Quartz

Have all their valence forces satisfied, as is shown by the high surface activity of silica. In each of the various crystalline forms of silica, there is a special pattern which is Arrangement of four oxygen atoms around each silicon persists in each crystalline form but the Repeated throughout the crystal in a regular definite crystal lattice. The regular tetrahedral Si-O-Si bond angles and the rotation about each Si-O bond are different in the different Polymorphic species.

When crystalline silica is heated sufficiently it melts to give a viscous liquid having a Random structure, presumably with the silicon atoms still on the average close to four oxygenA and the oxygen atoms close to two silicon atoms. When this liquid silica is cooled it does Not crystallize readily, but usually it under supercools tremendously and eventually becomes Rigid without having undergone orientation into a regular crystal pattern. This rigid, highly under Supercooled liquid is called vitreous silica or silica glass (frequently incorrectly referred to as Fused quartz).

3.5.3 Silicates and their Uses

The compounds derived from silicic acids are termed as silicates.

Sodium silicate, Na, SiO,

This is a sodium salt of metasilicic acid H,SiO, It is known as water glass or soluble Glass. It is prepared by fusing sodium carbonate with pure sand. The process is carried out in a Furnace called reverberatory furnace.

Properties:

Hydrolysis.

Sodium silicate is soluble in water and its solution is strongly alkaline due to the Hydrolysis.

• Chemical Garden :When crystals of soluble coloured salts like nickel chloride, ferrous sulphate, copper sulphate Or cobalt nitrate, etc. are placed in a solution of sodium silicate, they produce a very beautiful Growth, like plant, which is called chemical garden.

Uses of Sodium Silicate:

1. It is used as a filler for soap in soap industry.

2. It is used in textile as a fire proof agents.

3. It is used as furniture polish.

4. It is also used in calico printing,

Aluminium Silicate

Many important silicate rocks contain aluminium. The weathering of these rocks results In the disintegration of the complex silicates which they contain. The boiling and freezing ofW in the rocks, and the chemical action of water and carbon dioxide convert these compounds Into potassium carbonate, sand and clay. The following reaction explains the weathering of Potassium feldspar.

Pure clay, which has the formula shown above, is white and is called kaolin. It is used to Make porcelain and china wares. Ordinary clay contains compounds of iron and other metals and It has a yellow or reddish yellow colour.

Impure clays can be more easily fused because they contain oxides of iron, calcium, Magnesium and other metals which form easily fusiable silicates with sand. Such clays are used To make bricks, tiles, and stonewares. Due to the presence of ferric oxide, the articles of this clay Turn reddish when heated in a kiln.

Stoneware is usually glazed to give it a less porous surface by throwing salt (NaCl) upon The articles while they are hot. This treatment produces sodium aluminate and sodium aluminium Silicate, which melt readily and cover the entire surface. When the article cools, the covering Solidifies, producing a compact, smooth, waterproof surface. China wares are made from a Mixture of kaolin, bone ash, and feldspar; the mixture fuses when heated and fills the pores Between the grains of kaolin.

The use of clay in making pottery and other ceramic articles depends upon the plasticity Of the paste. When soaked in water the clay progressively hydrates, and the paste becomes more Plastic. When the clay is heated the water of hydration is lost, and a hard rock like mass is formed.

Tale or Soapstone

The magnesium silicate, Mg,H,(SiO₂),, is commonly known as talc or soapstone. It is Physically greasy to touch. Therefore it is used in making cosmetics. It is also used in making Household articles.

Asbestos

Asbestos is hydrated calcium magnesium silicate CaMg,(SiO₂). It is commonly used in Making incombustible fabrics and hardboard, etc.

Silicones

The chemistry of silicon is, in many respects, very much like the chemistry of carbon. Just as carbon forms the compounds carbon dioxide (CO₂), carbon tetrachloride (CCI), and Methane (CH,), similarly silicon forms silicon dioxide (SiO₂), silicon tetrachloride (SICI), and

Silane (SiH).

The silicon atom holds four methyl groups, Si(CH₂),, just as the carbon atom, C(CH₂),. If A compound of silicon containing chlorine atoms and methyl groups, SiCl,(CH₂)₂, is allowed to React with water, hydrogen chloride (HCI) comes out, and the silicon atoms join together through Oxygen atoms.

By this reaction we can make synthetically the silicon oxygen chains found in the Mineral silicates, which we have just been discussing. A difference is that here we have Groups instead of oxygen atoms joined to silicon as side chains. Such a compound is called a Silicone; this particular one is a methyl silicone. Other alkyl groups may also be substituted for The methyl groups and the molecular chain can be made of various lengths.

Properties and Uses of the Silicones

Some of the methyl silicones are oily liquids and they become more viscous as the chain Length increases. They are used as lubricants, either incorporated in greases or as oils, in bearings, Gears, etc. They are also used in hydraulic brakes and other hydraulic systems. The outstanding Physical attribute of silicone oil is its very small change in viscosity with change in temperature, Compared with the behaviour of other oils of similar viscosity.

If the temperature is dropped from 100°C to 0°C the viscosity of petroleum oil mayIncrease about one hundred folds, whereas that of silicone oil will increase less than four folds. In The presence of air or oxygen at temperature as high as 300°C, silicone oils remain free from acidFormation, oxidation and similar phenomena, which frequently limit the usefulness of petroleum Products and other synthetic organic liquids.

Methyl silicones of high molecular mass resemble rubber and are used in making rubber Like tubing and sheets.

Silicone molecules can be made in such a way that bridges, or cross linkages bind one Long molecule to another at several points along the chain. These compounds have resinousProperties and are extensively used in electrical insulation.

Another interesting and important application of silicones is their use in the treatment of Various surfaces to make them water repellent. A silicone film covers the surface and repels water Like a grease film. Much of the leak of electricity through the moisture film on ceramic electrical Insulators can be prevented by a silicone film;