Silicais widely distributed in nature and it is the common constituent of earth's crust . It exhibits allotropy . It is found in nature in different forms like crystalline and amorphous forms . Quartz , tridymite , and cristobalite are the crystalline allotropes of silicon dioxide to the crystal . Coloured crystals are used as gems .
Quartz , tridymite and cristobalite , the three crstalline polymorphs have a low temperature `alpha` form and a high temperature `beta` modification .
Low temperature α –quartz α –Tridymite α Cristobalite
845 K 400-435 K 475-550 K ↕475-550 K
High temperature β- quartz ↔ β- Tridymite ↔ β- Cristobalite
1140 K 1740 K ↕1985 K
The purest form of silica is quartz or rock crystal.Sand is the crushed form of quartz and sandstone is sand particles with iron oxide bound to them. Flint consists of amorphous silica associated with quartz. Kiesulguhr is a siliceous rock composed of the remains of minutesea organisms.
Amorphous forms of silica:-
Agate, onyx,jasper are the amorphous forms of silica. Silica is found in plants and animals both.
Silica can be prepared in the laboratory by the following methods:
1.Burning of silicon in air or oxygen gives silicon dioxide
Si + O2→SiO2
2. Silica is formedby the hydrolysis of silicon tetrafluoride or tetrachloride.
3SiF4 +4H2O→H4SiO4 +2H2SiF6
SiCl4 + 4H2O→ H4SiO4 +4HCl
OrthoSilicic acid obtained by the above reaction loses water at 1000⁰C and gives pure silica.
H4SiO4 → SiO2+ H2O
3. It is insoluble in water.
4. It does not react with HCl,HBr,HI. It react with HF and gives silicon tetra fluoride.
SiO2 +4HF →SiF4+ 2H2O
It reacts with alkalies to form silicates.
SiO2 +2NaOH →Na2Sio3 + H2O
At high temperatures , it reacts with metal oxides and gives silicates.
SiO2 +Na2 O →Na2Sio3
SiO2 +CaO →CaSio3
When silica is heated with carbon in an electric furnace, it is reduced to carborundam,SiC.
Whenheated to 1600°C , silica melts to form quartz glass. Red hot quartz glass when pluged into water does not break into pieces. Due to this nature , it is used in making optical instruments.
STRUCTURE OF SILICA:-
STRUCTURE OF SILICA
Likediamond silica also exists as giant molecules and has got a three dimensional structure. In this giant molecule ,each silicon atom is linked to four oxygen atoms by covelent bonds and they are arranged tetrahedrally around silicon. The crystal structure of silica is compared with the C-C bonds of diamond the Si-O-Si bonds in silica areweak.Hence, it is not as hard as diamond and melts at lower temperature. It exists in the solid state due to its giants molecular structure
Inthe giant molecular structure of silicaon dioxide each silicon atom hasonly half the share of each of the four oxygen atoms linked with it. Therefore silicon dioxide is given the formula SiO2. The ratio of silicon and oxygen atoms in silicon dioxide is 1:2.
uses of silica dioxide: