Chapter 79

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 constitutes 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 glass. 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 glass, infusible except in the oxy- hydrogen blow- pipe, and harder than glass. Rock crystal is massive 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 masscolloid--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 masses. 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 glass, sodium or potassium 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 glass.