silica gel, chemical compound. It is a colloidal form of silica, and usually resembles coarse white sand. It may be prepared by partial dehydration of metasilicic acid, H₂SiO₃. Because it has many tiny pores, it has great adsorptive power. It is used as a drying agent, as a catalyst or catalyst carrier, and in purifying various substances. Silica aerogel is fully dehydrated silica gel; it is very porous and is often used in insulation, e.g., for refrigerators.

silica or silicon dioxide, chemical compound, SiO₂. It is insoluble in water, slightly soluble in alkalies, and soluble in dilute hydrofluoric acid. Pure silica is colorless to white. It occurs in several forms and is widely and abundantly distributed throughout the earth, both in the pure state and in silicates, e.g., in quartz (agate, amethyst, chalcedony, flint, jasper, onyx, and rock crystal), opal, sand, sandstone, clay, granite, and many other rocks; in skeletal parts of various protists and animals, such as certain sarcodines (see Sarcodina), diatoms, and sponges, and in the stems and other tissue of higher plants. Silica has many important uses. It is used as a filler for paint and rubber; in making ordinary glass; in ceramics; in construction; and in the preparation of other substances, e.g., silicon carbide. Fused quartz is pure amorphous silica; it is used in special chemical and optical apparatus. Because it has a low thermal coefficient of expansion, it withstands sudden changes in temperature and can be used in parts that are subjected to wide ranges of heat and cold. Unlike ordinary glass, it does not absorb infrared and ultraviolet light.

Any of the forms of silicon dioxide (SiO₂), including quartz, tridymite, cristobalite, coesite, stishovite, melanophlogite, lechatelierite, and chalcedony. Various kinds of silica minerals have been produced synthetically.

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