Absorption is the process by which things are absorbed.
Adsorption, on the other hand, describes the act of gas or liquid molecules adhering to a surface.
Both words are commonly misspelled as absorbtion and adsorbtion. This spelling error is usually corrected automatically by a computer’s spell check.
The spelling change comes from pronunciation over time. The voiced consonant of /b/ is grouped with the voiceless /tion/. Another example are the words describe and description.