What is the difference between dissolution and absorption? Let us first look at the definitions from Gold Book:

Absorption:


  The process of one material (absorbate) being retained by another (absorbent); this may be the physical solution of a gas, liquid, or solid in a liquid, attachment of molecules of a gas, vapour, liquid, or dissolved substance to a solid surface by physical forces, etc


Dissolution:


  The mixing of two phases with the formation of one new homogeneous phase (i.e. the solution).


I've re-read them again and again, but have been unable to come up with the terminology to exactly differentiate between the two. In fact, I've reached some interesting counter-examples.

Here's one: we always says "sugar dissolves in water" and never "sugar absorbs in water" (Ngram). But, going by the first definition which reads: "physical solution of a gas, liquid, or solid in a liquid", can't we say that the physical solution of solid sugar in liquid water is an example of absorption as well? The water is retaining sugar particles in its bulk after all, so why couldn't we call water an an absorbent and sugar an adsorbate?

Another one: we say that "cotton absorbs water" but never "cotton dissolves water" (Ngram). But, going by the second definition: "mixing of two phases with the formation of one new homogeneous phase (i.e. the solution)" can't we say that the solid cotton mixed with liquid water to form a new homogenous phase? After all, the wet cotton needs to be dried to separate the two components, so they clearly are homogenous.

Interestingly enough, the Gold Book definitions seem to have been created to deliberately introduce confusion. Notice that the definition for "absorption" mentions both the words "solution" and "dissolution". So, why did the Gold Book need to refer to dissolution to explain absorption?

Dissolution and absorption definitely are different from each other, otherwise some explicit relation should've been mentioned in the Gold Book. For example, the definition for "chirality centre" reads "A chirality centre is thus a generalized extension of the concept of the asymmetric carbon atom to central atoms of any element" clearly highlighting their relation (the former is a superset). Such isn't a case in the definitions for dissolution and absorption. So, they are different.

So, to summarize, my questions are:


What exactly is the difference between absorption and dissolution? And, as a direct consequence, given a physical process: how would I identify if it is a dissolution or absorption?
In the two examples (sugar and cotton) I highlighted, where is my reasoning wrong in applying "dissolve" in place of "absorb" and vice-versa?
Is there any subset/superset relation between absorption and dissolution? If so, how's that apparent from the Gold Book's definition?
Why did the Gold Book need to refer to dissolution to explain absorption? Couldn't it have explained them both in separate terms without creating confusion?
Casual use of &quot;absorption&quot; is different from the normal, technical, use of the term
The key difference between &quot;absorption&quot; and &quot;solution&quot; is confused in your examples because you use the casual use of the term not the technical one. Even so, the two processes are very distinct.
Solutions are homogeneous. But when things are absorbed, either in casual use or technical use, there are distinct phases. Absorption is usually a technical term for a surface phenomenon where, for example, gas molecules stick to the surface of a catalyst. This is a widely studied process as it is a vital part of many major industrial processes. Water absorbed onto cotton wool is a more casual use of the term absorption but the result is still not a homogeneous mixture. Liquid water sticks to the surface of the fine fibres of cotton wool by surface tension. There are still two phases: liquid water and solid fibre.
Only in the most careless and casual use should you ever describe the process of dissolving sugar in water (or sugar taking up water) as one &quot;absorbing&quot; the other.