An Introduction to Chemical Science Part 19

Wood ashes originally formed the main source of K2CO3. From plants this substance is taken into the animal system, and makes a portion of its tissue. Sheep excrete it in sweat, which is then absorbed by their wool. Large quant.i.ties are now obtained by washing wool and evaporating the water. K2CO3 and other compounds of K are mainly derived from KCl, beds of which exist in Germany.

In the following list each K compound is prepared like the same Na compound, and the uses of each of the former are similar to those of the latter. K compounds are made in much smaller quant.i.ties than those of Na, as KCl is far less common than NaCl.

{ K KCl { K2SO4 { K2CO3 { KOH KNO3 { { HKCO3

Examine specimens of each, side by side with like Na compounds.

Describe in full their preparation, giving the reactions. Also, perform theexperiments given under Na, subst.i.tuting K therefor.

From KOH are made KClO3 and KCN.

KOH {KCl03 {KCN

245. Pota.s.sium Chlorate.--KCl03 is made by pa.s.sing Cl into a hot concentrated solution of KOH.

6 KOH + 6 Cl = KCl03 + 5 KCl + 3 H2O

Its uses are making O, and as an oxidizing agent.

246. Pota.s.sium Cyanide, KCN, is a salt from HCN--hydrocyanic or prussic acid. Each is about equally poisonous, and more so than any other known substance. A drop of pure HCN on the tongue will produce death quickly by absorption into the system. In examining these compounds take care not to handle them or to inhale the fumes. KCN is used as a solvent for metals in electro-plating, and is the source of many cyanides, i.e. compounds of CN and a metal. KCN is employed to kill insects for cabinet specimens. In a wide-mouthed bottle is placed a little KCN, which is covered with cotton, and over this a perforated paper. The bottle is inverted over the insect, and the fumes destroy life without injuring the delicate parts. HCN is made from KCN and H2SO4.

247. Gunpowder.--Gunpowder is a mixture of KNO3, C, and S. Heat or concussion causes a chemical change, and transforms the solids into gases. These gases at the moment of explosion occupy 1500 or more times the volume of the solids. Hence the great rending power of powder. If not confined, powder burns quietly but quickly. The appended reaction is a part of what takes place, but it by no means represents all the chemical changes.

2KNO3 + S + 3C =K2S + 2N + 3CO2.

From this equation compute the percentage, by weight, of each substance used to make gunpowder economically.

Thoroughly burned charcoal, distilled sulphur, and the purest nitre are powdered and mixed in a revolving drum,made into a paste with water, put under great pressure between sheets of gun metal, granulated, sifted, to separate the coa.r.s.e and fine grains, and glazed by revolving in a barrel which sometimes contains a little powdered graphite.

Experiment 119.--Pulverize and mix intimately 4 g. KNO3, l/2 g.

S, 1/2 g. charcoal. Pile the mixture on a brick, and apply a lighted match. The adhering product can be removed by soaking in water.

AMMONIUM COMPOUNDS.

248. Read the chapter on NH3. Also, review the experiments on bases. Examine NH4Cl, NH4NO3, (NH4)2SO4, (NH4)2CO3.

Ammonium, NH4, is too unstable to exist alone, but it forms salts similar to those of K and Na. NH3 dissolved in water forms NH4OH.

The food of plants, as well as that of animals, must contain N.

It has not yet been shown that they can make use of that contained in the air, but they do absorb its compounds from the soil. All fertilizers and manures contain a soluble compound of NH4. All NH4 compounds are now obtained either from coal, in making illuminating-gas, or from bones, by distillation.

Suppose the product obtained from the gas-house to be NH4OH, how would NH4Cl be made? (NH4)2SO4? NH4NO3? Write the reactions.

(NH4)2CO3 is made by heating NH4Cl with CaCO3. Give the reaction.

Chapter XLVI.

CALCIUM COMPOUNDS.

Examine CaCO3--marble, limestone, chalk, not crayon,--CaSO4 -- gypsum or selenite--CaCl2, CaO.

249. Occurrence.--The above are the chief compounds of Ca. The element itself is not found uncombined, is very difficult to reduce (page 141), is a yellow metal, and has no use. Its most abundant compound is CaCO3. Sh.e.l.ls of oysters, clams, snails, etc., are mainly CaCO3, and coral reefs, sometimes extending thousands of miles in the ocean, are the same. CaCO3 dissolves in water holding CO2, and thence these marine animals obtain it and therefrom secrete their bony framework. All mountains were first laid down on the sea bottom layer by layer, and afterwards lifted up by pressure. Rocks and mountains of CaCO3 were formed by marine animals, and all large ma.s.ses of CaCO3 are thought to have been at one time the framework of animals. Marble is crystallized, transformed limestone. The process, called metamorphism, took place in the depths of the earth, where the heat is greater than at the surface.

250. Lime.--If CaCO3 be roasted with C, CO2 escapes and CaO is left. CaCO3 - CO2 = ? This is called burning lime, and is a large industry in limestone countries. CaO is unslaked lime, quicklime or calcium oxide. It may be slaked either by exposure to the air, air-slaking, when it gradually takes up H2O and CO2; or by mixing with H2O, water-slaking. Ca0 + H2O = Ca(OH)2.

Great heat is generated in the latter case, though not so much as in the formation of KOH and NaOH. Like them, Ca(OH)2 dissolves in water, forming lime-water. Milk of lime, cream of lime, etc., consist of particles of Ca(OH)2 suspended in H2O.

