Surface Chemistry


Adsorption is the deposition of molecular species onto the surface. It differs from absorption which is a physical or chemical phenomenon in which atoms, molecules or ions enter some bulk phase. Let us know more about Absorption and its significance.

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Adsorption, Adsorbant and Adsorbate


Adsorption is the deposition of molecular species onto the surface. The molecular species that gets adsorbed on the surface is known as adsorbent and the surface on which adsorption occurs is known as the adsorbate. Common examples of adsorbents are clay, silica gel, colloids, metals etc. Thus, adsorption is a surface phenomenon. The process of removal of adsorbent from the surface of adsorbate is known as desorption

Mechanism of Adsorption

It is an exothermic process which means that energy is liberated during this process.  The amount of heat that gets evolved when one mole of the adsorbate is adsorbed on adsorbent is known as enthalpy. The change in enthalpy is denoted to be negative.The reason behind this is that when adsorbate molecules are adsorbed on the surface, freedom of movement of molecules become restricted and this results in a decrease in entropy. At constant temperature and pressure, adsorption occurs spontaneously.

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Types of Adsorption

The process can take place by two means:

  • Physisorption
  • Chemisorption

Physical Adsorption or Physisorption

This phenomenon involves the use of weak Van der Waal forces by means of which gas molecules get adsorbed on a solid surface. The characteristics of physisorption are:

  • There is no specificity as any gas can be adsorbed onto the surface
  • It has been observed that highly liquifiable gases are physically adsorbed more strongly.
  • It is reversible in nature and is dependent on pressure as well as temperature. An increase in pressure decreases the volume of gas and thus increases the adsorption of gas molecules. Conversely, a decrease in pressure will cause the removal of gas molecules from the solid surface.
  • Since the adsorption process is exothermic, the physical adsorption occurs readily at low temperature and decreases with increasing temperature (Le-Chatelier’s principle).
  • Porous substances are better adsorbents as an increased surface area promotes adsorption.
  • Does not require energy for activation.

Chemical Adsorption or Chemisorption

We can characterize it by the involvement of chemical bonds between the gas molecules and the adsorbent surface Moreover, it results in a unimolecular layer. The characteristics of chemisorption are:

  • The process specific in character means that it will occur only if there is a chemical bond formation between the adsorbent and adsorbate.
  • The process is irreversible in nature.
  • It is an exothermic process and the process is accompanied by an increase in temperature.
  • It occurs slowly at low temperature and occurs at a higher rate with increase in pressure.
  • Just as in case of physisorption, chemisorption is directly proportional to surface area and thus increases with increase in surface area.
  • Since the process involves chemical bond formation, the enthalpy in high.
  • It requires a certain energy of activation.

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Solved Questions For You

Que: Which of these is NOT a characteristic of chemical adsorption? It is…

  1. Irreversible
  2. Exothermic
  3. Requires energy for activation
  4. Does not require energy for activation.

Ans: The correct answer is option “D”. It does not require an energy for activation. This statement is incorrect.

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