Distillation: collect a solvent and separate liquid mixtures
Connect vaporisation, condensation and collection, then compare simple and fractional distillation using the mixture's supplied properties.
Section C12.4 · Separation and purification
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Distinguish evaporation, boiling and condensation.
- Use supplied boiling-point information and identify the desired recovered component.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which component is collected as distillate in the ideal salt-water model?
What you will learn
- Explain simple distillation for recovering a volatile solvent from non-volatile solute.
- Explain how a fractionating column improves separation of suitable miscible liquids with different boiling behaviour.
Collect the vapour after condensation
Simple distillation heats a suitable solution so the more volatile component vaporises, then cools that vapour in a condenser and collects liquid distillate. In the ideal salt-water model, water is volatile and the dissolved salt is non-volatile, so water can be collected while salt remains in the flask. Evaporation alone loses the solvent to the surroundings unless a collection stage is added.
The diagram shows a thermometer bulb near the vapour entrance to the condenser, rather than immersed in the boiling liquid. Cooling water enters the lower condenser connection and leaves the upper one to keep the jacket filled. The receiving vessel is open in the schematic; it is not a sealed heated pressure system. Actual apparatus assembly, heating and handling belong to an appropriate supervised laboratory procedure.
Choose the method from the mixture's behaviour
Fractional distillation adds a column where repeated vaporisation and condensation improve separation of suitable miscible liquids with different boiling behaviour, especially when their boiling points are relatively close. In a supplied simplified two-liquid model, the rising vapour becomes richer in the more volatile, lower-boiling component before condensation and collection.
A single statement that the first liquid is perfectly pure does not follow from different boiling points alone. Real separation depends on mixture behaviour and the method; a boiling-point match also needs the stated pressure and appropriate purity evidence. Immiscible liquids or an insoluble solid mixture may need another technique entirely. Select the target and relevant properties rather than saying fractional distillation is the best answer for every mixture.
Sources for this lesson
Pause and explain
What is the fractionating column's role in the supplied suitable liquid mixture?
Worked example
Choose a method to collect water from the ideal salt-water model and to improve separation of a supplied suitable miscible-liquid pair boiling at 78°C and 83°C.
Show the worked solution
- Water recovery from non-volatile salt needs vaporisation followed by condensation and collection, so use simple distillation.
- For the suitable miscible-liquid pair with close boiling points, a fractionating column provides repeated vaporisation and condensation.
- In the simplified pair, the first collected fraction is richer in the lower-boiling component; absolute purity is not guaranteed by the two numbers alone.
Answer Simple distillation for the ideal salt-water solvent; fractional distillation for the stated suitable close-boiling liquid pair, without assuming guaranteed perfect purity.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain simple distillation of the ideal non-volatile salt/volatile water solution, including the condenser.
Give me a hint
Track the solvent through gas and liquid states.
Compare my reasoning
- Heat using the appropriate supervised method so water vaporises while non-volatile salt remains.
- Cool the vapour in the condenser; cooling water enters lower and leaves upper.
- Collect the condensed water as distillate in the receiving vessel.
Vaporise water, condense its vapour and collect distillate; non-volatile salt remains in the ideal model.
Look for these in your work
- Both changes of state and the target are identified.
- The condenser's cooling-water direction is correct.
Explain why fractional distillation can improve separation of the supplied suitable miscible liquids boiling at 78°C and 83°C.
Give me a hint
Describe what the column repeats and how vapour composition changes.
Compare my reasoning
- The boiling points are close in this supplied suitable-mixture model.
- The column provides repeated vaporisation and condensation.
- The rising vapour becomes richer in the lower-boiling component, improving its separation before collection.
Repeated vaporisation/condensation enriches the more volatile 78°C component in the rising vapour; purity still needs evidence.
Look for these in your work
- The column mechanism is given.
- Improved separation is not overclaimed as guaranteed purity.
A proposal uses filtration to collect water from dissolved salt, then uses evaporation without a condenser when that fails. Evaluate both choices against the target.
Give me a hint
The target is collected solvent, not simply removal of solvent.
Compare my reasoning
- Ordinary filtration passes the salt solution rather than removing its dissolved solute.
- Evaporation without collection lets solvent vapour escape, so it does not recover liquid water.
- Simple distillation supplies the needed vaporisation, condensation and collection stages for this ideal model.
Neither proposed route collects the intended separated water; simple distillation does so under the ideal non-volatile-solute assumptions.
Look for these in your work
- Both unsuitable methods are explained.
- The alternative addresses the specified recovery target.
Common mistakes
- Using evaporation alone when the target is collected solvent.
- Reversing lower cooling-water inlet and upper outlet.
- Placing the thermometer bulb in the liquid when measuring vapour temperature in this setup.
- Assuming every first fraction must be a perfectly pure substance.
What can you explain now?
What change turns collected solvent vapour back into liquid?
Compare with the explanation
Condensation in the cooled condenser.
Heating and cooling serve different stages of the separation.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Label a simple-distillation diagram, explain cooling-water direction and distinguish the column's role from filtration.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.