NDA (II) 2023 General Ability Test · Question 115 of 146

Correct — C, Fractional distillation. Kerosene and petrol are both fractions of the same crude oil and they mix completely, so no method that relies on two visible layers or on a change of state of one component alone will work. What separates them is boiling point, and each is itself a blend of hydrocarbons whose boiling ranges run into one another. A single simple distillation would therefore give an impure cut. A fractionating column solves this by giving the rising vapour many successive cycles of condensation and re-evaporation, so the more volatile petrol fraction is progressively enriched at the top while the less volatile kerosene stays lower down. That is exactly how a refinery separates them from crude oil in the first place.

• (a)Sublimation — Sublimation separates a solid that passes straight to vapour — ammonium chloride, camphor or naphthalene — from a solid that does not. Both petrol and kerosene are liquids, so there is nothing to sublime.
• (b)Separating funnel — A separating funnel works only on immiscible liquids, which settle into two layers that can be run off one at a time, as with kerosene and water. Petrol and kerosene are miscible and form a single layer, so a funnel has nothing to separate.
• (d)Compressing and cooling — Compressing and cooling is how gases are liquefied, and it is the step that prepares a gas mixture such as air for separation. Petrol and kerosene are already liquids in the same phase, so compressing and cooling the mixture leaves it exactly as it was.

Choosing a separation technique is a matter of matching the method to the property that differs between the components. Immiscible liquids differ in density and separate in a funnel. A sublimable solid differs in its route from solid to vapour. Miscible liquids differ only in volatility, so they are separated by distillation — simple distillation where the boiling points are far apart, and fractional distillation, using a packed or trayed column, where they are close.

The stem says 'best separated', which is the examiner signalling that more than one method might be attempted but only one is appropriate. The tempting wrong answer is the separating funnel, because kerosene is the standard textbook partner for water in immiscible-liquid demonstrations — but the partner here is petrol, not water, and the two mix. The usual classroom rule is that fractional distillation is called for when boiling points differ by less than about 25 K; petrol and kerosene are not single compounds but overlapping bands of hydrocarbons, which is the same difficulty in a stronger form and the reason refineries use tall fractionating columns rather than a single flask.

• Fractional distillation separates a mixture into fractions by boiling point, using a column that provides repeated vaporisation and condensation.
• It is the method of choice for miscible liquids whose boiling points are close — the usual textbook cut-off is a difference of less than about 25 degrees.
• Petrol and kerosene are both fractions of crude oil and are drawn off at different heights of the same refinery column, the lightest fractions leaving from the top.
• A separating funnel works only on immiscible liquids, which form two visible layers.

• Reaching for the separating funnel because kerosene appears in the standard kerosene-and-water demonstration; here the partner liquid is miscible.
• Treating petrol and kerosene as single compounds with one boiling point each — both are bands of hydrocarbons.
• Confusing compressing and cooling, which liquefies gases, with a method of separating two liquids.

Asked as a match-the-method item — name the pair of substances and pick the technique that exploits the property in which they differ.

• practice — not a real PYQ

  Which one of the following pairs can be separated using a separating funnel?

  • (a)Petrol and kerosene
  • (b)Kerosene and water
  • (c)Ethanol and water
  • (d)Common salt and water

  Answer(b) Kerosene and water — they are immiscible and settle into two layers, whereas the other pairs are miscible or form a solution.

• practice — not a real PYQ

  In the fractional distillation of crude oil, the fraction collected nearest the top of the column is

  • (a)the one with the highest boiling point
  • (b)the one with the lowest boiling point
  • (c)the densest fraction
  • (d)the fraction with the largest molecules

  Answer(b) the one with the lowest boiling point — the lightest, most volatile fractions rise highest before condensing, while the heaviest leave from the bottom.