Activated charcoal is used in purification processes to remove colouring matter from solutions by which principal mechanism?

Introduction / Context:
This question explores the role of activated charcoal (also called activated carbon) in purification and decolourisation processes. Activated charcoal is used in laboratories and industries to remove coloured impurities from solutions, in gas masks to remove toxic vapours and in water filters to improve taste and odour. Understanding the mechanism by which it works is important in physical chemistry and environmental applications.

Given Data / Assumptions:

• The substance under discussion is activated charcoal or activated carbon.
• It is used to remove colouring matter and other impurities from pure substances.
• Options include bleaching, oxidation, adsorption, reduction and filtration.
• Activated charcoal has extremely high internal surface area due to its porous structure.

Concept / Approach:
Activated charcoal purifies by adsorption, not by absorption or simple filtration. Adsorption is the process in which molecules from a gas or liquid adhere to the surface of a solid. Because activated charcoal has a vast internal surface area with many pores, it can adsorb large quantities of coloured and odorous molecules from solutions or gases. This removes them from the bulk phase without necessarily changing their chemical identity. Bleaching by chemical reaction, oxidation or reduction would change the chemical structure of the impurities, which is not the primary mechanism here. Filtration alone would only remove suspended solids, not dissolved coloured molecules.

Step-by-Step Solution:
Step 1: Recall that activated charcoal is produced by heating carbonaceous materials to create a highly porous structure with enormous surface area. Step 2: When a coloured solution is shaken with activated charcoal, the coloured molecules are attracted to and held on the surface of the charcoal particles. Step 3: This surface attachment is called adsorption and depends on physical forces such as van der Waals interactions and sometimes specific chemical affinities. Step 4: After adsorption, the charcoal with attached impurities can be removed by filtration, leaving behind a decolourised or purified solution. Step 5: Bleaching in this context does not mean a chemical reaction with oxidising agents like chlorine; instead, it is primarily a physical process of adsorption. Step 6: Oxidation or reduction reactions may occur in specialised cases but are not the general mechanism in simple decolourisation with activated charcoal. Step 7: Therefore, the correct description is that activated charcoal removes colouring matter by adsorption onto its large surface area.

Verification / Alternative check:
The behaviour of activated charcoal in gas masks and water filters supports this explanation. Toxic vapours, organic compounds and odours are removed because they bind to the surface of the charcoal rather than being chemically destroyed. When the charcoal becomes saturated with adsorbed species, its effectiveness decreases and it must be replaced or regenerated. The concept of adsorption is widely discussed in physical chemistry, especially in the context of adsorption isotherms and surface phenomena. These topics describe how the amount of solute adsorbed depends on concentration and temperature, fitting the observed behaviour of activated charcoal.

Why Other Options Are Wrong:
Bleaching through chemical reaction usually involves specific oxidising agents such as chlorine or hydrogen peroxide, which alter the structure of coloured compounds. Activated charcoal does not act primarily as a strong oxidiser. Oxidation and reduction would require suitable reagents and reaction conditions, which are not provided by charcoal alone in simple decolourisation experiments. Simple filtration would only remove suspended solids, not dissolved coloured molecules; the key step is adsorption onto charcoal followed by filtration of the solid charcoal. Therefore, the main mechanism is adsorption, not bleaching, oxidation, reduction or mere filtration.

Common Pitfalls:
Students sometimes confuse adsorption with absorption because the words sound similar. Absorption refers to a substance being taken up into the bulk of another (like a sponge absorbing water), whereas adsorption involves accumulation on the surface. Another mistake is to assume that any removal of colour must be a chemical bleaching reaction. In fact, physical adsorption can effectively remove dyes and other coloured molecules without changing their chemical identity. To avoid confusion, remember that activated charcoal works mainly through adsorption due to its large surface area and porous structure.

Final Answer:
Activated charcoal removes colouring matter from solutions mainly by adsorption onto its large surface area.