Emulsion is known as a
- (a)colloidal solution of substances having different physical states
- (b)true solution
- (c)distillation mixture for making alcohols
- (d)colloidal solution of two liquids
Correct — D, a colloidal solution of two liquids. An emulsion is the particular kind of colloid in which both the dispersed phase and the dispersion medium are liquids, and the two do not mix on their own — one is broken into fine droplets and held suspended in the other. Milk is the standard example, tiny globules of fat dispersed through water; butter is the reverse arrangement, water dispersed through fat. Because the definition names both phases as liquids, option (d) is more precise than the vaguer wording of option (a), and precision is what the question is testing.
- (a)colloidal solution of substances having different physical states — True of colloids in general but not a definition of an emulsion. Every colloid has a dispersed phase and a dispersion medium, and they may be solid in liquid (a sol), liquid in gas (an aerosol) or gas in liquid (a foam). Naming only 'different physical states' fails to pick out the liquid-in-liquid case, which is what the word emulsion means.
- (b)true solution — The opposite of the truth. In a true solution the solute is broken down to individual molecules or ions, the mixture is genuinely homogeneous, it does not scatter a light beam and it cannot be separated by filtration. An emulsion has droplets thousands of times larger, scatters light and can be broken by standing or centrifuging.
- (c)distillation mixture for making alcohols — Distillation separates liquids by exploiting differences in boiling point; it has nothing to do with the colloidal state. This option is filler, and the wording does not correspond to any standard chemical term.
Mixtures are graded by the size of the dispersed particles. Below about one nanometre the mixture is a true solution; above about a thousand nanometres it is a suspension, whose particles settle out and can be filtered; in between lies the colloidal range. Colloids are named by the pair of phases involved — a sol is solid in liquid, an aerosol is liquid or solid in gas, a foam is gas in liquid, a gel is liquid in solid, and an emulsion is liquid in liquid. Emulsions are usually unstable and need an emulsifying agent to keep the droplets from coalescing; the casein in milk and the lecithin in egg yolk both do this job.
Options (a) and (d) are the real contest, and the trap is that (a) is not false — it is merely not specific. In a 'known as' question, the option that reproduces the actual definition wins over the option that describes the wider family. A second useful marker is the Tyndall effect: shine a torch through milk and the beam becomes visible because the droplets scatter light, whereas the same beam passes through a true solution such as salt water without showing its path. That single observation separates option (b) from option (d) at once.
- An emulsion is a colloid in which both the dispersed phase and the dispersion medium are liquids.
- Milk is an oil-in-water emulsion; butter and cold cream are water-in-oil emulsions.
- Colloidal particles lie roughly between 1 and 1000 nanometres, which is why colloids scatter light and true solutions do not.
- Emulsifying agents such as soap, casein and lecithin stabilise an emulsion by coating the droplets and stopping them from merging.
- The scattering of a light beam by colloidal particles is the Tyndall effect, the standard laboratory test for a colloid.
- Settling for the broadly true option instead of the precisely correct one when both look defensible.
- Calling milk a solution because it looks uniform — it is opaque precisely because it is not one.
- Forgetting that an emulsion can run either way, oil in water or water in oil, depending on which liquid is the medium.
The colloid family reaches NDA either as a definition item like this one or through the Tyndall effect, so learn the phase pairs by name and keep one everyday example ready for each.
Which among the following is NOT true with respect to colloidal solution ?
- (a) Particles are uniformly distributed throughout the solution
- (b) Colloidal solution is homogenous in nature
- (c) They show Tyndal effect
- (d) They do not settle down when kept undisturbed
Answer(b) Colloidal solution is homogenous in nature
Nails the property that separates a colloid from a true solution — a colloid only looks uniform and is in fact heterogeneous, which is exactly why option (b) of this 2016 item fails.
Tyndall effect is a phenomenon of
- (a) scattering of light by the colloidal particles.
- (b) refraction of light by the colloidal particles.
- (c) dispersion of light by dust particles.
- (d) refraction of light by dust particles.
Answer(a) scattering of light by the colloidal particles.
The laboratory test that identifies a colloid, and the quickest practical way to tell an emulsion such as milk from a true solution.
- practice — not a real PYQ
Which one of the following is an example of a water-in-oil emulsion?
- (a)Milk
- (b)Butter
- (c)Soap solution
- (d)Smoke
Answer(b) Butter — water droplets dispersed through fat, the reverse arrangement of milk.
- practice — not a real PYQ
A beam of light passed through a colloidal solution becomes visible as a bright path. This observation is called
- (a)the Raman effect
- (b)the Tyndall effect
- (c)total internal reflection
- (d)dispersion
Answer(b) the Tyndall effect — colloidal particles are large enough to scatter light, which is why the path shows up in milk but not in salt water.