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.