Packets of silica gel. 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. Packaged in a semi-permeable packet, silica gel is used as a desiccant to keep a variety of packaged goods dry. Although silica gel is inert and non-toxic, indicator substances are often added to it which change colour when moist. These substances are often toxic which is why the packets are usually labelled as such.