In chemistry, adsorption refers to the process by which atoms, ions, or molecules from a gas or liquid phase adhere to the surface of a solid or liquid substance. This phenomenon involves the accumulation of the adsorbate (the substance being adsorbed) at the interface between the adsorbent (the surface to which adsorption occurs) and the fluid phase.

Adsorption is a surface phenomenon and can occur due to various intermolecular forces, such as van der Waals forces, electrostatic interactions, and hydrogen bonding. The adsorption process is typically reversible, meaning that adsorbate molecules can detach from the surface under suitable conditions, such as changes in temperature or pressure.

Adsorption is classified into two main types based on the strength of the interaction between the adsorbate and the adsorbent:

1. Physical Adsorption (Physisorption): Also known as van der Waals adsorption, physical adsorption involves the weak intermolecular forces of attraction between the adsorbate and the adsorbent. These forces include London dispersion forces, dipole-dipole interactions, and induced dipole interactions. Physisorption typically occurs at relatively low temperatures and can be reversed by altering the conditions, such as increasing temperature or reducing pressure.
2. Chemical Adsorption (Chemisorption): Chemisorption involves the formation of chemical bonds between the adsorbate and the adsorbent surface. This type of adsorption is characterized by stronger interactions compared to physical adsorption. Chemisorption is specific and typically occurs at higher temperatures. The adsorption process involves the breaking of existing bonds on the adsorbate and the formation of new chemical bonds with the adsorbent surface. Chemisorption is generally not easily reversible.

Both physical and chemical adsorption play important roles in various chemical, biological, and industrial processes. Adsorption processes are utilized in applications such as gas separation, catalysis, purification, and wastewater treatment. Understanding adsorption is crucial in areas like surface science, materials science, and heterogeneous catalysis.