Deserts made with large deposits of sand (SiO2)
Silicon dioxide, SiO2:
The chemical name of silicon dioxide is silica (sand) and is found in nature in abundance as sand, quartz,
flint and precious stones like opal, jasper, agate etc.
Other names of Silicon dioxide are:
- Quartz
- Silica
- Silicic oxide
- Silicon(IV) oxide
- Crystalline silica
Pure silicon dioxide
Preparation:
In the laboratory it may be obtained by heating ammonium silicate or by the ignition of silicic acid
It is prepared by burning silicon in oxygen or by the decomposition of silicates.
It is also obtained by the hydrolysis of silicon tetrafluoride
Properties:
- Silica exists in two forms as crystalline and amorphous.
crystalline e.g. quartz, tridimite etc.
amorphous e.g. flint, opal, agate etc. - It is colorless and transparent, with specific gravity of 2.6 g/cm3
- It begins to melt at 1600°C and volatilize on an electric arc.
- Silica in any form is nonreactive.
- Silica is insoluble in water.
Sodium silicate (water glass)
With acids:
Silica reacts with hydrofluoric acid to form silicon tetrafluoride.
With alkalis:
Silica dissolves in hot concentrated alkalis form silicates.
With halogens:
Only fluorine attacks silica to give silicon tetrafluoride.
with basic oxides:
It combines with a number of basic oxides on heating with the formation of silicate.
Optical lenses made of quartz
Uses
- Transparent quartz is used for making optical instruments and lenses.
- The coloured varieties of quartz are used as gems, e.g. amethyst, jaspar and opal.
- The opaque quartz is used in making vessels which are acid and alkali resistant and can stand sudden changes of temperature.
- Sand is used in the manufacture of glass, mortar and porcelain. It forms an important construction material when mixed with lime or cement.
- Silica is used in making refractory bricks for furnace linings.
- Sand (finely ground) is used in the extraction of phosphorus from phosphates.
- Silica gel (solid) is used as a desiccant.