NDA (II) 2023 General Ability Test · Question 92 of 146

Correct — C, A-3 B-4 C-2 D-1. Acetone and water mix completely, so no funnel can part them; what separates them is the gap between their boiling points, about 56 degrees Celsius for acetone against 100 for water, and distillation exploits exactly that. Water and kerosene do not mix at all and settle into two layers of different density, which is the textbook case for a separating funnel. Cream is a suspension of fat globules dispersed through milk, and spinning the milk throws the denser skimmed portion outward and leaves the lighter cream behind, which is centrifugation. Plant pigments are several coloured compounds dissolved together, and chromatography separates them because each one clings to the stationary medium and travels with the solvent to a different extent. That gives A with 3, B with 4, C with 2 and D with 1.

• (a)A-3 B-2 C-4 D-1 — It gets acetone-water and the pigments right but swaps the middle pair. A separating funnel cannot take cream out of milk, because cream and milk are not two immiscible layers, and a centrifuge is the long way round for water and kerosene, which simply stand apart on their own.
• (b)A-4 B-2 C-1 D-3 — Every pair is misplaced. A separating funnel is useless on the miscible acetone-water mixture, chromatography does not skim cream, and distilling a plant extract would drive off the solvent and destroy the very pigments the question wants separated.
• (d)A-4 B-1 C-2 D-3 — Only cream and centrifugation are correctly paired. It sends the miscible acetone-water pair to a separating funnel, hands the immiscible water-kerosene pair to chromatography, and again tries to distil plant pigments.

Every separation method is chosen for the one physical property in which the components of the mixture differ. Miscible liquids differ in boiling point, so they are distilled. Immiscible liquids differ in density and refuse to mix, so they settle in a separating funnel and the lower layer is run off. A dispersed solid or a suspended phase differs in density from the liquid around it, and a centrifuge speeds up the settling that gravity would take hours to do. Components that dissolve together but stick to a surface to different degrees are separated by chromatography.

A four-by-four matching item is best cracked by finding the one pair you are certain about and striking out every code that disagrees. Here the safest anchor is D — pigments from a plant extract can only be chromatography, and that alone removes options (b) and (d). Between the two survivors the deciding pair is B: water and kerosene are immiscible, so they belong to a separating funnel, not a centrifuge, which leaves option (c). Notice that the two liquid-liquid pairs in this question are deliberately contrasted — one miscible, one not — because that is the distinction the examiner is really testing. On the acetone-water pair, the boiling points differ by more than 25 degrees, so a simple distillation is enough; a closer pair would call for fractional distillation.

• Distillation separates miscible liquids with different boiling points; acetone boils at about 56 degrees Celsius and water at 100.
• A separating funnel is used for immiscible liquids such as water and kerosene, which form two layers of different density.
• Centrifugation separates a dispersed or suspended phase from a liquid by spinning it, as when cream is taken off milk.
• Chromatography separates dissolved components, such as the pigments in a plant extract, by their differing attraction to a stationary medium.

• Reaching for a separating funnel whenever two liquids are named, without first asking whether they actually mix.
• Treating milk as a two-layer liquid; cream is a dispersed phase, which is why it needs a centrifuge and not a funnel.
• Answering distillation for the plant extract because the solvent boils off — that removes the solvent but does not separate the pigments from one another.

Asked as a four-pair matching item — link each mixture to the separation method that uses the property in which its components differ.

• practice — not a real PYQ

  Which method would you use to separate a mixture of oil and water?

  • (a)Distillation
  • (b)Separating funnel
  • (c)Chromatography
  • (d)Sublimation

  Answer(b) Separating funnel — oil and water are immiscible and form two layers of different density, so the lower layer can simply be run off.

• practice — not a real PYQ

  The technique used to separate the different coloured components of a dye is

  • (a)centrifugation
  • (b)crystallisation
  • (c)chromatography
  • (d)filtration

  Answer(c) chromatography — the components travel different distances because each is held by the stationary medium to a different extent.