Silica gel is a granular, porous form of silica made synthetically from sodium silicate. Despite the name, silica gel is a solid. It is usually distributed in the form of beads, which are packaged in a semi-permeable packet. It was created at Johns Hopkins University, Baltimore, Maryland in the 1920s. Its high porosity, around 800 m²/g, allows it to adsorb water readily, making it useful as a desiccant (drying agent). It can reduce the relative humidity in a closed space to around 40%. Once saturated with water, the gel can be regenerated (dried) by heating it to 150°C (300°F) for 1.5 hours per liter (about 1 dry quart measure or about 30oz weight) in a thick-walled Pyrex dish. Silica gel is non-toxic, non-flammable and chemically unreactive.
However, some of the beads may be doped with a moisture indicator, such as cobalt (II) chloride, a chemical that changes color when moist. Cobalt (II) chloride is deep blue when dry and pink when moist. Another common indicator in use becomes yellow when moist. While non-doped silica gel is non-toxic, the chemicals used to dope it can be toxic and are the reason most silica gel is labeled as dangerous or poisonous when eaten.
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