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Chemistry · Chapter 2: Separation and Purification
Solvent Extraction
Separating a substance with two immiscible solvents in a separating funnel, with iodine and hexane as the example.
Key termRememberExampleWatch out
01Concept
Solvent extraction is the method of separating a substance from a solution by shaking it with a second solvent in which it is more soluble. The two solvents must be immiscible (they do not mix and form two layers).
Example: iodine is only slightly soluble in water but very soluble in hexane. Shaking iodine solution with hexane moves the iodine into the hexane layer, which turns purple.
Steps
- Pour the aqueous iodine solution into a separating funnel and add some hexane.
- Put in the stopper and shake the funnel. Turn it upside down and open the tap now and then to release the pressure of vapour.
- Clamp the funnel and let it stand. Two layers form: hexane (less dense) on top, water at the bottom.
- Open the tap to run off the lower water layer into a beaker. Close the tap just as the boundary reaches it.
- Pour the upper (purple) hexane layer out through the top. Evaporate the hexane to get iodine.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Real World Examples
Decaffeinated tea and coffee
Caffeine is removed from coffee beans by extracting it with a solvent (or with supercritical carbon dioxide).
Edible oils
Cooking oil is extracted from soybean and cottonseed meal using the solvent hexane, which is then removed by distillation.
Perfumes
Fragrant oils are extracted from flowers with solvents in the perfume industry.
04Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.What is solvent extraction?Show answer
Answer
Solvent extraction is separating a substance from a solution by shaking it with a second, immiscible solvent in which the substance is more soluble.
Q2.What are immiscible liquids? Give an example.Show answer
Answer
Immiscible liquids do not mix and form separate layers. Example: hexane and water; oil and water.
Q3.Why is the tap of the separating funnel opened while shaking?Show answer
Answer
To release the pressure of the solvent vapour built up inside, so the stopper does not pop out.
Q4.Which layer is on top in a separating funnel?Show answer
Answer
The less dense liquid forms the upper layer.
Q5.Why are several small extractions better than one large one?Show answer
Answer
Because each extraction removes a fraction of what is left, so repeated small extractions together remove more of the solute than one extraction with the same total volume.
Long questions
Q6.Describe how iodine can be extracted from its aqueous solution using a separating funnel.Show answer
Answer
- Pour the aqueous iodine solution into a separating funnel and add hexane.
- Stopper and shake, releasing the pressure from time to time.
- Clamp and allow the layers to separate: purple hexane layer on top, nearly colourless water below.
- Run off the lower water layer through the tap.
- Collect the hexane layer and evaporate the hexane to obtain iodine.
- For better extraction, repeat with a fresh portion of hexane.