Q.Define the term colloid, and distinguish between a true solution, a colloidal solution and a suspension on the basis of the approximate size of their dispersed particles.

Fog is a colloid with liquid as the dispersed phase and gas (air) as the dispersion medium - a liquid aerosol. Correct option (c).

Colloids are classified by the physical states of dispersed phase and medium. In fog, fine liquid water droplets are suspended in air:

• Dispersed phase = liquid
• Dispersion medium = gas This is called a liquid aerosol (like clouds and mist).

(For comparison: smoke = solid in gas; foam = gas in liquid.)

So fog is a colloidal solution of liquid in gas. Classification of colloidal systems is a standard NCERT Class 12 Chemistry (Surface Chemistry) topic.

Peptization is the process of dispersing a freshly prepared precipitate into a colloidal solution using a small amount of a suitable electrolyte (peptizing agent). Correct option (b).

When a fresh precipitate is treated with a small amount of a suitable electrolyte, the precipitate particles adsorb the electrolyte ions, acquire like charges, repel one another and break up into colloidal-sized particles - forming a sol. This is peptization, and the electrolyte is the peptizing agent (e.g. Fe(OH)3 precipitate peptized by FeCl3).

• Dialysis is purification of a sol.
• Coagulation is the opposite (sol -> precipitate).
• Electrophoresis is migration of colloidal particles in an electric field.

So the process is peptization. This is a standard NCERT Class 12 Chemistry (Surface Chemistry) definition.

Dialysis removes dissolved ionic/small-molecule impurities from a sol using a semi-permeable membrane.

A freshly prepared colloidal sol usually contains small amounts of electrolyte impurities left over from its preparation, which if not removed can destabilise (coagulate) the sol. Dialysis is the process used to purify a colloid: the sol is placed in a bag of a semi-permeable (parchment/cellophane) membrane and suspended in a stream of fresh water/solvent. The membrane's pores are small enough to let ions and small molecules diffuse out into the surrounding water, while the much larger colloidal particles are held back inside the bag. Continuously running fresh water removes the impurities steadily, purifying the sol.

(Peptisation is used to convert a freshly precipitated solid into a colloid, not to purify one; coagulation destroys a sol; Bredig's arc method is used to prepare metal sols, not purify them.)

A colloid with liquid dispersion medium and gas dispersed phase is called a foam.

Colloidal systems are classified by the physical state of the dispersed phase and the dispersion medium:

Here the dispersion medium is a liquid and the dispersed phase is a gas, which by definition is a foam.

Below CMC, surfactant molecules stay dissolved individually; above CMC they spontaneously aggregate into micelles.

Surfactants (like soap or detergent molecules) have a hydrophilic (polar/ionic) head and a hydrophobic (long hydrocarbon) tail. At very low concentration in water, they exist as individual dissolved molecules or ions.

As the concentration is increased, at a certain characteristic concentration the molecules start spontaneously aggregating — the hydrophobic tails cluster together (away from water) while the hydrophilic heads face outward into the water — forming spherical or other aggregates called micelles.

The Critical Micelle Concentration (CMC) is defined as this minimum concentration of the surfactant above which micelle formation begins. Below the CMC there are essentially no micelles; above it, additional surfactant added mostly goes into forming more micelles rather than increasing the concentration of free (unassociated) surfactant molecules.

Lyophobic colloids have little affinity for the dispersion medium; metal sols such as gold sol are the classic example.

Colloids are classified by how strongly the dispersed phase interacts with the dispersion medium:

• Lyophilic ('solvent-loving') colloids — e.g. gum, starch, gelatin, proteins — form spontaneously and are stable because the dispersed particles interact strongly (often via H-bonding) with the solvent.
• Lyophobic ('solvent-hating') colloids — e.g. metal sols (gold, silver), sulphur sol — have little affinity for the medium, must be prepared by special methods, and are comparatively unstable, easily coagulated by small amounts of electrolyte.

Among the given options, gum, starch and gelatin are all naturally occurring macromolecules that disperse readily and are lyophilic. Gold sol is a metal sol with no real chemical affinity for water — it is lyophobic.

A sol is a colloidal system with a solid dispersed phase and a liquid dispersion medium, with particle sizes in the colloidal range (roughly 1-1000 nm).

Definition: A sol is a type of colloidal solution in which very finely divided solid particles (the dispersed phase, in the colloidal size range of about 1 to 1000 nm) are dispersed throughout a continuous liquid medium (the dispersion medium). The particles are small enough to remain suspended (not settling under gravity like a true suspension) but large enough that they scatter light (Tyndall effect) and cannot pass through animal membranes, distinguishing a sol from a true solution.

Example: Starch sol — starch particles dispersed in water. Other common examples: gold sol (colloidal gold in water), As2S3 sol, Fe(OH)3 sol.

Gum is a solvent-loving natural polymer, the classic lyophilic colloid.

Lyophilic ('solvent-loving') colloids are those where the dispersed phase has a strong affinity for the dispersion medium and forms colloids reversibly and spontaneously — examples include starch, gelatin, proteins and gum. Gold sol is a classic lyophobic ('solvent-hating') colloid (metal sols need stabilisers). Milk and blood are typically classified as other colloid types (emulsion/biological dispersion) rather than the textbook lyophilic example.