Silica gel is a highly porous form of silicon dioxide with a bulk density of about 0.7 grams per millilitre (137 pounds per cubic foot). It is able to adsorb large quantities of water at room temperature. This makes it a valuable industrial product, used for removing moisture from gases and liquids and thickening chemicals. It is also an excellent desiccant. Small paper envelopes containing silica gel, usually with a “do not eat” warning, are often included in dry food packages to absorb any humidity that might spoil the contents.
The gel is generally prepared by acidifying a silicate solution or mineral, washing out soluble impurities and drying the resultant gelatinous mass, which then can be crushed into coarse granules. The adsorption characteristics are determined by the free oxygen atoms on the surface of the gel, which can be bound to hydrogen (Si-OH, silanol) or to another silicon atom in a bonding site (Si-O-Si, siloxane).
High-density silica is used in a variety of applications including refractories and in gas and liquid chromatography. It is also a raw material in the production of polymers such as polyethylene and polypropylene.
To improve the adsorption performance of silica gel, adding carbon materials such as expanded graphite and multi-walled carbon nanotubes with high intrinsic thermal conductivity is a common approach.  However, preparing a high-density silica sorbent requires complex and time-consuming processing steps.