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.

Which of the following forces is involved in physical adsorption?

van der Waals force

The correct answer is option 3. van der Waals force.

The following force is involved in physical adsorption: van der Waals force

Physical adsorption (physisorption): This type of adsorption involves the weak, non-specific interaction between the adsorbate (the substance being adsorbed) and the surface of the adsorbent. The forces involved are typically van der Waals forces, which include dipole-dipole interactions, London dispersion forces, and other weak intermolecular forces. These forces are relatively weak compared to chemical bonds and do not involve the sharing or transfer of electrons.

Gravitational force: This is the force of attraction between masses. It is not involved in the adsorption process at the molecular level.

Magnetic force: This force arises from the interaction between magnetic fields and magnetic dipoles. It is not relevant to the process of adsorption, which involves intermolecular interactions.

Electromagnetic force: While this term encompasses a wide range of interactions, including those between electric charges and magnetic fields, it is not specific to the weak intermolecular forces involved in physisorption.

Therefore, van der Waals force is the correct answer, as it describes the weak interactions that characterize physical adsorption.