MPSC 2026 Prelims Paper-I · Question 65 of 100

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.

• (1)Solution — 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.
• (2)Foam — 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.
• (4)Gel — 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.

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.

Naming colloids by the two states generates the whole standard table, and every entry has a familiar example. Solid in liquid is a sol (paint, cell fluid); solid in gas is an aerosol (smoke, dust); solid in solid is a solid sol (coloured glass and some alloys). Liquid in liquid is an emulsion (milk); liquid in solid is a gel (cheese, jellies); liquid in gas is again an aerosol (fog, mist, cloud, spray). Gas in liquid is a foam (lather, froth, whipped cream); gas in solid is a solid foam (pumice, foam rubber). There is no gas-in-gas colloid, because gases mix freely at the molecular level and give a true solution. Emulsions are the entry with the most everyday reach — milk and dairy products, cosmetics and creams, and the entire action of soaps and detergents, which emulsify oily dirt so that water can carry it away.

• An emulsion is a colloidal dispersion of one liquid in another liquid with which it is immiscible. Milk, in which liquid fat is dispersed through water, is the standard example.
• Emulsions are of two types: oil-in-water, where oil droplets are dispersed in water (milk, vanishing cream), and water-in-oil, where water droplets are dispersed in oil (cold cream).
• An emulsion needs an emulsifying agent to stay stable, since the immiscible droplets would otherwise coalesce and the layers separate. Soaps and detergents act as emulsifiers, which is the basis of their cleaning action; the proteins in milk stabilise milk.
• Colloidal particle size runs roughly from 1 to 1000 nanometres, between a true solution (below about 1 nm, homogeneous, no light scattering) and a suspension (above about 1000 nm, particles settle out).
• Colloids are named as dispersed phase in dispersion medium: sol (solid in liquid), gel (liquid in solid), foam (gas in liquid), solid foam (gas in solid), aerosol (solid or liquid in gas), solid sol (solid in solid). There is no gas-in-gas colloid.

• Assuming an emulsion is any oily mixture. The definition needs both liquids to be liquid and the mixture to be colloidal, not merely cloudy; two layers standing apart are not an emulsion.
• Swapping the two members of a pair — gel is liquid in solid while sol is solid in liquid, and a candidate who has learnt the names without the order will get both wrong together.
• Marking 'Solution' because the stem describes a mixture that looks uniform. A true solution is homogeneous and shows no Tyndall effect, and immiscible liquids cannot form one.

This topic is asked in three shapes and all three come from the same table. The definition-to-name question, as here: describe the two states and ask for the term. The example-to-name question, which is commoner in state papers: milk, fog, smoke, whipped cream, pumice stone or cheese is named and the colloid type asked for. And the property question, which asks what distinguishes a colloid from a true solution and expects the Tyndall effect, or asks about Brownian motion or coagulation. Because the whole area is generated by one two-way classification, the efficient preparation is to write the eight-cell table out once with an example in each cell and be able to read it in either direction.

No directly related past PYQ was found.

• practice — not a real PYQ

  Milk is a familiar example of which one of the following types of colloidal system?

  • (a)Gas dispersed in a liquid
  • (b)Liquid dispersed in a liquid
  • (c)Solid dispersed in a liquid
  • (d)Liquid dispersed in a solid

  Answer(b) Liquid dispersed in a liquid — that is, an emulsion, in which droplets of liquid fat are dispersed through water and are held apart by the proteins of the milk acting as an emulsifying agent. Gas in liquid would be a foam, solid in liquid a sol, and liquid in solid a gel.

• practice — not a real PYQ

  The scattering of a beam of light by the particles of a colloidal dispersion, which makes the path of the beam visible, is known as :

  • (a)Brownian motion
  • (b)Tyndall effect
  • (c)Coagulation
  • (d)Electrophoresis

  Answer(b) Tyndall effect — colloidal particles are large enough to scatter light, so the beam's path can be seen, while a true solution shows nothing. It is the simplest practical test for telling a colloid from a true solution. Brownian motion is the random movement of the particles themselves, and coagulation is the clumping together that destroys the colloid.