Q.What is the difference between a sol and a gel?

Butter is a semi-solid colloidal system (a gel, liquid-in-solid type), the correctly matched pair among the options; the other three pairings mismatch the colloid type with its example.

Going through each option using the standard dispersed-phase/dispersion-medium classification: (a) 'Foam - mist' is wrong — foam is gas dispersed in liquid (e.g. whipped cream, soap lather) or gas in solid (e.g. rubber, pumice), while mist is actually an example of an aerosol (liquid dispersed in gas).

(b) 'Emulsion - smoke' is wrong — an emulsion is liquid dispersed in liquid (e.g. milk, cream), while smoke is an aerosol (solid dispersed in gas).

(c) 'Sol - whipped cream' is wrong — a sol is solid dispersed in liquid (e.g. paints, cell fluids), while whipped cream is a foam (gas dispersed in liquid). …

Fog is an aerosol formed by fine water-droplet dispersion (liquid) in air (gas), so it is classified as a liquid-in-gas colloidal system, the same class as mist and clouds.

Colloids are classified by the physical state of the dispersed phase and the dispersion medium. In fog, the dispersed phase is minute liquid water droplets and the dispersion medium is the surrounding air (a gas); this liquid-dispersed-in-gas combination is the aerosol type called a 'liquid aerosol,' of which fog, mist, and cloud are common examples. Smo …

Colloid classification by dispersed-phase/dispersion-medium: cheese has liquid (whey/moisture, fat) dispersed throughout a solid protein (casein) network, giving it a semi-solid, jelly-like consistency — the classic hallmark of a gel.

An aerosol (a) is a liquid or solid dispersed in a gas (e.g. fog, smoke) — not applicable to cheese. A foam (b) is a gas dispersed in a liquid or solid (e.g. whipped cream, bread) — cheese is not gas-dispersed. A 'solid sol' (c) is a solid dispersed in a solid (e.g. coloured glass, some alloys) — cheese's dispersed phase is liquid, not solid. C …

Gelatin is a lyophilic colloid because it dissolves directly in water to form a stable, reversible colloidal sol due to strong particle-solvent affinity.

Lyophilic colloids ('solvent-loving') are formed when the dispersed phase has a strong affinity for the dispersion medium; such sols form spontaneously just by mixing the substance with the solvent, are generally reversible (the sol can be converted back to the solid by evaporation and reconstituted again by adding solvent), and are relatively stable, needing no special stabiliser. Common examples are gelatin, starch, gum, and proteins in water.

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