Milk is an example of :

Question

UnlockIAS · Accepted Answer

Correct — D, Emulsion. Colloids are named by a single rule: take the physical state of the dispersed phase and the physical state of the dispersion medium, and the pair of states gives you the name. Milk is fat dispersed through water. The fat is liquid butterfat, the medium is liquid water, and a liquid dispersed in a liquid is called an emulsion — more precisely an oil-in-water emulsion. Cow's milk carries roughly three to four per cent fat, present not as a dissolved substance but as separate globules a few micrometres across, which is why milk is not a true solution. Left alone, those globules would float upwards and separate as a layer of cream; they do not, because milk's own proteins, chiefly the caseins together with whey proteins, coat each globule and act as natural emulsifying agents, and because commercial homogenisation forces the milk through fine openings to break the globules much smaller so they stay dispersed. That is the emulsion the key marks. One honest refinement. Milk is in truth a composite system: an emulsion of fat in water, plus a colloidal dispersion of casein micelles (which behaves like a sol), plus lactose and mineral salts genuinely in true solution. Chemistry textbooks and every exam key classify milk by its defining feature, the fat-in-water emulsion, and that is why (b) Sol is the strongest distractor rather than an outright error of the setter. Answer 'emulsion', and know why 'sol' is not absurd.

Why the other options are wrong — (a) A foam is a GAS dispersed in a liquid — soap lather, shaving foam, whipped cream, the froth on boiling milk or on a glass of beer. Milk in the glass contains no dispersed gas phase at all. You can turn milk into a foam by whipping air into it, as in the frothed milk of a cappuccino, but that is something done to milk, not what milk is.  (b) A sol is a SOLID dispersed in a liquid — paints, muddy water, starch stirred into water, cell fluid. This is the most defensible wrong answer, because milk really does carry casein protein micelles dispersed through water, which is sol-like behaviour, and those micelles contribute to milk's whiteness. But the classification a chemistry paper is asking for keys on the dispersed FAT, which is liquid, not solid; milk's textbook identity is the oil-in-water emulsion, and it is listed as the standard example of an emulsion in every syllabus table.  (c) In an aerosol the dispersion MEDIUM is a gas: a liquid dispersed in gas gives fog, mist, clouds and insecticide sprays, and a solid dispersed in gas gives smoke and dust. Milk's medium is plainly water, a liquid, so the option fails at the first test. This distractor works only on a candidate who remembers the word 'aerosol' as meaning 'a fine dispersion' without remembering which phase is which.

Concept: Mixtures are graded by particle size. In a true solution, such as sugar in water, the dissolved particles are smaller than about one nanometre, the mixture is transparent and nothing settles. In a suspension, such as chalk powder in water, the particles are larger than about one micrometre, the mixture is visibly cloudy and the particles settle on standing. Between the two lies the colloid, with particles roughly one nanometre to one thousand nanometres: too small to settle, too large to let light pass undisturbed. A colloid always has two parts — the dispersed phase and the dispersion medium — and the eight classical types are named by the states of those two: emulsion (liquid in liquid), foam (gas in liquid), sol (solid in liquid), gel (liquid in solid), aerosol (liquid or solid in gas), solid sol, solid foam and solid aerosol.

State PCS 2024 Q65 — Science & Technology PYQ with Answer

State PCS 2024 Q65 — Science & Technology PYQ with Answer