Adhesion of atoms, ions, biomolecules or molecules of gas, liquid or dissolved solids to a surface is called adsorption. The substance on which the adsorption takes place is known as the adsorbent and the substance which is adsorbed is known as adsorbate.

Types of Adsorption: The adsorption of a gas into solid surface is mainly two types: 
Difference Between Physical Adsorption and Chemical Adsorption || Surface Chemistry

Physical Adsorption or Physisorption
1. This is usually takes place at low temperature and decreases with increase in temperature.
2. The forces of attraction holding the adsorbate are Vander Waal’s forces
3. Heat of adsorption is low
4. It is reversible
5. It is not very specific
6. It forms multimolecular layer
7. It does not require any activation energy.
8. Depends on nature of gas. Easily liquefiable gases are absorbed easily.
9. Low heat adsorption usually in the range of 20-40 kJmol-1. 
Chemical Adsorption or Chemisorption
1. This takes place at high temperature
2. The forces of attraction holding the adsorbate are chemical bond forces
3.Heat of adsorption is high
4. It is irreversible
5. It is highly specific
6. It forms monomolecular layer
7. It requires activation energy
8. Much more specific than physical adsorption.
9. High heat of adsorption in the range of 40-400 kJ mol-1
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