When a gas is adsorbed, the freedom of movement of its molecules become restricted. This amounts to decrease in the entropy of the gas after adsorption, i.e. Entropy change is negative.

But now, I am unable to understand why it isn't a violation of the second law of thermodynamics, which states that the entropy change of a system can never be negative. Please guide.

• Oh, no not again that... Entropy decreases locally by but raises globally - If this wouldn't be possible you wouldn't exist Feb 5, 2015 at 17:38
• This I think is the most commonly asked question about entropy! (Not the exact question, but the concept) Feb 5, 2015 at 20:29

$$\mathrm{d}S > 0$$
$$\Delta S_\mathrm{sys} < 0 \qquad \Delta S_\mathrm{surr} > 0$$ $$|{\Delta S_\mathrm{surr}}| > |{\Delta S_\mathrm{sys}}|$$
This is because adsorption is an exothermic process and so the surroundings are heated up and therefore increase in entropy. If you consider the Gibbs energy change for adsorption it will be negative because the negative $$\Delta H$$ term is larger than the positive $$-T\Delta S$$ term. This chem.uic.edu page goes through the maths of this very nicely.