How does absorption differ from adsorption in hygroscopy

In hygroscopy, there are two processes to understand: absorption and adsorption.

• Absorption happens when water goes inside a material and spreads evenly throughout it—like a sponge soaking up water.
• Adsorption happens when water only sticks to the surface of a material without going inside—like ink staying on the surface of chalk.

The key difference: Absorption works within the bulk (inside), while adsorption works only on the surface (outside).

Absorption

• Mechanism: Water enters deep into the material and spreads through its entire volume.
• Concentration: More water is found inside the bulk than on the surface.
• Example: A sponge soaking up water or a towel absorbing liquid.

Adsorption

• Mechanism: Water molecules attach only to the outer surface of the material. They do not move inside.
• Concentration: More water is found on the surface than in the inner part.
• Example: Activated charcoal trapping impurities on its surface or chalk holding ink only on the outside.

Key Differences Summarized

• Phenomenon: Absorption = bulk process. Adsorption = surface process.
• Location of water: Absorption = inside and spread throughout. Adsorption = stays on the surface.
• Concentration: Absorption = higher inside. Adsorption = higher on the surface.