a highly porous, noncrystalline form of silica used to remove moisture from gases and liquids, to thicken liquids, to impart a dull surface to paints and synthetic films, and for other purposes.

Silica gel was known as early as 1640, but it remained a curiosity until its adsorbent properties were found useful in gas masks during World War I. It is generally prepared by acidification of a solution of a silicate, such as water glass; the resulting silicic acid forms either a rigid mass or a gelatinous precipitate from which soluble materials are removed by washing with water. The water is finally removed by heating, leaving a glassy, granular solid. For highest activity as a desiccant, the gel is not completely dehydrated but is left with a small percentage of combined water.

The true density of silica is 2.2 grams per millilitre (137 pounds per cubic foot), but the porosity of silica gels gives them much lower bulk densities. A common commercial form has a bulk density of about 0.7 gram per millilitre and a surface area of about 750 square metres per gram, more than five acres per ounce.