Question
What is the difference between adsorption and absorption? Give examples of each.
This is a direct 2-mark question in CBSE Class 12 board exams and appears as a definitional MCQ in JEE Main. Getting the examples right is what separates full marks from partial.
Solution — Step by Step
Adsorption is a surface phenomenon — the adsorbate (substance getting adsorbed) accumulates only on the surface of the adsorbent, not inside it.
Think of it this way: when silica gel absorbs moisture from inside a medicine bottle, the water molecules are sticking to the outer surface of silica gel particles. Nothing goes inside.
Absorption is a bulk phenomenon — the absorbate gets distributed uniformly throughout the entire volume of the absorbent.
Classic example: a sponge soaking up water. The water doesn’t just sit on the surface — it penetrates the whole sponge uniformly.
| Property | Adsorption | Absorption |
|---|---|---|
| Where it occurs | Surface only | Entire bulk |
| Distribution | Non-uniform | Uniform |
| Nature | Surface phenomenon | Bulk phenomenon |
| Example | Silica gel, charcoal | Sponge, anhydrous CaCl₂ |
| Rate | Fast initially, slows down | Steady rate |
When both processes occur simultaneously, we call it sorption. Anhydrous CaCl₂ is a good example — it adsorbs moisture on its surface and also absorbs it into the bulk.
This distinction matters in JEE Main MCQs where they specifically ask whether a given situation involves adsorption, absorption, or sorption.
Why This Works
The key is the Latin roots. Ad- means “at” or “on” — so adsorption is happening AT the surface. Ab- means “into” — absorption happens INTO the material. Once this is locked in, you’ll never confuse the two again.
Adsorption is also further classified as physical adsorption (physisorption, held by van der Waals forces, reversible) and chemical adsorption (chemisorption, involves chemical bond formation, irreversible at room temperature). This sub-classification carries weightage in both CBSE and JEE.
The thermodynamics also differ: adsorption is always exothermic (entropy decreases as molecules become ordered on the surface, so enthalpy must decrease to keep negative). This is a favourite NCERT-based MCQ.
Alternative Method
For exam conditions, use the mnemonic approach:
AdSorption → Surface
AbSorption → Soak (bulk)
Or think of it as: you adsorb things onto you, you absorb things into you. A towel absorbs water (it goes in). Activated charcoal adsorbs gases (they stick to the surface).
In questions asking for “mechanism of action” for activated charcoal in gas masks, always write adsorption — not absorption. The poisonous gases stick to the porous surface of charcoal. This comes up in both CBSE long answers and JEE assertion-reason questions.
Common Mistake
Students write “silica gel absorbs moisture” in answers — this costs marks every time. Silica gel adsorbs moisture on its surface. It is a desiccant that works by surface adsorption, which is why it can be regenerated (dried out and reused) — reversible physisorption. If it absorbed moisture into the bulk like a sponge, you couldn’t reuse it so easily. Always check: is it surface or bulk?
Final Answer:
- Adsorption = accumulation of adsorbate on the surface of the adsorbent (surface phenomenon). Example: activated charcoal adsorbing gases.
- Absorption = uniform distribution of absorbate throughout the bulk of the absorbent (bulk phenomenon). Example: a sponge absorbing water.