An Elementary Study of Chemistry by William McPherson (best english books to read for beginners .txt) 📕
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A number of elements act in this same way, their hydroxides under some conditions being weak acids and under others weak bases.
ALLOYSSome metals when melted together thoroughly intermix, and on cooling form a homogeneous, metallic-appearing substance called an alloy. Not all metals will mix in this way, and in some cases definite chemical compounds are formed and separate out as the mixture solidifies, thus destroying the uniform quality of the alloy. In general the melting point of the alloy is below the average of the melting points of its constituents, and it is often lower than any one of them.
Antimony forms alloys with many of the metals, and its chief commercial use is for such purposes. It imparts to its alloys high density, rather low melting point, and the property of expanding on solidification. Such an alloy is especially useful in type founding, where fine lines are to be reproduced on a cast. Type metal consists of antimony, lead, and tin. Babbitt metal, used for journal bearings in machinery, contains the same metals in a different proportion together with a small percentage of copper.
BISMUTHOccurrence. Bismuth is usually found in the uncombined form in nature. It also occurs as oxide and sulphide. Most of the bismuth of commerce comes from Saxony, and from Mexico and Colorado, but it is not an abundant element.
Preparation. It is prepared by merely heating the ore containing the native bismuth and allowing the melted metal to run out into suitable vessels. Other ores are converted into oxides and reduced by heating with carbon.
Physical properties. Bismuth is a heavy, crystalline, brittle metal nearly the color of silver, but with a slightly rosy tint which distinguishes it from other metals. It melts at a low temperature (270°) and has a density of 9.8. It is not acted upon by the air at ordinary temperatures.
Chemical properties. When heated with the blowpipe on charcoal, bismuth gives a coating of the oxide Bi2O3. This has a yellowish-brown color which easily distinguishes it from the oxides formed by other metals. It combines very readily with the halogen elements, powdered bismuth burning readily in chlorine. It is not very easily acted upon by hydrochloric acid, but nitric and sulphuric acids act upon it in the same way that they do upon copper.
Uses. Bismuth finds its chief use as a constituent of alloys, particularly in those of low melting point. Some of these melt in hot water. For example, Wood's metal, consisting of bismuth, lead, tin, and cadmium, melts at 60.5°.
Compounds of bismuth. Unlike the other elements of this group, bismuth has almost no acid properties. Its chief oxide, Bi2O3, is basic in its properties. It dissolves in strong acids and forms salts of bismuth:
The nitrate and chloride of bismuth can be obtained as well-formed colorless crystals. When treated with water the salts are decomposed in the manner explained in the following paragraph.
HYDROLYSISMany salts such as those of antimony and bismuth form solutions which are somewhat acid in reaction, and must therefore contain hydrogen ions. This is accounted for by the same principle suggested to explain the fact that solutions of potassium cyanide are alkaline in reaction (p. 210). Water forms an appreciable number of hydrogen and hydroxyl ions, and very weak bases such as bismuth hydroxide are dissociated to but a very slight extent. When Bi+++ ions from bismuth chloride, which dissociates very readily, are brought in contact with the OH- ions from water, the two come to the equilibrium expressed in the equation
For every hydroxyl ion removed from the solution in this way a hydrogen ion is left free, and the solution becomes acid in reaction.
Reactions of this kind and that described under potassium cyanide are called hydrolysis.
DEFINITION: Hydrolysis is the action of water upon a salt to form an acid and a base, one of which is very slightly dissociated.
Conditions favoring hydrolysis. While hydrolysis is primarily due to the slight extent to which either the acid or the base formed is dissociated, several other factors have an influence upon the extent to which it will take place.
1. Influence of mass. Since hydrolysis is a reversible reaction, the relative masses of the reacting substances influence the point at which equilibrium will be reached. In the equilibrium
the addition of more water will result in the formation of more bismuth hydroxide and hydrochloric acid. The addition of more hydrochloric acid will convert some of the bismuth hydroxide into bismuth chloride.
2. Formation of insoluble substances. When one of the products of hydrolysis is nearly insoluble in water the solution will become saturated with it as soon as a very little has been formed. All in excess of this will precipitate, and the reaction will go on until the acid set free increases sufficiently to bring about an equilibrium. Thus a considerable amount of bismuth and antimony hydroxides are precipitated when water is added to the chlorides of these elements. The greater the dilution the more hydroxide precipitates. The addition of hydrochloric acid in considerable quantity will, however, redissolve the precipitate.
Partial hydrolysis. In many cases the hydrolysis of a salt is only partial, resulting in the formation of basic salts instead of the free base. Most of these basic salts are insoluble in water, which accounts for their ready formation. Thus bismuth chloride may hydrolyze by successive steps, as shown in the equations
The basic salt so formed may also lose water, as shown in the equation
The salt represented in the last equation is sometimes called bismuth oxychloride, or bismuthyl chloride. The corresponding nitrate, BiONO3, is largely used in medicine under the name of subnitrate of bismuth. In these two compounds the group of atoms, BiO, acts as a univalent metallic radical and is called bismuthyl. Similar basic salts are formed by the hydrolysis of antimony salts.
