Before checking the options, fix the difference between adsorption and absorption. Adsorption is a surface phenomenon: molecules stick only to the outer surface of a solid, so a substance needs a large surface area, usually from pores or a layered structure, to be a good adsorbent. Absorption is a bulk phenomenon: molecules go all the way into the material, not just its surface. The question is really asking which substance works by absorption instead of adsorption.
- Silica gel: This is a highly porous solid with an enormous internal surface area, and it is one of the standard adsorbents used to pull moisture and gases onto its surface. It is a true adsorbent.
- Mica sheet: Mica is made of thin stacked layers, and gas or vapour molecules can stick onto these layered surfaces. It behaves as an adsorbent.
- Alumina: Activated alumina is porous and is commonly used as a stationary phase in chromatography precisely because it adsorbs different substances on its surface to different extents. It is an adsorbent.
- Calcium chloride: $CaCl_2$ is hygroscopic, meaning it takes up water, but it does this by reacting with water throughout its bulk to form hydrates like $CaCl_2 \cdot 6H_2O$, not by holding water molecules on its outer surface. This is absorption, not adsorption.
Since calcium chloride takes up moisture by bulk hydrate formation rather than surface attachment, it is the one substance here that is not an adsorbent.
Let's summarize:
- Adsorption happens only at the surface; absorption happens throughout the bulk of the material.
- Silica gel, alumina, and mica sheet all work through surface adsorption, while calcium chloride works by forming hydrates in bulk.
The substance that is not an adsorbent is calcium chloride.