When the two components of a mixture are both liquids, separation becomes a subtler problem than pulling a solid out of water — and a far more important one. The petrochemical industry runs on it, spirits distillers have practised it for a thousand years, and pharmaceutical and perfume chemistry depend on it daily. The technique is distillation: exploiting the fact that different liquids boil at different temperatures.
Ethanol boils at 78.37 °C, water at 100 °C. Heat a mixture of the two and the vapour that rises is richer in ethanol than the liquid it came from, because at any temperature the more volatile component escapes the liquid more readily. Capture and condense that vapour and you hold a fraction enriched in ethanol; keep heating, and once the ethanol is largely gone the temperature climbs until the water distils in its turn. A thermometer tracks the whole story — plateaus in the temperature curve mark the phase changes, and the rise between them signals the changeover from one component to the other.
In this laboratory you will separate a 25% v/v ethanol–water solution — the strength of a typical mouthwash — by two-stage simple distillation, collecting an ethanol-rich first fraction in one test tube at 85 °C and the remaining water in a second at 105 °C. Note that without a fractionating column the first fraction is ethanol-enriched rather than pure ethanol, so the temperature rises through the ethanol–water range instead of holding at 78 °C — a limitation you will observe directly in the temperature curve.
Educational Goals
Familiarization with the laboratory environment
- Identify the layout and equipment of the distillation laboratory, including the hot plate with magnetic stirrer, the stand and clamps, the thermometer, the timer, the ice bath and the temperature-vs-time results table.
Assembly and reconfiguration of a distillation apparatus
- Set up the vapour path from the stoppered Erlenmeyer flask through the glass elbow and connector to a chilled collection tube, taking care with the fragile glass elbow.
- Swap the receiving test tube between fractions without interrupting the run — the manipulation that turns one distillation into a two-stage separation.
Controlled two-stage heating
- Run the first stage at 85 °C to drive off the ethanol-rich fraction, then raise the target to 105 °C to distil the water.
- Judge when a fraction is finished, using the flow into the test tube and the temperature curve.
Reading a temperature–time curve
- Identify the plateaus in the results table, connect each to the boiling of one component, and explain the temperature rise between them.
Understanding volatility and mixtures
- Explain why the vapour above a boiling mixture is richer in the more volatile component, and why a single vaporisation step enriches but does not purify.
- Use the terms distillate, fraction, volatility and plateau correctly.
Protocol
- Using the 70 mL graduated cylinder, measure approximately 70 mL of 25% ethanol solution.
- Pour the measured liquid into the 250 mL Erlenmeyer flask.
- Insert the magnetic stir bar into the Erlenmeyer flask.
- Close the Erlenmeyer flask with the two-hole stopper including the glass elbow.
- Insert the thermometer into the hole of the stopper with a glass elbow. The thermometer bulb should sit in the vapor just above the surface of the liquid, level with the glass elbow, and must not touch the bottom of the flask.
Warning! The glass elbow can break if it is not inserted in line with the hole.
- Place the Erlenmeyer flask on the hot plate.
- Fill the 600 mL beaker containing the ice two-thirds full with cold tap water.
- Place the beaker to the right of the hot plate.
- Attach a universal clamp to the support, at a height of approximately 20 cm (bottom position).
- Attach test tube 1 to the clamp.
- Attach the other clamp to the support, at a height of approximately 30 cm (top position).
- Attach the purple connector to the top clamp.
- Start the magnetic stirrer.
- Start the stopwatch.
- Adjust the temperature of the heating plate to 85°C.
You can monitor the results in the Temperature vs. Time tab in the results table.
- When the liquid no longer rises in test tube 1 (after approximately 2 minutes following reaching 85°C) :
- detach the purple connector from the test tube and monitor the condensation,
- remove the universal clamp from the top,
- detach test tube 1 and place it on the test tube rack.
- Attach test tube 2 to the bottom clamp.
- Reattach the other clip to the holder, at a height of approximately 30 cm (top position).
- Attach the purple connector to the top clamp.
- Adjust the temperature of the heating plate to 105°C.
- Monitor the appearance of another temperature plateau in the results table.
- When the solution has completely evaporated from the Erlenmeyer flask :
- detach the purple connector from the test tube and monitor the condensation,
- remove the universal clamp from the top,
- detach test tube 2 and place it on the test tube rack.
- Turn off the stirrer and reduce the target temperature of the hot plate to 15°C.
Anticipated Outcomes
Expected results. The 70 mL of mouthwash-strength solution separates into an ethanol-rich first fraction and a water second fraction, collected in separate test tubes.
| Stage | Hot plate target | What distils | Collected in |
|---|---|---|---|
| 1 | 85 °C | ethanol-rich vapour (ethanol boils at 78.37 °C) | test tube 1 |
| 2 | 105 °C | water (boils at 100 °C) | test tube 2 |
Why the ethanol comes off first. In a liquid mixture, every component contributes to the vapour above it in proportion to both its abundance and its volatility. Ethanol molecules escape the liquid far more readily than water molecules at the same temperature, so the vapour rising from the boiling solution carries a higher proportion of ethanol than the liquid below — and it is this vapour, led through the elbow and condensed in the chilled tube, that becomes the first fraction. As ethanol leaves, the remaining liquid grows steadily poorer in it, and progressively higher temperatures are needed to keep the mixture boiling.
What the plateaus mean. While a component is boiling off, the heat supplied by the plate is consumed as latent heat of vaporisation rather than raising the temperature, so the curve flattens. The first plateau appears near, but above, ethanol’s boiling point of 78.37 °C; the second sits at 100 °C while the water distils. The climb between them is the changeover: ethanol nearly exhausted, water not yet boiling — the signal to swap receiving tubes.
Why the first fraction is not pure ethanol. A single vaporisation step enriches, it does not purify: the vapour over a 25% solution is richer in ethanol than 25%, but far from 100%, and its composition shifts continuously as the run proceeds. Industrial and laboratory stills solve this with a fractionating column, which stacks many vaporisation–condensation cycles on top of one another. Without one, this experiment’s first fraction is best described as ethanol-enriched — which is why the temperature drifts upward through the ethanol–water range instead of holding at 78 °C.
Summary of Assignment by Grade Range
Grade 9–10
- Focus: two liquids can be separated by their boiling points, and a temperature curve tells you what is happening in the flask.
- Activities: assemble the apparatus and run both stages following the protocol; watch the temperature curve and note when each plateau appears; swap the receiving tube at the changeover; name the two fractions and state which component each contains.
Grade 11
- Focus: volatility, latent heat and the shape of the temperature curve.
- Activities: calculate the volumes of ethanol and water in 70 mL of 25% v/v solution; explain each plateau in terms of latent heat of vaporisation; explain why the vapour is richer in ethanol than the liquid; predict how the curve would change for a 50% solution.
Grade 12 / College Level
- Focus: vapour–liquid equilibrium and the limits of simple distillation.
- Activities: explain enrichment using the relative volatility of ethanol and water and sketch the reasoning behind a fractionating column; account for the azeotrope at 95.6% by mass and its consequence for purification; estimate the composition drift of the first fraction over the course of stage 1; carry out a mass balance on the recovered fractions and locate the losses.
Laboratory essentials
Instruments
- Beaker (50 mL and 600 mL)
- Erlenmeyer (250 mL)
- Graduated Cylinders (70 mL)
- Hot plate
- Lab Stand & Clamps
- Magnetic stirrer
- Plastic connector
- Test tubes
- Thermometer
- Timer
Products
- 25% v/v ethanol (solution)