EXERCISES1. Name all the elements so far studied which possess allotropic forms.
2. What compounds would you expect phosphorus to form with bromine and iodine? Write the equations showing the action of water on these compounds.
3. In the preparation of phosphine, why is coal gas passed into the flask? What other gases would serve the same purpose?
4. Give the formula for the salt which phosphine forms with hydriodic acid. Give the name of the compound.
5. Could phosphoric acid be substituted for sulphuric acid in the preparation of the common acids?
6. Write the equations for the preparation of the three sodium salts of orthophosphoric acid.
7. Why does a solution of disodium hydrogen phosphate react alkaline?
8. On the supposition that bone ash is pure calcium phosphate, what weight of it would be required in the preparation of 1 kg. of phosphorus?
9. If arsenopyrite is heated in a current of air, what products are formed?
10. (a) Write equations for the complete combustion of hydrosulphuric acid, methane, and arsine. (b) In what respects are the reactions similar?
11. Write the equations for all the reactions involved in Marsh's test for arsenic.
12. Write the names and formulas for the acids of antimony.
13. Write the equations showing the hydrolysis of antimony trichloride; of bismuth nitrate.
14. In what respects does nitrogen resemble the members of the phosphorus family?
CHAPTER XXI SILICON, TITANIUM, BORONGeneral. Each of the three elements, silicon, titanium, and boron, belongs to a separate periodic family, but they occur near together in the periodic grouping and are very similar in both physical and chemical properties. Since the other elements in their families are either so rare that they cannot be studied in detail, or are best understood in connection with other elements, it is convenient to consider these three together at this point.
The three elements are very difficult to obtain in the free state, owing to their strong attraction for other elements. They can be prepared by the action of aluminium or magnesium on their oxides and in impure state by reduction with carbon in an electric furnace. They are very hard and melt only at the highest temperatures. At ordinary temperatures they are not attacked by oxygen, but when strongly heated they burn with great brilliancy. Silicon and boron are not attacked by acids under ordinary conditions; titanium is easily dissolved by them.
SILICONOccurrence. Next to oxygen silicon is the most abundant element. It does not occur free in nature, but its compounds are very abundant and of the greatest importance. It occurs almost entirely in combination with oxygen as silicon dioxide (SiO2), often called silica, or with oxygen and various metals in the form of salts of silicic acids, or silicates. These compounds form a large fraction of the earth's crust. Most plants absorb small amounts of silica from the soil, and it is also found in minute quantities in animal organisms.
Preparation. The element is most easily prepared by reducing pure powdered quartz with magnesium powder:
Properties. As would be expected from its place in the periodic table, silicon resembles carbon in many respects. It can be obtained in several allotropic forms, corresponding to those of carbon. The crystallized form is very hard, and is inactive toward reagents. The amorphous variety has, in general, properties more similar to charcoal.
Compounds of silicon with hydrogen and the halogens. Silicon hydride (SiH4) corresponds in formula to methane (CH4), but its properties are more like those of phosphine (PH3). It is a very inflammable gas of disagreeable odor, and, as ordinarily prepared, takes fire spontaneously on account of the presence of impurities.
Silicon combines with the elements of the chlorine family to form such compounds as SiCl4 and SiF4. Of these silicon fluoride is the most familiar and interesting. As stated in the discussion of fluorine, it is formed when hydrofluoric acid acts upon silicon dioxide or a silicate. With silica the reaction is thus expressed:
It is a very volatile, invisible, poisonous gas. In contact with water it is partially decomposed, as shown in the equation
The hydrofluoric acid so formed combines with an additional amount of silicon fluoride, forming the complex fluosilicic acid (H2SiF6), thus:
Silicides. As the name indicates, silicides are binary compounds consisting of silicon and some other element. They are very stable at high temperatures, and are usually made by heating the appropriate substances in an electric furnace. The most important one is carborundum, which is a silicide of carbon of the formula CSi. It is made by heating coke and sand, which is a form of silicon dioxide, in an electric furnace, the process being extensively carried on at Niagara Falls. The following equation represents the reaction
The substance so prepared consists of beautiful purplish-black crystals, which are very hard. Carborundum is used as an abrasive, that is, as a material for grinding and polishing very hard substances. Ferrosilicon is a silicide of iron alloyed with an excess of iron, which finds extensive use in the manufacture of certain kinds of steel.
Manufacture of carborundum. The mixture of materials is heated in a large resistance furnace for about thirty-six hours. After the reaction is completed there is left a
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