CDS (II) 2024 General Knowledge · Question 11 of 120

Correct — D, Shaving cream. A colloid is described by two things: the dispersed phase, which is broken up and scattered, and the dispersing medium, which surrounds it. Shaving cream is a lather — gas bubbles scattered through a liquid — so the dispersed phase is a gas and the dispersing medium is a liquid, which is what the item asks for. Test the others the same way. Mist is fine droplets of liquid floating in air, so its medium is a gas, not a liquid. Jelly is a gel, in which liquid is held within a solid network, so its medium is a solid. That leaves the second and fourth options, both of which describe frothy material. Read strictly, a foam is also a gas dispersed in a liquid, so it would qualify as well; read in the everyday Indian sense of foam as the solid, spongy material of a mattress or a packing sheet, it is a gas in a solid and does not. Shaving cream is the option that needs no such reading, and that is what makes it the safe answer.

• (a)Mist — Mist is tiny droplets of water suspended in air. The dispersed phase is a liquid and the medium is a gas, so it is an aerosol, the reverse of what the item wants. Fog and cloud belong to the same box.
• (b)Foam — This option is the loose one. Where foam means a lather such as whipped cream or soap suds, it is gas in liquid and would also fit; where it means the solid, spongy foam of a mattress or pumice-like material, it is gas in solid and does not. Because the word can be read either way, it cannot be the intended single answer — but a candidate who marked it was not reasoning badly.
• (c)Jelly — Jelly is a gel: liquid trapped inside a solid network that holds its shape. The dispersed phase is the liquid and the medium is the solid, so it is the exact opposite of what the item asks for. Cheese and butter sit in the same box.

A colloid sits between a true solution and a suspension. Its particles are larger than those of a solution, so they scatter a beam of light, but small enough that they do not settle out on standing. Because both the scattered phase and the surrounding medium can be solid, liquid or gas, colloids are classified in a grid of combinations, each with its own name — aerosol for liquid or solid in gas, sol for solid in liquid, emulsion for liquid in liquid, foam for gas in liquid, gel for liquid in solid, and solid foam for gas in solid. Gas dispersed in gas gives no colloid, because gases mix completely.

The reliable way to attack any item of this kind is to name the two phases in that order — what is scattered, and what surrounds it — before looking at the options at all. Mist scatters liquid in air, jelly holds liquid in a solid, a lather scatters gas in liquid. This particular item is worth a second look, because two of its four options describe frothy material and the word foam is used in more than one sense in ordinary speech. Standard reference tables treat shaving cream as an example of foam, which puts the second and fourth options in the same box; the item works only if foam is taken in its everyday sense of the solid spongy material. Where a question turns on a word used loosely, the safe strategy in the hall is to mark the option that needs no interpretation.

• A colloid is named by its dispersed phase and its dispersing medium, in that order.
• Aerosol is liquid or solid dispersed in a gas — fog, mist, cloud, smoke.
• Foam is gas dispersed in a liquid — whipped cream, soap lather, shaving cream.
• Gel is liquid dispersed in a solid — jelly, cheese, butter.
• Solid foam is gas dispersed in a solid — styrofoam, foam rubber, pumice, aerogel.
• Colloidal particles scatter a light beam, which is the Tyndall effect, and they do not settle out on standing.

• Reversing the two phases; the name always states the dispersed phase first and the medium second.
• Treating a colloid as a mixture that will settle if left alone; it does not, which is what separates it from a suspension.
• Assuming an everyday word maps onto exactly one box — foam and cream are both used loosely.

Usually as an example-to-type match, or as an odd-one-out on the dispersing medium; the Tyndall effect and the solution-colloid-suspension ladder are the commonest companions.

• practice — not a real PYQ

  Milk is an example of which one of the following types of colloid?

  • (a)Aerosol
  • (b)Emulsion
  • (c)Gel
  • (d)Foam

  Answer(b) Emulsion — milk is fine droplets of fat dispersed in water, a liquid scattered through another liquid.

• practice — not a real PYQ

  Fog and cloud are colloids in which the dispersed phase and the dispersing medium are respectively

  • (a)liquid and gas
  • (b)gas and liquid
  • (c)solid and gas
  • (d)liquid and solid

  Answer(a) liquid and gas — water droplets are scattered through air, which makes fog and cloud aerosols.