Preparing and purifying organic liquids

Heating under reflux

A condenser fitted vertically above a round-bottomed flask lets the mixture be heated for a long time; vapours condense and drip back, so volatile reactants and products are not lost. Use anti-bumping granules for smooth boiling, and heat with a water bath or electric heater for flammable liquids.

Separating and washing

In a separating funnel, two immiscible layers separate; the denser layer is at the bottom. Washing with sodium hydrogencarbonate solution removes acid (release the pressure from CO2 often). Washing with water removes soluble impurities.

Drying and distillation

• Drying: add an anhydrous salt (anhydrous sodium sulfate, magnesium sulfate or calcium chloride) to the organic liquid until it is clear and the solid moves freely; then decant or filter.
• Distillation: heat the liquid with the thermometer bulb at the side arm opening, and collect the fraction that distils at (or within a degree or two of) the product's boiling point.

Checking purity

A pure liquid boils at a sharp, fixed temperature. Measure the boiling point and compare it with a data-book value.

Worked example

A student makes 1-bromobutane. List the steps after the reaction to obtain a pure, dry product.

1. Distil the crude product from the reaction mixture.
2. Wash in a separating funnel with NaHCO3(aq), then with water; keep the organic layer.
3. Dry with anhydrous sodium sulfate; decant.
4. Redistil, collecting the fraction at the boiling point of 1-bromobutane.

Answer: Distil, wash, separate, dry, redistil and check the boiling point.

Check yourself

1. What is the purpose of heating under reflux?
   • To heat a mixture for a long time without losing volatile substances
   • To separate liquids by boiling point
   • To remove water from a liquid
   • To cool a reaction quickly
   Show answer

   To heat a mixture for a long time without losing volatile substances. The vertical condenser returns vapour to the flask.

2. Which substance is used to dry an organic liquid?
   • Sodium hydrogencarbonate solution
   • Concentrated hydrochloric acid
   • Anhydrous calcium chloride
   • Distilled water
   Show answer

   Anhydrous calcium chloride. Anhydrous salts absorb water.

3. Where should the thermometer bulb be placed during distillation?
   • At the opening of the side arm leading to the condenser
   • In the liquid at the bottom of the flask
   • In the receiving flask
   • At the top of the condenser
   Show answer

   At the opening of the side arm leading to the condenser. It must measure the temperature of the vapour that is distilling.

4. Why must a separating funnel be vented when shaking with sodium hydrogencarbonate solution?
   • The liquid boils
   • Oxygen is released
   • The funnel gets too cold
   • Carbon dioxide gas builds up pressure
   Show answer

   Carbon dioxide gas builds up pressure. NaHCO3 reacts with acid to give CO2.

5. How can you show a liquid product is pure?
   • It is colourless
   • It boils at a sharp temperature that matches the data-book value
   • It is denser than water
   • It dissolves in ethanol
   Show answer

   It boils at a sharp temperature that matches the data-book value. Impurities make the boiling point vary over a range.

6. Why are anti-bumping granules added before heating?
   • To catalyse the reaction
   • To dry the liquid
   • To neutralise acid
   • To make boiling smooth and prevent violent bumping
   Show answer

   To make boiling smooth and prevent violent bumping. They provide sites for small bubbles to form.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.