Silica Gel is a granular, vitreous, highly porous form of silica made synthetically from sodium silicate. Despite its name, Silica Gel is a solid. It is a naturally occurring mineral that is purified and processed into either granular or beaded form. As a desiccant, it has an average pore size of 2.4 nanometers and has a strong affinity for water molecules.

Silica Gel is most commonly encountered in everyday life as beads packed in a vapor-permeable plastic. In this form, it is used as a desiccant to control local humidity in order to avoid spoilage or degradation of some goods. Because of poisonous dopants (see below) and their very high absorption of moisture, Silica Gel packets usually bear warnings for the user not to eat the contents. If consumed, the pure Silica Gel is unlikely to cause acute or chronic illness. Food-grade desiccant should not include any poisons which would cause long-term harm to humans if consumed in the quantities normally included with the items of food.

For desiccant uses, Silica Gel may be used to keep the relative humidity (RH) inside a high frequency radio or satellite transmission system waveguide as low as possible. Excessive moisture buildup within a waveguide can cause arcing inside the waveguide itself, damaging the power amplifier feeding it. Also, the beads of water that form and condense inside the waveguide change the characteristic impedance and frequency, degrading the signal. It is common for a small compressed air system (similar to a small home aquarium pump) to be employed to circulate the air inside the waveguide over a jar of silica gel.

Silica Gel is also used to dry the air in industrial compressed air systems. Air from the compressor discharge flows through a bed of silica gel beads. The Silica Gel absorbs moisture from the air, preventing damage at the point of use of the compressed air due to condensation or moisture. The same system is used to dry the compressed air on railway locomotives, where condensation and ice in the brake air pipes can lead to brake failure.

Silica Gel is sometimes used as a preservation tool to control relative humidity in museum and library exhibitions and storage.

For chemical uses, Silica Gel is used in chromatography as a stationary phase. In column chromatography, the stationary phase is most often composed of silica gel particles of 40-63 μm. Different particle sizes are used for achieving a desired separation of certain molecular sizes. Silica Gel is also applied to aluminium, glass, or plastic sheets for thin layer chromatography.