There are several examples, which reveal that the surface of a solid has the tendency to attract and retain the molecules of the phase with which it comes into contact. These molecules remain only at the surface and do not go deeper into the bulk. The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.

Adsorption is essentially a surface phenomenon. Solids, particularly in finely divided state, have large surface area and therefore, charcoal, silica gel, alumina gel, clay, colloids, metals in finely divided state, etc. act as good adsorbents.

What is the process called when the molecules of a substance are retained at the surface of a solid or a liquid?

Adsorption

The correct answer is option 2. Adsorption.

Let us look at each of the given processes:

1. Absorption:

Absorption involves the uptake of molecules of a substance (absorbate) into the bulk phase of another substance (absorbent), distributing throughout the absorbent. This process involves the internalization of the absorbate into the absorbent, often accompanied by diffusion. A sponge absorbing water, where water penetrates the entire structure of the sponge.

2. Adsorption:

Adsorption is the process by which molecules of a substance (adsorbate) adhere to the surface of a solid or liquid (adsorbent) without penetrating its internal structure. It primarily involves weak van der Waals forces or sometimes chemical bonds if it's chemisorption. For instance, activated charcoal adsorbing gases or impurities from liquids. Gas masks use activated charcoal to adsorb toxic gases from the air, trapping them on the charcoal's surface.

3. Sorption:

Sorption is a general term that encompasses both adsorption and absorption. It describes the process of one substance becoming attached to another, either on the surface or within the bulk. Soil can sorb pollutants, either by absorbing them into its pores or adsorbing them onto its particles.

4. Desorption:

Desorption is the process by which adsorbed molecules are released from the surface of the adsorbent back into the surrounding phase. This can occur naturally when the adsorbent reaches saturation, or it can be induced by changing conditions like temperature or pressure. When a contaminated activated charcoal filter is heated, the adsorbed substances can be released, regenerating the filter.

Key Differences:

Surface vs. Bulk: Adsorption occurs at the surface, while absorption involves the entire volume of the absorbent.

Interaction Strength: Adsorption involves surface forces (van der Waals forces or chemical bonds), whereas absorption involves solubility and diffusion into the bulk.

Process Nature: Adsorption is often a faster process since it occurs on the surface, whereas absorption can be slower due to the diffusion step into the bulk material.

By understanding these distinctions, it's clear why adsorption specifically refers to the retention of molecules at the surface of a solid or liquid.