A physical adsorption at low temperature may pass into chemisorption as the temperature is increased.

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Chemisorption involves a high energy of activation. Sometimes these physisorption and chemisorption occur simultaneously and it is not easy to determine the type of adsorption. A physical adsorption at low temperature may pass into chemisorption as the temperature is increased. For example, dihydrogen is first adsorbed on nickel by vander Waals’ forces. Molecules of hydrogen then dissociate to form hydrogen atoms which are held on the surface by chemisorption.