An Introduction to Chemical Science Part 17

Examine metallic a.r.s.enic, realgar, orpiment, a.r.s.enopyrite, a.r.s.enic trioxide, copper a.r.s.enite.

The compounds of a.r.s.enic are very poisonous if taken into the system, and must be handled with care.

207. Separation. Experiment 115.--Draw out into two parts in the Bunsen flame a piece of gla.s.s tubing 20cm long and 1 or 2cm in diameter. Into the end of one of the ignition tubes thus formed, when it is cool, put one-fourth of a gram of a.r.s.enic trioxide, As2O3, using paper to transfer it. Now put into the tube a piece of charcoal, and press it down to within 2 or 3cm of the AS2O3 (Fig. 45). Next heat the coal red-hot, and then at once heat the As203. Continue this process till you see a metallic sublimate- metallic mirror-on the tube above the coal. Break the tube and examine the sublimate. It is As. Heat vaporizes the As2O;3.

Explain the chemical action. What is the agency of C in the experiment? Of As2O3? 2 As2O3 + 3 C = ?

208. Tests.-Experiments 115 and 116 are used as tests for the presence of a.r.s.enic.

Experiment 116.--Prepare a H generator, - a flask with a thistle- tube and a philosopher's lamp tube (Fig. 46), put in some granulated Zn, water, and HCl. Test the purity of the escaping gas (Experiment 23), and when pure, light the jet of H. H is now burning in air. To be sure that there is no As in the ingredients used, hold the inside of a porcelain evaporating-dish directly against the flame for a minute. If no silvery-white mirror is found, the chemicals are free from As. Then pour through the thistle-tube, while the lamp is still burning, 1cc.solution of AS2O3 in HCl or H2O a bit of As2O3 not larger than a grain of wheat in 10 cc. HCl.

See whether the color of the flame changes; then hold the evaporating-dish once more in the flame, and notice a metallic deposit of As. Set away the apparatus under the hood and leave the light burning.

This experiment must not be performed unless all the cautions are observed, since the gas in the flask (AsH3) is the most poisonous known, and a single bubble of it inhaled is said to have killed the discoverer. By confining the gas inside the flask there is no danger.

Instead of using As2O3 solution, a little Paris green, wall paper suspected of containing a.r.s.enic, green silk, or green paper labels, etc., may be soaked in HCl, and tested.

209. Explanation.--The chemical changes are as follows: The compounds of As, in this case As2O3, in presence of nascent H, are immediately converted into the deadly hydrogen a.r.s.enide (arsine, a.r.s.eniuretted hydrogen), AsH3. As2O3 + 12 H = 2 AsH3 + 3 H2O. The AsH3 mixed with excess of H tends to escape and is burned to As2O3 and H2O, and thus is rendered comparatively harmless as it pa.s.ses into the air. This is why the flame must be burning when the a.r.s.enic compound is introduced. 2 AsH3 + 6 O = As2O3 + 3 H2O.

In the combustion of AsH3, H burns at a lower point than As. The introduction of a cold body like porcelain cools the flame below the kindling-point of As, and this is deposited, while H burns, in exactly the same way as lamp- black was collected in Experiment 26.

210. Expert a.n.a.lysis.--A modification of this experiment is employed by experts to test for AS2O3 poisoning. The organs.-- stomach or liver--are cut into small pieces dissolved by nascent Cl, or HClO, made from KC1O3 and HCl, and the solution is introduced into a H generator, as above. AS2O3 preserves the tissues it comes in contact with, for a long time, and the test can be made years after death. All the chemicals must be pure, since As is found in small quant.i.ties in most ores, and the Zn, HCl, and H2SO4 of commerce are very likely to contain it. The above is called Marsh's test, and is so delicate that a mere trace of a.r.s.enic can be detected.

211. Properties and Occurrence.--As is a grayish white solid, of metallic l.u.s.ter, while a few of its characters are non-metallic.

It is very widely distributed, being sometimes found native, and sometimes combined, as a.s.s, realgar, As2S8, orpiment, and Fea.s.s, a.r.s.enopyrite. Its chief source is the last, the fine powder of which is strongly heated, when As separates and sublimes. It has the odor of garlic, as may be observed by heating a little on charcoal with the blow-pipe.

212. Atomic Volume.--As is peculiar in that its atomic volume, so far as the volume can be determined, is only half that of the H atom. Its vapor density is 150, which gives 300 for the molecular weight, while its least combining or atomic weight is 75. 300, the molecular weight = 75, the atomic weight =4, the number of atoms in the molecule. All gaseous molecules being of the same size, represented by two squares, the atomic volume of As must be one-fourth of this size, represented by half of one square. Of what other element is this true? 213. Uses of As2O3.-a.r.s.enic is used in shot-manufacture, for hardening the metal. Its most important compound is As2O3, a.r.s.enic trioxide, called also a.r.s.enious anhydride, a.r.s.enious acid, white a.r.s.enic, etc. So poisonous is this that enough could be piled on a one-cent piece to kill a dozen persons. Taken in too large quant.i.ties it acts as an emetic. The antidote is ferric hydrate Fe2(OH)6 and a mustard emetic, followed by oil or milk.

