Why does the 'b' in absorb change to a 'p' in absorption?

The question pretty much says it all. Why is "absorbtion" an incorrect spelling?

Solution 1:

Voicing Assimilation is the technical term for what happened here.

In English (and Latin, and most Indo-European languages, among many others), /b/ and /p/ are identical in pronunciation (both are bilabial stops), differing only in their Voice parameter; /b/ is Voiced, while /p/ is Voiceless.

It is a fact about the human vocal tract that consonant clusters that differ in Voice are difficult to pronounce, because changing from Voiced to Voiceless consonants requires independent movement of the larynx, which can be very hard to switch on and off at the millisecond timing required for consonant clusters.

Therefore there is a universal tendency in human language for consonant clusters to be either all Voiced or all Voiceless. However, what determines voicing when a cluster is formed from two dissimilarly-voiced consonants varies from language to language.

In Russian, for example, it's Regressive assimilation -- the Voice parameter of the final consonant in a cluster becomes the parameter of the whole cluster. So the Russian preposition в /v/ 'in' is pronounced /f/ when its object starts with a voiceless consonant.

English, however, mostly favors Progressive assimilation -- that's why the {-Z} suffixes of noun plural, noun possessive, and verb 3sgpresent vary between /s/ and /z/; they take their voicing parameter from the sound that comes before them. The {-D} verb past tense suffix varies between /d/ and /t/ in the same way.

Solution 2:

The main reason is most likely pronunciation. There is another example of this, where the difficulty may be more prominent: describe. It would be rather difficult to say describtion, as compared to description, which flows off of one's tongue rather nicely. The same goes for absorption; it is simply easier than absorbtion.

Etymonline suggests that this change occurred in the Latin roots (for both absorption and description). Nowadays, hoi polloi are more eloquent than the commoners who spoke Latin; while we today may not find it difficult to say these, they would have.