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.
Silica gel's high surface area (around 800 m²/g) allows it to absorb water readily, making it useful as a desiccant (drying agent). Once saturated with water, the gel can be regenerated by heating it to 120 °C (250 °F) for two hours. Some types of silica gel will "pop" when exposed to enough water
A solution of sodium silicate is acidified to produce a gelatinous precipitate that is washed, then dehydrated to produce colorless silica gel. When a visible indication of the moisture content of the silica gel is required, ammonium tetrachlorocobaltate(II) (NH4)2CoCl4 or cobalt chloride CoCl2 is added. This will cause the gel to be blue when dry and pink when hydrated.
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