The vapor density of this compound shows that its symbol should be As4O6, but the improper one, As2O3, is likely to remain in use. Another oxide, As2O5, a.r.s.enic pentoxide, exists, but is less important. Show how the respective acid formulae are obtained from these anhydrides. See page 50.

AS2O3 is used in making Paris green; in many green coloring materials, in which it exists as copper a.r.s.enite; in coloring wall papers, and in fly and rat poisons. It is employed for preserving skins, etc. Fashionable women sometimes eat it for the purpose of beautifying the complexion, to which it imparts a ghastly white, unhealthy hue. Mountaineers in some parts of Europe eat it for the greater power of endurance which it is supposed to give them. By beginning with small doses these a.r.s.enic-eaters finally consume a considerable quant.i.ty of the poison with apparent impunity; but as soon as the habit is stopped, all the pangs of a.r.s.enic-poisoning set in. Wall paper containing a.r.s.enic is said to be injurious to some people, while apparently harmless to others.

Chapter XLI.

SILICON, SILICA, AND SILICATES.

214. Comparison of Si and C.--The element Si resembles carbon in valence and in allotropic forms. It occurs in three forms like C, a diamond form, a graphite, and an amorphous. C forms the basis of the vegetable and animal world; Si, of the mineral. Most soils and rocks, except limestone, are mainly compounds of O, Si, and metals. While O is estimated to make up nearly one- half of the known crust of the earth, Si const.i.tutes fully a third. The two are usually combined, as silica, SiO2, or silicates, SiO2 combined with metallic oxides. This affinity for O is so strong that Si is not found uncombined, and is separated with great difficulty and only at the highest temperatures. No special use has yet been found for it, except as an alloy with Al. Its compounds are very important.

215 Silica.--Examine some specimens of quartz, rock crystal, white and colored sands, agate, jasper, flint, etc.; test their hardness with a knife blade, and see whether they will scratch gla.s.s. Notice that quartz crystals are hexagonal or six-sided prisms, terminated by hexagonal pyramids. The coloring matters are impurities, often Fe and Mn, if red or brown. When pure, quartz is transparent as gla.s.s, infusible except in the oxy- hydrogen blow- pipe, and harder than gla.s.s. Rock crystal is ma.s.sive Si02. Sand is generally either silica or silicates.

The common variety of Si02 is not soluble in water or in acids, except HF. An amorphous variety is to some extent soluble in water. Most geysers deposit the latter in successive layers about their mouths. Agate, chalcedony, and opal have probably an origin similar to this. A solution of this variety of SiO2 forms a jelly-like ma.s.scolloid--which will not diffuse through a membrane of parchment -dialyzer--when suspended in water. Crystalloids will diffuse through such a membrane, if they are in solution.

This principle forms the basis of dialysis.

All substances are supposed to be either crystalloids, i.e.

susceptible of crystallization, or colloids-jelly-like ma.s.ses.

HCl is the most diffusible in liquids of all known substances; caramel is one of the least so. To separate the two, they would be put into a dialyzer suspended in water, when HCl will diffuse through into the water, and caramel will remain. As2O3, in cases of suspected poisoning, was formerly separated from the stomach in this way, as it is a crystalloid, whereas most of the other contents of the stomach are colloidal.

216. Silicates.--Si is a tetrad. SiO2 + 2 H2O =? Si02 + H2O =? In either case the product is called silicic acid. Replace all the H with Na, and name the product. Replace it with K; Mg; Fe; Ph; Ca.

Na4SiO4 and Na2SiO3 are typical silicates of Na, but others exist.

217. Formation of SiO2 from Sodium Silicate. Experiment 117.--To 5cc.Na4SiO4 in au evaporating-dish add 5cc. HCl. Describe the effect. Pour away any extra HCl. Heat the residue gently, above a flame, till it becomes white, then cool it and add water. In a few minutes taste a drop of the water, then pour it off, leaving the residue. Crush a little in the fingers, and compare it with white sand, SiO2. Apply to the experiment these equations: - Na4SiO4 + 4 HCl = 4 NaCl + H4SiO4. H4SiO4 + 2 H2O = Si02. Why was H4Si04 heated? Why was water finally added?

Water gla.s.s, sodium or pota.s.sium silicate, used somewhat for making artificial stone, is made by fusing SiO2 with Na2CO3 or K2CO3, and dissolving in water. Silicic acid forms the basis of a very important series of compounds, - the silicates. The above two are the only soluble ones, and may be called liquid gla.s.s.

Chapter XLII.

GLa.s.s AND POTTERY.

Examine white sand, calcium carbonate, sodium carbonate, smalt; bottle, window, Bohemian and flint gla.s.s.

