Adsorption on a Vast Internal Surface
Silica gel is amorphous silicon dioxide (SiO₂) processed into a rigid bead riddled with microscopic pores. Its power is surface area: a single gram has an internal surface of roughly 700 to 800 m². Water vapour molecules are adsorbed, physically held, onto this internal surface by Van der Waals forces. The gel stays solid and does not change chemically; it simply fills its pores with water. That gives it a clean, non-dusting, non-corrosive behaviour that suits small sealed packs, but it also caps its capacity: once the pores are full, at roughly 25 to 40% of its weight, it is saturated and adsorbs no more.
Silica gel performs best in the 30 to 70% RH mid-range and at moderate temperatures. It is ideal for protecting a camera, an instrument, a medication bottle or a small retail pack, anywhere the enclosure is small and the humidity load is modest. It is the wrong tool for a shipping container, where the moisture load is measured in litres and the high-humidity peaks of a long voyage exhaust its limited capacity long before arrival.