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Silicon dioxide (SiO2), also known as silica, is a common element found in nature as quartz and as the main constituent of sand. It is also used as a raw material for producing glass.
It does not conduct electricity either as a solid or as a liquid. The reason is that it has no mobile electrons or ions. It is also non-metallic.
Phosphorus, sulphur and chlorine all form oxides which consist of molecules. These molecules have strong attractions between them which requires a lot of heat energy to break. As a result, they have high melting and boiling points.
Carbon dioxide, on the other hand is a gas with a low boiling point. The reason is that carbon dioxide has a molecular structure. Its atoms are bonded together through van der Waals forces which have weaker attraction than the covalent bonds of SiO2.
The SiO2 molecule has a macromolecular structure. Its atoms are connected to each other through thousands of covalent bonds which need much more heat energy to break than van der Waals forces. The SiO2 melts at 1700°C and boils at 2230°C.
As the temperature increases, pyrite-type silica undergoes a series of phase transitions into coesite, stishovite, cotunnite and finally into Fe2P. These changes in the crystal structure can have significant effects on the melting temperature. Recent ab initio calculations24 of the melting curve of SiO2 at the highest pressures available to date – up to 160 GPa23 – give excellent agreement with experimental data23,25.