NDA (I) 2023 General Ability Test · Question 75 of 149

Correct — C, Separating funnel. Benzene and water are two immiscible liquids of different density — benzene is lighter and floats as a distinct upper layer. A separating funnel exploits exactly this: the lower, denser water layer is run out through the stopcock, leaving the benzene behind. It is the standard apparatus for separating two immiscible liquids.

• (a)Round bottom flask — A round-bottom flask is just a container used for heating or holding liquids; it has no mechanism to draw off one layer, so it cannot separate the two liquids.
• (b)Conical flask — A conical (Erlenmeyer) flask is a general-purpose container for mixing and titrations; it cannot separate immiscible layers.
• (d)Dean and Stark apparatus — The Dean-Stark apparatus is used to collect and measure water removed during a reaction by azeotropic distillation, not to separate a simple benzene-water mixture by density.

Immiscible liquids do not mix and settle into separate layers by density. The right way to separate them is a separating funnel: let the layers settle, then open the stopcock to run off the lower layer while the upper layer stays in the funnel.

The trap is the Dean-Stark apparatus, which sounds technical and does involve water — but it removes water during a reaction, not from a plain benzene-water mixture. Round-bottom and conical flasks are mere containers. Match 'immiscible liquids' to 'separating funnel.'

• Benzene and water are immiscible; benzene (density about 0.88) floats on water.
• A separating funnel separates immiscible liquids using their density difference.
• The lower layer is drained through the stopcock; the upper layer remains.
• Miscible liquids (e.g. alcohol and water) instead need fractional distillation.

• Choosing the Dean-Stark apparatus because it mentions water.
• Confusing immiscible-liquid separation with distillation (used for miscible liquids).

Asked as a 'which apparatus/method separates this mixture' item, testing the separating funnel.

No directly related past PYQ was found.

• practice — not a real PYQ

  A separating funnel is most suitable for separating

  • (a)two miscible liquids
  • (b)two immiscible liquids
  • (c)a solid dissolved in a liquid
  • (d)two gases

  Answer(b) two immiscible liquids — it drains the denser lower layer off first.

• practice — not a real PYQ

  A mixture of two miscible liquids with different boiling points is best separated by

  • (a)a separating funnel
  • (b)fractional distillation
  • (c)sublimation
  • (d)filtration

  Answer(b) fractional distillation — a separating funnel only works for immiscible liquids.