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K324 / 2027
Experimental Chemistry

Lesson 4 of 6

Distillation and liquid layers

Use boiling-point differences or immiscibility.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can describe separation methods.Syllabus

    Syllabus K324, 1.2(a). Describe separation methods

  • I can choose separation methods from properties.Syllabus

    Syllabus K324, 1.2(b). Choose separation methods from properties

Distillation and liquid layers

Use boiling-point differences or immiscibility.

Make a guess. You are not marked.

A mixture of ethanol (boiling point 78 degrees C) and water (boiling point 100 degrees C) is heated slowly in fractional distillation. Which liquid collects first?

  1. Both together, in the same ratio as the mixture
  2. Mostly ethanol, as it boils at a lower temperature
  3. Mostly water, as there is more of it in the mixture
Show the answer

Mostly ethanol, as it boils at a lower temperature

Ethanol has the lower boiling point, so its vapour reaches the top of the column first while most water condenses and drips back.

Simple distillation recovers a solvent from a solution containing a non-volatile solute. Heat salt water: water vaporises, enters a condenser, cools and becomes liquid distillate. Salt remains in the flask. Place the thermometer bulb near the side-arm to measure the vapour entering the condenser. Cooling water enters the condenser at the lower end and exits at the upper end to keep the jacket full.

Fractional distillation separates miscible liquids with different boiling points, especially when those boiling points are close. A fractionating column provides repeated evaporation and condensation. The vapour reaching its top is richer in the more volatile, lower-boiling component. Collect fractions over suitable temperature ranges. Crude oil is separated into groups of hydrocarbons this way; a fraction is generally a mixture, not one pure compound.

Pure only

For immiscible liquids such as oil and water, use a separating funnel. Let two layers settle, remove the stopper and open the tap to drain the denser lower layer. Close the tap at the boundary. Density tells you which layer is lower; do not assume the substance you want is always on top.

Check your understanding

You need pure water from seawater. Why choose distillation rather than evaporation to dryness?

  1. Distillation condenses and collects the water vapour; evaporation lets it escape.
  2. Evaporation leaves salt dissolved in the water, so the water that remains is impure.
  3. Distillation removes the salt faster, because it heats the seawater to a higher temperature.
Show the answer

Distillation condenses and collects the water vapour; evaporation lets it escape.

Both methods boil off the water, but only distillation cools the vapour back into liquid and collects it. Evaporation keeps the salt and loses the water.