assuming that the steam, in the first instance, produces hydrate - from the carbonate - of lime, which latter then at once breaks up into its two components. More probably, however, the steam acts only by producing a quasi vacuum, that is, by clearing out the carbonic acid which, if allowed to stagnate even at high temperatures, would react on the quicklime produced, thus preventing the decomposition of a portion of the carbonate.





Quicklime acts readily and energetically on water, with evolution of much heat (269 units per unit weight of lime, Berthelot) and formation of a bulky white powder of the hydrate Ca011,0 or Ca(OH),. This powder readily mixes with water into a smooth paste, which may be diluted to a milky liquid - milk of lime. This, when filtered through paper, yields " lime-water," a strongly alkaline liquid containing about _,Guth of its weight of lime (calculated as Ca0). When boiled it deposits a part of its dissolved lime as such, and when exposed to ordinary air it quickly draws a skin of carbonate of lime. Hence its application as a reagent for carbonic acid, and the extensive use of milk of lime (whitewash) as a cheap white pigment in wall-painting. Lime paste, as every one knows, is most extensively used as a mortar or cement for bricks and stones in building. For this purpose it is always mixed with a certain proportion of sand. This admixture in all probability was originally intended only to save lime and prevent shrinking. But it is now generally assumed to have a chemical function, causing the formation of a hard silicate of lime pervading and thus strengthening the mortar. Some chemists deny the practical importance though not the occurrence of this silication ; what admits of no doubt is that the hardening of mortar involves the very gradual conversion of the original hydrate into carbonate of lime. Under the name of plaster, a fine smooth paste of lime and sand, with short hair to increase the tenacity of the mixture, is a most important material for coating the internal walls and roofs of ordinary buildings.

Hydraulic Centents. - Ordinary mortar, on account of the solubility of lime in water, is unfit for aquatic masonry; for this purpose hydraulic cements must be used. Of these there are a great variety, which, however, mostly agree in this that they consist of calcined mixtures of limestone and clay (preferably alkaliferous clay) and other silicates. By calcining such mixtures at temperatures short of that at which a glass would be produced, the lime becomes caustic, and part of the caustic lime, by uniting with the clay (and silicate generally), forms a silicate sufficiently basic to be disintegrable by acids and even by water. When such cement as a powder, is mixed with water, the lime acts upon the silicate of alkali and the gelatinous silica-hydrate transitorily produced, and with the silica and alumina and oxide of iron unites into a hard, waterproof, very complex, silicate mixture.

IL Ste Claire Devine having found that magnesia has hydraulic properties, hydraulic cements have been made by calcining dolomites of the proper composition so far as to decompose only the carbonate of magnesia (into Mg0 and CO). See CEMENT, vol. v. p. 328.

Lime, being the cheapest of powerful bases, is largely used in chemical manufacturing. It serves for the caustic-icing of soda, for the preparation of ammonia from ammonia salts, and for the manufacture of bleaching powder. It also enters into the composition of certain kinds of glass, and is used (as lime or as carbonate), in the making of soda ash.

Line Salts. - These can in general be prepared by the saturation of the respective acids with lime hydrate. Thus the (pure) carbar/ate CaCO3 may be prepared by passing carbonic acid into lime-water. But a more convenient method is to decompose a solution of pure chloride of calcium with excess of carbonate of ammonia, preferably at 70-80' C., when the carbonate assumes the form of a crystalline precipitate which settles readily and is easily washed with hot water. The sulphate (artificial gypsum) appears as a voluminous white precipitate, consisting of minute colourless needles, when sulphuric acid is added to a not too dilute solution of chloride of calcium or other lime salt. The precipitate CaSO4.2H,0 is appreciably soluble in water, 1000 parts of which at 0", 35°, and 100° C. dissolve 2.05, 2.54, and a little over 2 parts respectively of gypsum. The hydrated sulphate at temperatures exceeding 110" C. loses its water. The anhydrous sulphate, if formed below about 200° C., readily recombines with water into compact gypsum (plaster of Paris). By exposure to high temperatures (500° C. and upwards) sulphate of lime loses its power of recombining with water ; at very high temperatures it fuses. A naturally anhydrous sulphate of lime (anhydrite) occurs in association with rock salt, and otherwise, as a not very common mineral.





The well-known favourable action of gypsum as a manure, more especially for clover (see AGRICULTURE), has lately been explained by Deherain on the strength of analyses and vegetation experiments of his own, by assuming that it converts the carbonate of potash of the soil into sulphate, which, being less obstinately retained by the soil, more readily finds its way into the roots of the plants.

