Topic 10 · page 19 of 27

Separating, drying and checking a liquid

A crude liquid product is separated from water with a separating funnel, dried with an anhydrous salt, and then distilled. The boiling temperature shows whether it is pure.

12 min read10.1910.19(ii)10.19(iv)10.19(v)

Read first: Heating under reflux and distillation

Extraction with a separating funnel 10.1910.19(ii)

After a preparation, the liquid product is mixed with water, acid, unreacted starting material and by-products. If the product does not mix with water, the two liquids form separate layers, and a separating funnel can split them.

A separating funnel with a stopper at the top and a tap at the bottom. It holds two layers. The upper layer is labelled organic layer, less dense. The lower layer is labelled aqueous layer, more dense. A beaker below the tap catches the lower layer.
A separating funnel. The denser liquid is the lower layer and leaves through the tap first.

PracticalUsing a separating funnel

  1. Check that the tap is closed, then pour the mixture in with the stopper out. Put the stopper in.
  2. Shake gently, then turn the funnel upside down and open the tap to release the pressure. Close the tap. Repeat.
  3. Stand the funnel in a ring and let the layers settle.
  4. Remove the stopper, so that air can enter. Run off the lower layer through the tap into a flask.
  5. Pour the upper layer out through the top. Keep both layers until you know which one holds the product.

The denser liquid is the lower layer. Water is the reference: an organic liquid with a density lower than water floats on top, and one denser than water sinks. Do not guess: in Core Practical 6 the chloroalkane is less dense than water and is the upper layer, but a halogenoalkane with several halogen atoms is often denser than water.

Drying with an anhydrous salt 10.1910.19(iv)

The separated liquid still holds a trace of dissolved water, and may look cloudy. An anhydrous salt removes it by taking the water into its crystals.

MethodDrying a liquid

  1. Put the liquid in a dry flask.
  2. Add a spatula of an anhydrous salt: calcium chloride, magnesium sulfate or sodium sulfate. Swirl.
  3. If the salt clumps together, there is still water present. Add more until some salt stays free-flowing, like a loose powder, and the liquid is clear.
  4. Decant or filter the liquid into a clean, dry flask.

Clumping salt means there is water left to remove. Free-flowing salt means the liquid is dry.

Checking purity by boiling temperature 10.1910.19(v)

Distil the dried liquid and note the temperature on the thermometer while the liquid distils. A pure liquid boils at a single, fixed temperature (at a given pressure). An impure liquid boils over a range, and often at a different temperature from the pure compound. Compare the value with a data book.

What you find What it means
a steady temperature that matches the data book the liquid is pure
a temperature that rises over several degrees a mixture: impurities are present
a temperature that is steady but different from the data book a different compound from the one expected

Collect only the fraction that comes over within a narrow range around the expected boiling temperature. The next page, on Core Practical 6, shows all three techniques in one preparation.

A flow chart of the purification of a liquid product in five boxes joined by arrows. Reaction mixture, then separating funnel to remove the aqueous layer, then wash with sodium hydrogencarbonate to remove acid, then dry with an anhydrous salt, then distil and collect the fraction at the expected boiling temperature.
The purification of a liquid, in order. Each step removes a different impurity.

Check yourself

  1. Explain how a separating funnel separates two liquids, and state which liquid leaves first.
  2. Describe two precautions when using a separating funnel.
  3. Name two anhydrous salts used to dry an organic liquid.
  4. How can you tell when enough drying agent has been added?
  5. A liquid distils over the range 47–58 °C. The data book gives 51 °C. What does this suggest?
  6. Explain why the boiling temperature of a pure liquid is a single value.

Answers cover these until you have tried the questions

  1. The two liquids do not mix and have different densities, so they form separate layers. The denser (lower) layer is run off first through the tap.
  2. Release the pressure by opening the tap while the funnel is inverted; remove the stopper before running off the lower layer; keep both layers until the product is identified.
  3. Any two of anhydrous calcium chloride, anhydrous magnesium sulfate and anhydrous sodium sulfate.
  4. The salt stops clumping and some of it stays free-flowing, and the liquid becomes clear.
  5. The liquid is impure: it boils over a range, and the range is wider and different from the pure value. Another liquid or water is present.
  6. In a pure liquid all the molecules are the same, so all need the same energy to escape, and the liquid boils at a fixed temperature. Impurities change the boiling behaviour.