251. Uses of Lime--CaO is infusible at the highest temperatures.

If it be introduced into the oxy-hydrogen blow-pipe (page 28), a brilliant light, second only to the electric, is produced. Mortar is made by mixing CaO, H2O, and Si02. It hardens by evaporating the extra H2O, absorbing CO2 from the air, and uniting with Si02 to form calcium silicate. It often continues to absorb CO2 for hundreds or thousands of years before being saturated, as is found in the Egyptian pyramids. Hence the tenacity of old mortar.

Hydraulic mortar contains silicates of Al and Ca, and is not affected by water. What are the uses of mortar? Being the important const.i.tuent of mortar and plaster, lime is the most useful of the bases.

252. Hard Water.--Review Experiment 76. The solubility of CaCO3 in water that contains CO2 leads to important results. Much dissolves in the waters of all limestone countries; and the water, though perfectly transparent, is hard; i.e. soap has little action on it. See page 187. Such water may be softened by boiling, a deposit of CaCO3 being formed as a crust on the kettle. Such water is called water of temporary hardness. MgCO3 produces a similar effect, and water containing it is softened in the same way. Permanently hard waters contain the sulphates of Ca and Mg, which cannot be removed by boiling, but may be by adding (NH4)2CO3. 253. The Formation of Caves in limestone rocks is due also to the solubility of CaCO3. Water collects on the mountains and trickles down through crevices, dissolving, if it contains CO2, some of the CaCO3, and thus making a wider opening, and forcing its way along fissures and lines of least resistance into the interior of the earth, or out at the base of the mountain.

Its channel widens as it dissolves the rock, and the stream enlarges until in the course of ages an immense cavern may be formed, with labyrinths extending for miles, from the entrance of which a river often issues. In the long ages which elapsed during the slow formation of Mammoth Cave its denizens lost many of the characters of their ancestors, and eyeless fish and also eyeless insects now abound there.

254. Reverse Action.--Drops of water on the roofs of these caverns lose their CO2, and deposit CaCO3. Thus long, pendant ma.s.ses of limestone, called stalact.i.tes, are slowly formed on the roofs like icicles. From these, water charged with CaCO3 drops to the bottom, loses CO2 and deposits CaCO3, which forms an upward- growing ma.s.s, called stalagmite. In time it may meet the stalact.i.te and form a pillar. Notice that the same action which formed the cave is filling it up; i.e. the solubility of CaCO3 in water charged with CO2.

255. Famous Marbles.--The marble from Carrara, Italy, is most esteemed on account of a pinkish tint given by a trace of oxide of iron. The best of Grecian marble was from Paros, one of the Cyclades. The isles of the Mediterranean are of limestone, or of volcanic, origin, often of both. 256. Calcium Sulphate occurs in two forms, (1) with water of crystallization--gypsum, CaSO4 + 2 H2O, --(2) without it--anhydrite, CaSO4. The former, on being strongly heated, gives up its water, and is reduced to a powder-- plaster of Paris. This, on being mixed with water, again takes up 2 H2O, and hardens, or sets, without crystallizing. If once more heated to expel water, it will not again absorb it. When plaster of Paris sets, it expands slightly, and on this account is admirable for taking casts.

257. Uses.--Gypsum finds use as a fertilizer and as an adulterant in coloring-materials, etc. CaSO4 is employed in making casts, molds, statuettes, wall-plaster, crayons, etc.

How can CaCl2 be made? What is its use? See page 27. What else is used for a similar purpose?

Symbolize and name the acid represented by Ca(ClO)2, and name this salt (page 107). It is one of the const.i.tuents of bleaching- powder, the symbol of which, though still under discussion, may be considered Ca(ClO)2 + CaCl2. This is made by pa.s.sing Cl over Ca(OH)2 2 Ca(OH)2 + 4 Cl = Ca(ClO)2 + CaCl2 + 2 H2O.

CHAPTER XLVII.

MAGNESIUM, ALUMINIUM, AND ZINC.

MAGNESIUM AND ITS COMPOUNDS.

Examine magnesite, dolomite, talc, serpentine, hornblende, meerschaum, magnesium ribbon, magnesia alba, Epsom salt.

258. Occurrence and Preparation.--Mg is very widely distributed, but does not occur uncombined. Its salts are found in rocks and soils, in sea water and in the water of some springs, to which they impart a brackish taste.

The most common minerals containing Mg are magnesite, MgCO3, dolomite, MgCO3 + CaCO3, and talc, serpentine, hornblende, and meerschaum. The last four are silicates, and often are unctious to the touch. What proportion of the earth's crust is composed of Mg? See page 173.

259. Metallic Mg is prepared by fusing MgCl2 with Na. Why is the process expensive? Write the reaction.

Experiment 120.--With forceps hold a short strip of Mg ribbon in a flame. Note the brilliancy of the light, and give the reaction.

Examine and name the product.

Photographs of the interior of caverns, where sunlight does not penetrate, are taken by Mg light. Gun-cotton sprinkled with powdered Mg has recently been employed for that purpose. Mg tarnishes slightly in moist air. Compounds of Mg.--MgO, magnesia, like CaO, is very infusible, and is used for crucibles. Magnesia alba, a variable mixture of MgCO2 and Mg(OH)2, is employed in medicine, as is also Epsom salt, MgSO4 + 7 H2O.

ALUMINIUM AND ITS COMPOUNDS.

Examine aluminium, aluminium bronze, corundum, emery, feldspar, argillite, clay. Note especially the color, l.u.s.ter, specific gravity and flexibility of Al.

What elements are more common in the earth than Al? What metals?