Absorption:

Absorption is a physical or chemical phenomenon in which the atoms, ions or molecules of a substance enter into the bulk of another substance, a solid or liquid. The substance which gets absorbed is called absorbate and the substance which absorbs is called the absorbent. For example, when we use a paper towel to clean the spilled water, the paper absorbs the water, so in this case paper is absorbent and water is the absorbate.

Absorption can be a chemical or a physical process:

Chemical absorption: In this type of absorption, a chemical reaction takes place between absorbate and absorbent during absorption. For example, when hydrogen sulphide is removed from biogas streams and converted into solid sulphur through absorption; aqueous sodium hydroxide is used as the absorbent to dissolve an acid gas.

Physical absorption: It is a non-reactive process that occurs between two phases of matter, e.g. a liquid absorbs a gas or a solid absorbs a liquid. For example, when oxygen present in the air dissolves in water and the clay pot which is used to store water absorbs some of the water. This process depends on the gas and liquid phases and on physical properties like solubility, pressure and temperature.

Adsorption:

The phenomenon of adhesion of the molecules of a substance on the surface of a liquid or solid is known as adsorption. The substance which gets adsorbed on a surface is called the adsorbate and the substance on which it is adsorbed is known as an adsorbent. The surface where the process takes place is called interface.

The term "adsorption" was coined by Heinrich Kayser in 1881. The reverse of adsorption when the adsorbed substance is removed from the surface of adsorbent is known as desorption. Some of the commonly used adsorbents are zeolites, activated carbon, silica gel, resins etc.

Adsorption can be of two types: physical and chemical

Physical adsorption (Physisorption): In this type of adsorption, weak van der waals forces exist between adsorbate and adsorbent.

Chemical adsorption (Chemisorption): In this type of adsorption, chemical bonds (mostly the covalent bonds) are formed between adsorbate and adsorbent.

Uses of Adsorption:

  • Activated carbon removes impurities from water through adsorption.
  • Silica gel and alumina gels remove moisture from rooms through adsorption.
  • Silica gel protects electronics and clothing from moisture by adsorbing moisture.
  • Non-stick coatings are applied on surfaces through adsorption.
  • Zeolites are used as adsorbents to remove carbon dioxide from natural gas and carbon monoxide from reforming gas.

Based on the above information, some of the key differences between absorption and adsorption are as follows:

Absorption

Adsorption

A substance diffuses into another substance (liquid or solid).

Gas or liquid particles bind to the solid or liquid surface.

It is a bulk phenomenon, occurs throughout the absorbent.

It is a surface phenomenon.

The absorbate is uniformly distributed throughout the body of the solid or liquid.

The adsorbate remains on the surface and does not enter into the bulk or interior of the solid or liquid.

It occurs at a uniform rate.

It increases steadily.

Concentration of absorbate is same throughout the absorbent.

Concentration of adsorbate is more at surface than other parts of the adsorbent.

It is not affected by temperature.

It is affected by temperature.

It is an endothermic process.

It is an exothermic process.

Absorption is comparatively slow than adsorption. The equilibrium takes place slowly.

It is a rapid process, the equilibrium takes place quickly.