DESCRIPTION

Silicon Dioxide occurs as an amorphous substance that shows a noncrystalline pattern when examined by X-ray diffraction. It is produced synthetically, either by a vapor-phase hydrolysis process, yielding fumed silica, or by a wet process, yielding precipitated silica, silica gel, colloidal silica, or hydrous silica. Fumed silica is produced in an essentially anhydrous state, whereas the wet-process products are obtained as hydrates or contain surface-adsorbed water. Fumed silica occurs as a white, fluffy, non-gritty powder of extremely fine particle size and is hygroscopic. The wet-process silicas occur as white, fluffy powders or as white, microcellular beads or granules and are hygroscopic or absorb moisture from the air in varying amounts. All of these forms of Silicon Dioxide are insoluble in water and in organic solvents, but they are soluble in hydrofluoric acid and in hot, concentrated solutions of alkalies.