Why is it difficult to remove a curry stain

Each molecule, ion or atom of solute is bonded to the surface of the adsorbate. If we wish to remove the adsorbate from its substrate then we must overcome the enthalpy of adsorption A//(ads). So, removing adsorbate requires the input of energy. If A //(ads i is large, then more energy is needed to overcome it than if AH,adsi is small. It is difficult, therefore, to remove a layer of adsorbed curry stain without expending energy, which we experience physically as a difficulty in removing the stain the red colour won t just wash off . 

There is a wide range of adsorption enthalpies AH(adsi, ranging from effectively zero to as much a 600 kJ per mole of adsorbate. The adsorptive interaction cannot truly be said to be a bond if the enthalpy is small the interaction will probably be more akin to van der Waals forces, or maybe hydrogen bonds if the substrate bears a surface layer of oxide. We call this type of adsorption physical adsorption, which is often abbreviated to physisorption. At the other extreme are adsorption processes for which A//(ads) is so large that real chemical bond(s) form between the substrate and adsorbate. We call this type of adsorption chemical adsorption, although we might abbreviate this to chemisorption. 

As a good generalization, most aromatic and unsaturated species adsorb readily. Their adsorption is facilitated if the substrate is electronically conductive, being either 

A centroid is the location of a physicochemical phenomenon or effect or quantity. A charge centroid, therefore, represents the part of a molecule or ion having the highest charge density. 

The two extremes of physical adsorption and chemical adsorption are abbreviated to physisorption and chemisorption respectively. 

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