Adsorption is the process where molecules of a substance (adsorbate) accumulate on the surface of a solid (adsorbent). For methylene blue adsorption on activated charcoal at 25°C, let's analyze the statements:

Step 1: Understanding Physical Adsorption
This is an example of physical adsorption (physisorption), which involves weak van der Waals forces. Key characteristics:

• It does not require activation energy and occurs readily at low temperatures.
• It is reversible in nature.
• It is exothermic, so enthalpy change (ΔH) is negative.
• It decreases with an increase in temperature (as per Le Chatelier's principle, since it is exothermic).

Step 2: Evaluating the Options

• "The adsorption requires activation at 25°C." – Incorrect. Physisorption does not require activation energy.
• "The adsorption increases with increase of temperature." – Incorrect. Adsorption decreases with temperature increase for physisorption.
• "The adsorption is irreversible." – Incorrect. Physisorption is reversible.
• "The adsorption is accompanied by a decrease in enthalpy." – Correct. Since the process is exothermic, ΔH is negative (decrease in enthalpy).

Final Answer: The correct statement is "The adsorption is accompanied by a decrease in enthalpy."

Related Topics & Formulae

Adsorption Isotherms: The Freundlich isotherm is often used: $\frac{x}{m}=k{P}^{1/n}$, where x is mass adsorbed, m is mass of adsorbent, P is pressure, k and n are constants.

Enthalpy Change: For physisorption, $\mathrm{\Delta }H<0$ (exothermic).

Effect of Temperature: Adsorption decreases with temperature increase for physisorption due to the exothermic nature.