The family. Tin and lead, together with silicon and germanium, form a family in Group IV of the periodic table. Silicon has been discussed along with the non-metals, while germanium, on account of its rarity, needs only to be mentioned.

The other family of Group IV includes carbon, already described, and a number of rare elements.

TIN

Occurrence. Tin is found in nature chiefly as the oxide (SnO₂), called cassiterite or tinstone. The most famous mines are those of Cornwall in England, and of the Malay Peninsula and East India Islands; in small amounts tinstone is found in many other localities.

Metallurgy. The metallurgy of tin is very simple. The ore, separated as far as possible from earthy materials, is mixed with carbon and heated in a furnace, the reduction taking place readily. The equation is

The metal is often purified by carefully heating it until it is partly melted; the pure tin melts first and can be drained away from the impurities.

Properties. Pure tin, called block tin, is a soft white metal with a silver-like appearance and luster; it melts readily (235°) and is somewhat lighter than copper, having a density of 7.3. It is quite malleable and can be rolled out into very thin sheets, forming tin foil; most tin foil, however, contains a good deal of lead.

Under ordinary conditions it is quite unchanged by air or moisture, but at a high temperature it burns in air, forming the oxide SnO₂. Dilute acids have no effect upon it, but concentrated acids attack it readily. Concentrated hydrochloric acid changes it into the chloride

With sulphuric acid tin sulphate and sulphur dioxide are formed:

Concentrated nitric acid oxidizes it, forming a white insoluble compound of the formula H₂SnO₃, called metastannic acid:

Uses of tin. A great deal of tin is made into tin plate by dipping thin steel sheets into the melted metal. Owing to the way in which tin resists the action of air and dilute acids, tin plate is used in many ways, such as in roofing, and in the manufacture of tin cans, cooking vessels, and similar articles.

Many useful alloys contain tin, some of which have been mentioned in connection with copper. When tin is alloyed with other metals of low melting point, soft, easily melted alloys are formed which are used for friction bearings in machinery; tin, antimony, lead, and bismuth are the chief constituents of these alloys. Pewter and soft solder are alloys of tin and lead.

Compounds of tin. Tin forms two series of compounds: the stannous, in which the tin is divalent, illustrated in the compounds SnO, SnS, SnCl₂; the stannic, in which it is tetravalent as shown in the compounds SnO₂, SnS₂. There is also an acid, H₂SnO₃, called stannic acid, which forms a series of salts called stannates. While this acid has the same composition as metastannic acid, the two are quite different in their chemical properties. This difference is probably due to the different arrangement of the atoms in the molecules of the two substances. Only a few compounds of tin need be mentioned.

Stannic oxide (SnO₂). Stannic oxide is of interest, since it is the chief compound of tin found in nature. It is sometimes found in good-sized crystals, but as prepared in the laboratory is a white powder. When fused with potassium hydroxide it forms potassium stannate, acting very much like silicon dioxide:

Chlorides of tin. Stannous chloride is prepared by dissolving tin in concentrated hydrochloric acid and evaporating the solution to crystallization. The crystals which are obtained have the composition SnCl₂·2H₂O, and are known as tin crystals. By treating a solution of stannous chloride with aqua regia, stannic chloride is formed:

The salt which crystallizes from such a solution has the composition SnCl₄·5H₂O, and is known commercially as oxymuriate of tin. If metallic tin is heated in a current of dry chlorine, the anhydrous chloride (SnCl₄) is obtained as a heavy colorless liquid which fumes strongly on exposure to air.

The ease with which stannous chloride takes up chlorine to form stannic chloride makes it a good reducing agent in many reactions, changing the higher chlorides of metals to lower ones. Thus mercuric chloride is changed into mercurous chloride:

If the stannous chloride is in excess, the reaction may go further, producing metallic mercury:

Ferric chloride is in like manner reduced to ferrous chloride:

The chlorides of tin, as well as the alkali stannates, are much used as mordants in dyeing processes. The hydroxides of tin and free stannic acid, which are easily liberated from these compounds, possess in very marked degree the power of fixing dyes upon fibers, as explained under aluminium.

LEAD

Occurrence. Lead is found in nature chiefly as the sulphide (PbS), called galena; to a much smaller extent it occurs as carbonate, sulphate, chromate, and in a few other forms. Practically all the lead of commerce is made from galena, two general methods of metallurgy being in use.

Metallurgy. 1. The sulphide is melted with scrap iron, when iron sulphide and metallic lead are formed; the liquid lead, being the heavier, sinks to the bottom of the vessel and can be drawn off:

2. The sulphide is roasted in the air until a part of it has been changed into oxide and sulphate. The air is then shut off and the heating continued, the reactions indicated in the following equations taking place:

The lead so prepared usually contains small amounts of silver, arsenic, antimony, copper, and other metals. The silver is removed by Parkes's method, as described under silver, and the other metals in various ways. The lead of commerce is one of the purest commercial metals, containing as a rule only a few tenths per cent of impurities.

