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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:

  1. 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?
  2. In the two examples (sugar and cotton) I highlighted, where is my reasoning wrong in applying "dissolve" in place of "absorb" and vice-versa?
  3. Is there any subset/superset relation between absorption and dissolution? If so, how's that apparent from the Gold Book's definition?
  4. 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?
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  • $\begingroup$ "Absorption" is for chemical engineers. $\endgroup$ – Mithoron Mar 18 '18 at 18:04
  • $\begingroup$ A bathing sponge absorbs water, but we cannot say it dissolves water, nor water dissolves the sponge. OTOH, liquid absorbs water vapor or gases, but it also dissolves them. I would say dissolution is a very specific kind of absorption. $\endgroup$ – Poutnik Sep 30 at 5:55
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Okay, I'm just reading the definitions that you posted and what I am getting as the important part is the phase of the materials before and after combining. Sugar and water are combining a solid and a liquid to produce a homogeneous phase - sugar water. This would be dissolution, the sugar goes into a liquid phase - two phases originally and only one new one afterwards.

Your example of cotton and water is also combining a solid and liquid, but the new combination is still two phases although it may seem homogeneous. Neither changes phase.

Let's go back to the sugar example. If the sugar is in a moist environment, it may absorb moisture from the air. It doesn't change phase, but water molecules may physically attach to the solid surface of the sugar crystals.

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