Colloids — Important Questions

Al³⁺

Step 1: The Hardy-Schulze Rule states that the greater the valence (charge) of the coagulating ion, the greater its coagulating power. Step 2: For a negatively charged sol, cations (positive ions) are needed for coagulation. Step 3: Among the given options: Na⁺ has charge +1, K⁺ has charge +1, Ba²⁺ has charge +2, and Al³⁺ has charge +3. Step 4: Higher the charge, stronger is the attraction towards the negatively charged colloidal particles, neutralising their charge more effectively. Step 5: Therefore, Al³⁺ with the highest charge (+3) has the greatest coagulating power. Order: Al³⁺ > Ba²⁺ > N

Lyophilic colloids are reversible and more stable than lyophobic colloids

Step 1: 'Lyophilic' means solvent-loving — the dispersed phase has great affinity for the dispersion medium. Step 2: Lyophilic sols (like starch, gelatin, gum) form easily by direct mixing and are reversible — separated phases can reform the sol on remixing. They are more stable. Step 3: 'Lyophobic' means solvent-hating — the dispersed phase has no affinity for the medium (e.g., gold sol, silver sol). Step 4: Lyophobic sols require special methods (Bredig's arc, chemical methods) and are irreversible. Once precipitated, they do not easily reform. Step 5: Options A, C, and D incorrectly swap th

Peptisation

Step 1: Peptisation is the reverse of coagulation — it converts a precipitate back into colloidal form. Step 2: A suitable electrolyte called the peptising agent is added. For example, FeCl₃ is the peptising agent for Fe(OH)₃ precipitate. Step 3: Fe³⁺ ions from FeCl₃ are preferentially adsorbed on Fe(OH)₃ particles, giving them a positive charge. Step 4: These positively charged particles repel each other and disperse in the medium, forming a reddish-brown colloidal solution. Step 5: Dialysis is purification, coagulation is settling, and emulsification is stabilising emulsions — all are differ

Parchment or cellophane membrane

Step 1: Colloidal solutions contain impurities in the form of electrolytes (ions) which destabilise the sol. Step 2: Dialysis removes these ionic impurities by exploiting size-selective diffusion. Step 3: Colloidal particles (1–100 nm) are too large to pass through parchment or cellophane membranes. Step 4: Smaller ions and molecules pass through freely into the surrounding water, purifying the sol. Step 5: Filter paper allows even colloidal particles to pass through, so it cannot separate them. Silica and cotton membranes are not used for dialysis.