Properties. Lead is a heavy metal (den. = 11.33) which has a brilliant silvery luster on a freshly cut surface, but which soon tarnishes to a dull blue-gray color. It is soft, easily fused (melting at 327°), and quite malleable, but has little toughness or strength.

It is not acted upon to any great extent by the oxygen of the air under ordinary conditions, but is changed into oxide at a high temperature. With the exception of hydrochloric and sulphuric acids, most acids, even very weak ones, act upon it, forming soluble lead salts. Hot, concentrated hydrochloric and sulphuric acids also attack it to a slight extent.

Uses. Lead is employed in the manufacture of lead pipes and in large storage batteries. In the form of sheet lead it is used in lining the chambers of sulphuric acid works and in the preparation of paint pigments. Some alloys of lead, such as solder and pewter (lead and tin), shot (lead and arsenic), and soft bearing metals, are widely used. Type metal consists of lead, antimony, and sometimes tin. Compounds of lead form several important pigments.

Compounds of lead. In nearly all its compounds lead has a valence of 2, but a few corresponding to stannic compounds have a valence of 4.

Lead oxides. Lead forms a number of oxides, the most important of which are litharge, red lead or minium, and lead peroxide.

1. Litharge (PbO). This oxide forms when lead is oxidized at a rather low temperature, and is obtained as a by-product in silver refining. It is a pale yellow powder, and has a number of commercial uses. It is easily soluble in nitric acid:

2. Red lead, or minium (Pb₃O₄). Minium is prepared by heating lead (or litharge) to a high temperature in the air. It is a heavy powder of a beautiful red color, and is much used as a pigment.

3. Lead peroxide (PbO₂). This is left as a residue when minium is heated with nitric acid:

Pb₃O₄ + 4HNO₃ = 2Pb(NO₃)₂ + PbO₂ + 2H₂O.

It is a brown powder which easily gives up a part of its oxygen and, like manganese dioxide and barium dioxide, is a good oxidizing agent.

Soluble salts of lead. The soluble salts of lead can be made by dissolving litharge in acids. Lead acetate (Pb(C₂H₃O₂)₂·3H₂O), called sugar of lead, and lead nitrate (Pb(NO₃)₂) are the most familiar examples. They are while crystalline solids and are poisonous in character.

Insoluble salts of lead; lead carbonate. While the normal carbonate of lead (PbCO₃) is found to some extent, in nature and can be prepared in the laboratory, basic carbonates of varying composition are much more easy to obtain. One of the simplest of these has the composition 2PbCO₃·Pb(OH)₂. A mixture of such carbonates is called white lead. This is prepared on a large scale as a paint pigment and as a body for paints which are to be colored with other substances.

Lead sulphide (PbS). In nature this compound occurs in highly crystalline condition, the crystals having much the same luster as pure lead. It is readily prepared in the laboratory as a black precipitate, by the action of hydrosulphuric acid upon soluble lead salts:

It is insoluble both in water and in dilute acids.

Other insoluble salts. Lead chromate (PbCrO₄) is a yellow substance produced by the action of a soluble lead salt upon a soluble chromate, thus:

It is used as a yellow pigment. Lead sulphate (PbSO₄) is a white substance sometimes found in nature and easily prepared by precipitation. Lead chloride (PbCl₂) is likewise a white substance nearly insoluble in cold water, but readily soluble in boiling water.

EXERCISES

1. How could you detect lead if present in tin foil?

2. Stannous chloride reduces gold chloride (AuCl₃) to gold. Give equation.

3. What are the products of hydrolysis when stannic chloride is used as a mordant?

4. How could you detect arsenic, antimony, or copper in lead?

5. Why is lead so extensively used for making water pipes?

6. What sulphates other than lead are insoluble?

7. Could lead nitrate be used in place of barium chloride in testing for sulphates?

8. How much lead peroxide could be obtained from 1 kg. of minium?

9. The purity of white lead is usually determined by observing the volume of carbon dioxide given off when it is treated with an acid. What acid should be used? On the supposition that it has the formula 2PbCO₃·Pb(OH)₂, how nearly pure was a sample if 1 g. gave 30 cc. of carbon dioxide at 20° and 750 mm.?

10. Silicon belongs in the same family with tin and lead. In what respects are these elements similar?

11. What weight of tin could be obtained by the reduction of 1 ton of cassiterite?

12. What reaction would you expect to take place when lead peroxide is treated with hydrochloric acid?

13. White lead is often adulterated with barytes. Suggest a method for detecting it, if present, in a given example of white lead.