218. Gla.s.s is an Artificial Silicate.--Si02 alone is almost infusible, as is also Ca0; but mixed and heated the two readily fuse, forming calcium silicate. Ca0 + SiO2 = ? Notice that Si02 is the basis of an acid, while CaO is essentially a base, and the union of the two forms a salt. There are four princ.i.p.al kinds of gla.s.s: (1) Bohemian, a silicate of K and Ca, not easily fused, and hence used for chemical apparatus where high temperatures are required; (2) window or plate gla.s.s, a silicate of Na and Ca; (3) bottle gla.s.s, a silicate of Na, Ca, Al, Fe, etc., a variety which is impure, and is tinged green by salts of Fe; (4) flint gla.s.s, a silicate of K and Pb, used for lenses in optical instruments, cut gla.s.s ware, and, with B added, for paste, or imitation diamonds, etc. Pb gives to gla.s.s high refracting power, which is a valuable property of diamonds, as well as of lenses.

219. Manufacture.--Pure white sand, Si02, is mixed with CaCO3 and Na2CO3, some old gla.s.s - cullet - is added, and the mixture is fused in fire-clay crucibles. For flint gla.s.s, Pb304, red lead, is employed. If color is desired, mineral coloring matter is also added, but not always at this stage. CoO, or smalt, gives blue; uranium oxide, green; a mixture of Au and Sn of uncertain composition, called the "purple of Ca.s.sius," gives purple. MnO2 is used to correct the green tint caused by FeO, which it is supposed to oxidize. Opacity, or enamel, as in lamp-shades, is produced by adding As2O3, Sb2O3, SnO2, cryolite, etc. The gla.s.s- worker dips his blowpipe--a hollow iron rod five or six feet long--into the fused ma.s.s of gla.s.s, removes a small portion, rolls it on a smooth surface, swings it round in the air, blowing meanwhile through the rod, and thus fashions it as desired, into bottles, flasks, etc. For some wares, e.g. common goblets, the gla.s.s is run into molds and stamped; for others it is blown and welded. All gla.s.s must be annealed, i.e. cooled slowly, for several days. The molecules thus arrange themselves naturally. If not annealed, it breaks very easily. It may be greatly toughened by dipping, when nearly red-hot, into hot oil. Cut gla.s.s is prepared at great expense by subsequent grinding. Gla.s.s may be rendered semi-opaque by etching either with HF, or with a blast of sand.

220. Importance.--Few manufactured articles have more importance than gla.s.s. Without it the sciences of chemistry, physics, astronomy, microscopic anatomy, zoology, and botany, not to mention its domestic uses, would be almost impossible.

221. Porcelain and Pottery.--Genuine porcelain and china-ware are made of a fine clay, kaolin, which results from the disintegration of feldspathic rocks. Bricks are baked clay. The FeO in common clay is oxidized to Fe2O3, on heating, a process which gives their red color. Some clay, having no Fe, is white; this is used for fire-bricks and clay pipes. That containing Fe is too fusible for fire-clay, which must also have much SiO2. The electric arc, however, will melt even this, and the most refractory vessels are of calcium oxide or of graphite. Pottery is clay, molded, baked, and either glazed, like crockery, or unglazed, like flower-pots. Jugs and coa.r.s.e earthenware are glazed by volatilizing NaCl in an oven which holds the porous material. This coats the ware with sodium silicate. To glaze china, it is dipped into a powder of feldspar and SiO2 suspended in water and vinegar, and then fused. If the ware and glaze expand uniformly with heat, the latter does not crack.

Chapter XLIII.

METALS AND THEIR ALLOYS.

222. Comparison of Metals and Non-Metals.--The majority of elements are metals, only about a dozen being non-metallic in their properties. The division line between the two cla.s.ses is not very well defined; e.g. As has certain properties which ally it to metals; it has other properties which are non-metallic. H occupies a place between the two cla.s.ses. The following are the more marked characteristics of each group: -

METALS.

1. Metals are solid at ordinary temperatures, and usually of high specific gravity.

Exceptions: Hg is liquid above -39.5 degees; Li is the lightest solid known; Na and K will float on water.

2. Metals reflect light in a way peculiar to themselves. They have what is called a metallic l.u.s.ter.

3. They are white or gray. Exceptions: Au, Ca, Sr are yellow; Cu is red.

4. In general they conduct heat and electricity well.

NON-METALS. 1. Non-metals are either gaseous or solid at ordinary temperatures, and of low specific gravity. Exceptions: Br is a liquid; I has the heaviest known vapor.

2. Non-metallic solids have different l.u.s.ters, as gla.s.sy, resinous- silky, etc. Exceptions: I, B, and C have metallic l.u.s.ter.

3. Non-metals have no characteristic color.

4. They are non-conductors of heat and electricity. Exceptions: C and some others are conductors. 5. They are usually malleable and ductile.

6. They form alloys, or "chemical mixtures," with one another, similar to other solutions. Exceptions: Some, as Ph and Zn, will not alloy with one another.

7. Metals are electro-positive elements, and unite with O and H to form bases. Exceptions: Some of the less electro-positive metals, with a large quant.i.ty of O, form acids, as Cr, As, etc.

Numbers 2, 6, and 7 are the most characteristic and important properties.