NDA (II) 2023 General Ability Test · Question 114 of 146

Correct — B, Colloidal solution is homogenous in nature. A colloid always has two distinct parts — a dispersed phase of particles roughly between one nanometre and one micrometre across, and a continuous medium they are spread through — so it is classed as a heterogeneous mixture even though the eye cannot pick out the two parts. That is precisely the line the question is drawing: a true solution is uniform right down to the level of individual molecules and is a single phase, whereas a colloid only looks that way. The other three statements are the standard textbook properties of a colloid and are all true, which is what makes this one the odd statement out.

• (a)Particles are uniformly distributed throughout the solution — This is true of a colloid, not false. The dispersed particles are spread evenly through the medium, which is exactly why a colloid looks uniform and why students are tempted to call it homogeneous.
• (c)They show Tyndal effect — Also true. Colloidal particles are large enough to scatter light, so a beam passing through a colloid becomes visible as a bright path — the property used to tell a colloid from a true solution. The paper prints the name with a single l; the effect is named after John Tyndall.
• (d)They do not settle down when kept undisturbed — True as well. Colloidal particles are small enough that the constant jostling of Brownian motion keeps them suspended, so a colloid is stable on standing. It is a suspension, with its much larger particles, that settles out.

Mixtures are graded by the size of the dispersed particles. Below about one nanometre the particles are individual molecules or ions and the mixture is a true solution — one phase, uniform, non-scattering, and unable to be filtered or settled. Between roughly one nanometre and one micrometre it is a colloid — two phases, uniform to the eye, scattering light, and stable on standing. Above that it is a suspension — visibly two phases, scattering, and settling under gravity.

The trap is built into the word 'looks'. Milk, fog, ink and soap solution all appear perfectly uniform, and school texts themselves say that a colloid appears homogeneous. The examiner is testing whether you carry the second half of that sentence — that it appears homogeneous but is actually heterogeneous, because a dispersed phase and a dispersion medium can still be told apart. Usage is not perfectly tidy across sources, and a few loosely describe colloids as homogeneous mixtures; the classification the exam and the school syllabus use is the one given here. Two printed slips are worth noting only so they do not throw you: 'homogenous' is a variant spelling of homogeneous, and 'Tyndal' is missing an l.

• Colloidal particles have at least one dimension roughly between one nanometre and one micrometre.
• A colloid has a dispersed phase and a dispersion medium, which is why it is classed as heterogeneous.
• The Tyndall effect is the scattering of a light beam by colloidal particles, making the path of the beam visible.
• Colloids do not settle out on standing, because Brownian motion offsets gravity; suspensions, with larger particles, do settle.

• Reading 'appears homogeneous' as 'is homogeneous' — the question turns on exactly that gap.
• Assuming anything called a 'solution' must be a true solution; 'colloidal solution' is an old name for a colloid.
• Expecting a colloid to settle like muddy water; it does not, which is why it needs light scattering to reveal it.

Asked as a NOT-true item — three textbook properties of colloids and one inverted definition slipped in among them.

• practice — not a real PYQ

  Which one of the following is used to distinguish a colloid from a true solution?

  • (a)Filtration through ordinary filter paper
  • (b)The Tyndall effect
  • (c)Measuring the boiling point
  • (d)Letting the mixture stand undisturbed

  Answer(b) The Tyndall effect — a colloid scatters a beam of light and makes its path visible, while a true solution does not.

• practice — not a real PYQ

  Which one of the following is a suspension rather than a colloid?

  • (a)Milk
  • (b)Fog
  • (c)Chalk powder stirred into water
  • (d)Soap solution

  Answer(c) Chalk powder stirred into water — its particles are large enough to be seen and to settle out on standing, which a colloid's are not.