Q.Write a note on Helmholtz electrical double layer.

For an ion to coagulate a colloid at all, its charge must be opposite in sign to that of the colloidal particles (only then does it neutralise the particles' charge); and given that correct sign, its coagulating power then increases sharply with the magnitude of its charge — so both the sign and the magnitude of the ion's charge together determine coagulating power.

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Since the colloidal particles move to the cathode, the sol is positively charged, so coagulation requires oppositely-charged (anionic) electrolytes; the Hardy–Schulze rule states coagulating power increases sharply with the valency of the effective (oppositely-charged) ion, ranking $[Fe(CN)_6]^{4-}$ highest and $Cl^-$ lowest.

Step 1 — identify the charge on the sol: in electrophoresis, colloidal particles migrate towards the electrode of opposite charge. Since the particles move towards the cathode (the negative electrode), the particles themselves must be positively charged.

Step 2 — identify the effective coagulating ion: for a positively charged sol, coagulation is caused by the anion of the added electrolyte (the ion of charge opposite to the sol).

Step 3 — apply the Hardy–Schulze rule: the coagulating power of an ion increases sharply (not linearly) with its valency/charge. Identifying the anion and its charge in each electrolyte:

• (i) $K_2SO_4$ → $SO_4^{2-}$ (charge $-2$)
• (ii) $Na_3PO_4$ → $PO_4^{3-}$ (charge $-3$) …

Colloidal particles are large enough (1–1000 nm) to scatter visible light, which is what makes a light beam's path visible as it crosses a colloidal solution.

When a beam of light passes through a colloidal solution, the dispersed colloidal particles — being of a size (roughly 1 to 1000 nm) comparable to the wavelength of visible light — scatter the light in all directions. This scattering makes the PATH of the light beam visible from the side, an effect called the Tyndall effect. It does not occur (or is negligible) in a true solution, since dissolved particles/ions there are far too small to scatter visible …

Sulphide sols like As₂S₃ and Sb₂S₃ preferentially adsorb S²⁻/HS⁻ ions from solution and become negatively charged.

Colloidal particles acquire charge mainly by preferential adsorption of a particular type of ion from the dispersion medium onto their surface.

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Colloids are defined by an intermediate particle-size range - too large to be a true solution but too small to be a coarse suspension - roughly 1 nm to 1000 nm.

Mixtures are classified by the size of the dispersed particles:

• TRUE SOLUTIONS: particle size less than about 1 nm (e.g., dissolved ions/small molecules) - do not scatter light, cannot be filtered out. …

The Tyndall effect — visible scattering of a light beam — occurs specifically in colloidal solutions.

The Tyndall effect is the scattering of light by particles in a medium, making the path of a light beam visible when viewed from the side (e.g., a beam of sunlight through dust in a room, or a car headlight beam in fog).

Why colloids show it: Colloidal particles (1 nm – 1000 nm) are large enough to scatter visible light effectively, but small enough to stay suspended rather than settle. This size range is exactly what causes strong Tyndall scattering.

Why the other options don't:

• True solutions — particles (ions/small molecules, <1 nm) are too small to scatter visible light appreciably; the beam passes through unseen. …

Colloidal particles are large enough to scatter visible light in all directions, and this scattering is what produces the visible Tyndall cone/beam.

True solutions do not show this effect because their solute particles (ions/small molecules) are too small to scatter light appreciably, whereas colloidal particles (1 nm – 1000 nm) are comparable in size to the wavelength of visible light and scatter it strongly — this is Rayleigh-type scattering, distinct from simple absorption or specular reflection. The presence of a c …

Peptisation redisperses a precipitate into a colloidal sol, which is the opposite of coagulation (which converts a sol into a precipitate).

Delta formation occurs when colloidal clay carried by a river coagulates on meeting electrolytes in seawater; alum purifies drinking water by coagulating suspended colloidal impurities; tanning uses tannins to coagulate the collagen protein in animal hide into leather — all three are genuine coagulation processes. Peptisation, in contrast, is deliberately used to disperse a freshly prepared precipitate ba …