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
SnO2 + C = Sn + CO2.
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
Sn + 2HCl = SnCl2 + 2H.
With sulphuric acid tin sulphate and sulphur dioxide are formed:
Sn + 2H2SO4 = SnSO4 + SO2 + 2H2O
Concentrated nitric acid oxidizes it, forming a white insoluble compound of the formula H2SnO3, called metastannic acid:
3Sn + 4HNO3 + H2O = 3H2SnO3 + 4NO.
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:
SnO2 + 2KOH = K2SnO3 + H2O.
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:
SnCl2 + 2Cl = SnCl4.
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:
SnCl2 + 2HgCl2 = SnCl4 + 2HgCl.
If the stannous chloride is in excess, the reaction may go further, producing metallic mercury:
SnCl2 + 2HgCl = SnCl4 + 2Hg.
Ferric chloride is in like manner reduced to ferrous chloride:
SnCl3 + 2FeCl3 = SnCl4 + 2FeCl2.
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.