Cambridge O Level Chemistry · Syllabus 5070 · Experimental Techniques and Chemical Analysis
Fractional Distillation
What is Fractional Distillation?
A separation method for two or more liquids that mix completely and have different boiling points. The mixture is boiled and the vapour passes up a fractionating column, where it repeatedly condenses and re-boils on the packing. Each cycle enriches the rising vapour in the component with the lower boiling point, so that component reaches the top and distils over first while the higher-boiling component runs back into the flask. The thermometer at the top of the column shows a steady reading while one component is distilling over.
This definition is part of the Experimental Techniques and Chemical Analysis chapter in Cambridge O Level Chemistry.
Common mistakes with Fractional Distillation
- 11. “The residue is what you collect at the end of the distillation.” Why it fails What is collected is the distillate. The residue is what stays behind in the heated flask. Correct Distillate: condensed vapour, in the receiver. Residue: not vaporised, in the flask. Check In a fractional distillation of ethanol and water, which one is the ethanol at the start? The distillate — and the water is the residue until it in turn distils.
- 27. “Use a separating funnel to separate the mixture.” Why it fails A separating funnel is not one of the five methods named in statement 12.4.1, so it is not the expected answer — and it only works at all for liquids that do not mix. Correct Choose from a suitable solvent, filtration, crystallisation, simple distillation and fractional distillation, and justify the choice from the property data. Check Ethanol and water are miscible, so no layers form; fractional distillation is the method.
Examiner tips on Fractional Distillation
- The three-part answer. A full-credit 12.4.2 answer names the method, says which property difference makes it work, and states what you keep. “Fractional distillation” on its own is one third of an answer. “Fractional distillation, because the two liquids are miscible but have different boiling points, and the one boiling at \(65\ ^\circ\mathrm{C}\) is collected first” is all of it.
Questions students ask about Fractional Distillation
How should you choose a separation technique on an unfamiliar substance?
Match the technique to the property difference the question gives you: use a suitable solvent or filtration when solubility differs, crystallisation to recover a dissolved solid, simple distillation to separate a solvent from a dissolved solid or non-volatile component, and fractional distillation when two or more miscible liquids have different boiling points. A full-credit answer names the method, states the property difference that makes it work, and says what is kept.