A colloidal dispersion of one liquid in another immiscible with it is called _________.

Question

UnlockIAS · Accepted Answer

Correct — option (3), Emulsion. Colloids are named by a single rule: state of the dispersed phase, in state of the dispersion medium. Read the stem against that rule and it answers itself. 'One liquid' dispersed in 'another immiscible with it' is liquid-in-liquid, and the liquid-in-liquid colloid is called an emulsion. Milk is the standard example — droplets of liquid fat dispersed through water. Because the two liquids do not mix, the droplets would coalesce and the system separate into two layers if nothing held them apart; what keeps an emulsion stable is a third substance called an emulsifying agent or emulsifier, which coats the droplet surface. In milk that role is played by its proteins; in a soap solution lifting grease off cloth it is played by the soap itself, and that is precisely how detergents work. Emulsions come in two kinds depending on which liquid is inside and which outside: oil-in-water, where oil droplets are dispersed in water, as in milk and vanishing cream; and water-in-oil, where water droplets are dispersed in oil, as in cold cream. Note the two words in the stem that make the answer exact — 'colloidal', which excludes a true solution, and 'immiscible', which is why the mixture must be a two-phase dispersion in the first place.

Why the other options are wrong — (a) A true solution is not a colloid at all. It is homogeneous, its solute particles are of molecular or ionic size — conventionally below about one nanometre — and it does not scatter a beam of light, so it shows no Tyndall effect. Colloidal particles are far larger, roughly one to a thousand nanometres, big enough to scatter light and to keep the system heterogeneous even though it looks uniform. Beyond that, the stem says the two liquids are immiscible, and immiscible liquids by definition cannot form a solution in each other: they form two layers, or, when broken into fine droplets and stabilised, an emulsion.  (b) A foam is a gas dispersed in a liquid, not a liquid dispersed in a liquid — soap lather, the froth on a beaten liquid, whipped cream. The dispersed phase is wrong by one state. Keep the neighbouring cases in the same table so the confusion cannot arise: gas in solid gives a solid foam, such as pumice stone or foam rubber; liquid in gas gives the aerosol we see as fog, mist and cloud; solid in gas gives smoke and dust. The whole set is generated from two questions — what is dispersed, and what is it dispersed in.  (d) A gel is a liquid dispersed in a solid — the medium is solid, so the system holds its shape. Cheese and table jellies are the standard examples. Here it is the dispersion medium that is wrong: the stem specifies that the second substance is also a liquid, which fixes the medium as liquid and the answer as emulsion. The complementary case, solid dispersed in a liquid, is called a sol, and covers paint and the fluid inside a cell. Gel and sol are two sides of the same pair and are worth learning together, since a system can be made to pass between them.

Concept: Mixtures are sorted by the size of the dispersed particles. Below about one nanometre the mixture is a true solution — homogeneous, stable, non-scattering, and its particles pass through filter paper and parchment alike. Above about a thousand nanometres it is a suspension, whose particles are visible or settle out under gravity and can be filtered off. Between the two lies the colloidal state: particles too big to be molecular, too small to settle, which pass through ordinary filter paper but are held back by finer membranes. A colloid therefore looks homogeneous to the eye but is heterogeneous in fact, having a dispersed phase and a dispersion medium. The practical test is the Tyndall effect — shine a narrow beam through the mixture and a colloid scatters it, making the path of the beam visible, while a true solution leaves it invisible.

State PCS 2026 Q65 — Science & Technology PYQ with Answer

State PCS 2026 Q65 — Science & Technology PYQ with Answer