Deliquescence: Absorption, Not Adsorption
Most desiccants adsorb, water clings to the surface of a porous solid, and capacity is capped by surface area. Calcium chloride (CaCl₂) does something different: it absorbs. It is deliquescent, meaning it chemically attracts water vapour, reacts with it and dissolves into a calcium chloride brine. Because the reaction continues until all the available calcium chloride has been consumed, the capacity is governed by how much active salt is present, not by a fixed surface area. This is why high-purity calcium chloride absorbs up to 300% of its own weight (and more than 400% under optimal conditions) in water, against roughly 25 to 40% for silica gel.
The second crucial behaviour is the kinetics. The absorption rate is driven by the vapour-pressure difference between the brine and the surrounding air, and that difference grows as humidity rises. So calcium chloride absorbs fastest precisely at the 80 to 100% RH peaks where container rain forms, the opposite of silica gel, which slows as it approaches saturation. A calcium chloride desiccant is most aggressive exactly when the moisture threat is greatest.