Chloride of Calcium. (muriatt of lime), CaCI„ is prepared by dissolving marble or limestone in aqueous muriatic acid. The iron and manganese generally present as impurities can be eliminated, after peroxidation by chlorine water, by digestion with hydrate of lime, which also, if allowed sufficient time, removes the magnesia. The filtrate is acidified with hydrochloric acid and concentrated by evaporation, so far that, on cooling, it deposits part of the dissolved salt as crystals. These have the composition CaCI„ 6H2O. They are very easily soluble in water and alcohol, and highly hygroscopic. Hence the salt is used occasionally to keep textile fibres moist and in a fit state for being woven. The crystals when kept in a basin at about 200° lose about two-thirds of their water, and leave that porous kind of chloride of calcium which analytical chemists prefer for the drying of gases. This substance, when heated to redness - which must be done in platinum to prevent contamination - loses the rest of the water and (at 723° C., Carnelley) fuses into the anhydrous salt CaCl2, which on cooling hardens into a stone-like mass. In this final process of dehydration, however, part of the chlorine goes off as hydrochloric acid, so that the product obtained is contaminated with some oxycbloride. This can be prevented by igniting the salt with sal-ammoniac or - more surely - by effecting the dehydration in a current of anhydrous hydrochloric acid gas. Anhydrous chloride of calcium is witch used in laboratories as a powerful dehydrating agent, It combines with ammonia gas into a solid compound. It dissolves in methyl alcohol and in ethyl alcohol, forming crystallizable " alcoholates " (Graham), compounds of CaCl2 with "crystal alcohol."

Nitrate of Calcium., Ca(NO3)2, crystallizing with 41-1,0, is a very hygroscopic salt, soluble in even absolute alcohol. It is mentioned here as a material for the convenient preparation of pure lime - by simple ignition of the salt in a platinum crucible. Regarding bleaching powder, a double salt of hypochlorite and chloride of calcium CI – Ca – (C10), see CI11.0 !UNE, VOL v. p. 678.

Fluoride of Calcium, CaF5, obtainable by precipitation of chloride of calcium with an alkaline fluoride., occurs in nature as FraronsPan (q.v.). Fluoride of calcium is widely disseminated throughout the mineral kingdom ae an admixture with other minorals. All native forms of phosphate of lime contain it ; some in considerable quantity. Traces of it are found in bones and in the ashes of most plants.

Metallic Calcium cannot be prepared by the reduction of the oxide with charcoal. It may be produced, however, by the electrolysis of the fused chloride or - more conveniently - by heating the iodide Cal., (seven parts) with sodium (one part) in an iron crucible. The metal has a yellow colour; it is somewhat harder than lead, and very malleable and ductile (Lies-Bodart and Jobin). The specific gravity is 1.578 (Bunsen and Matthieson). It does not tarnish in dry air, but readily decomposes water, with evolution of hydrogen and formation of hydrate of lime. It is practically non-volatile. When heated in air or oxygen it burns with a most brilliant light into oxide, CaO.

Tests. - Solutions of ordinary calcium salts are not affected visibly bysulphuretted hydrogen, sulphide of ammonium, or pure ammonia. Carbonate of ammonia, even in the presence of sabammoniac, precipitates the carbonate. So far calcium behaves like barium and strontium. From the former it is distinguished by its not being precipitated by either hydrofluosilieic acid or bichroMate of potash, and from both by its spectrum and the relatively large solubility of its sulphate in water. The latter is obtained from any calcium solution by addition of sulphuric acid and alcohol. The sulphate is washed with alcohol on a filter. When then boiled with water it yields a solution which, dilute as it is, gives a very distinct pre.

cipitate with oxalate of ammonia (barium and strontium sulphates in these circumstances give negative results). Oxalate of ammonia is the most delicate precipitant for calcium ; the precipitate is insoluble in water, in ammonia and ammonia salts, and in acetic acid. From solutions (in acids) of phosphate or oxalate of calcium ammonia and likewise sulphide of ammonium precipitate the metal as phosphate or oxalate. To detect it in such a precipitate, dissolve in hydrochloric acid and add sulphuric acid and alcohol. The calcium is precipitated as sulphate, winch can be identified as just explained.

For the phosphates of lime, see PHOSPHATES. (WV. D.)



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