PAUL RAPSON / SCIENCE PHOTO LIBRARY PAUL RAPSON / SCIENCE PHOTO LIBRARY
Silica gel on a watch glass. Silica gel is a granular form of silica, synthetically manufactured from sodium silicate. It has an extremely high porosity, making it a highly effective adsorbent material. The large internal surface area enables each granule to attract and hold moisture through physical adsorbtion and capillary condensation. Usually packaged in a semi-permeable packet, silica gel is used as a desiccant to keep a variety of packaged goods dry. Indicator substances are added to silica gel which change colour when moist. Cobalt (II) chloride has been added to the granules on the watch glass. It is blue when dry and changes to pink when moist. Silica gel itself is inert and non-toxic, but the cobalt (II) chloride is toxic and carcinogenic.
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