Learning focus
Develop safe and reproducible practical methods, record precise observations and measurements, process evidence and evaluate experimental quality.
Purpose
Fractional distillation separates miscible liquids with different but relatively close boiling points.
Fractionating column
The column provides a temperature gradient and large surface area. Vapours condense and revaporise repeatedly, enriching the rising vapour in the more volatile component.
Collection
The lower-boiling fraction reaches the condenser first. Temperature remains near its boiling point while it distils, then rises before the next fraction.
Comparison
Simple distillation lacks effective repeated separation and is suitable when one component is non-volatile or boiling points differ greatly. Fractional distillation provides greater separation for close boiling points.
Practical or data skill
Use boiling-point data to decide whether simple or fractional distillation is appropriate.
Examination tip
Explain the fractionating column, not merely that it is present.
Review questions and suggested answers
Question 1
Which component distils first?
Suggested answer
The component with the lower boiling point.
Question 2
What occurs repeatedly in the column?
Suggested answer
Condensation and revaporisation.
Question 3
When is fractional distillation preferred?
Suggested answer
For miscible liquids with relatively close boiling points.
