Topic 20 · page 12 of 15

Drying, distillation and steam distillation

An anhydrous salt removes the last traces of water from a liquid. Distillation then separates liquids by their boiling temperatures, and steam distillation takes a volatile, water-insoluble compound out of a mixture below 100 degrees Celsius.

15 min read20.5(v)20.5(vi)20.5(vii)

Read first: Reflux, washing and solvent extraction

Drying a liquid 20.5(v)

After washing, the organic liquid still holds a little dissolved water, and may look cloudy. It is dried with an anhydrous salt, which takes up water into its crystals.

MgSOX4(s)+7 HX2O(l)→MgSOX4 ⋅ 7 HX2O(s) \ce{MgSO4(s) + 7H2O(l) -> MgSO4.7H2O(s)}

MethodDrying an organic liquid

  1. Run the liquid into a dry conical flask.
  2. Add a spatula of an anhydrous drying agent: calcium chloride, magnesium sulfate or sodium sulfate. Stopper and swirl.
  3. If the solid clumps together, water is still present. Add more until some of the solid stays free-flowing and the liquid is clear.
  4. Decant or filter the liquid into a clean, dry flask.

Clumping means there is still water to remove. A free-flowing powder in a clear liquid means it is dry.

Distillation 20.5(vi)

Distillation separates liquids with different boiling temperatures. The mixture is heated, the liquid with the lowest boiling temperature boils first, and its vapour is condensed and collected.

A simple distillation apparatus. A round-bottomed flask holds the liquid and anti-bumping granules and is heated. A still head carries a thermometer with its bulb level with the side arm. A Liebig condenser slopes downwards with cooling water in at the bottom and out at the top. The distillate runs through an adapter into a receiver flask.
Distillation. The thermometer bulb is level with the side arm, so it reads the temperature of the vapour that enters the condenser.
Feature Reason
thermometer bulb level with the side arm it measures the temperature of the vapour leaving, which is the boiling temperature of what is distilling
water in at the bottom of the condenser, out at the top the jacket stays full, so the vapour condenses fully
anti-bumping granules smooth boiling
apparatus open at the receiver end a sealed system would build up pressure
collect the fraction that distils over a narrow range the product comes over at a steady temperature; other liquids come over at other temperatures

Worked exampleDistilling an aldehyde as it forms

Propan-1-ol is oxidised by acidified dichromate. The boiling temperatures are: propanal 49 °C, propan-1-ol 97 °C and propanoic acid 141 °C.

  1. Heat the mixture gently, with the still head fitted.
  2. Propanal has the lowest boiling temperature, because it cannot hydrogen bond to itself. It distils over at about 49 °C as soon as it forms, and is collected.
  3. Because it leaves the hot, oxidising mixture, it cannot be oxidised further to propanoic acid.
  4. The alcohol and acid have higher boiling temperatures and stay behind in the flask.

Steam distillation 20.5(vii)

Some compounds would decompose if heated to their boiling temperatures. If such a compound is volatile and does not mix with water, it can be distilled with steam instead.

A steam distillation apparatus. A steam generator, a flask of boiling water, is connected by a tube to a two-necked flask holding the mixture. The steam tube reaches below the surface of the mixture. A still head and a condenser lead the steam and vapour to a receiver. The distillate collects as two layers.
Steam distillation. Steam passes through the mixture and carries the volatile compound into the condenser. The distillate separates into two layers.

The mixture boils when the combined vapour pressure of the water and the organic liquid equals atmospheric pressure. That happens below the boiling temperature of water, so the organic compound distils at a temperature below 100 °C, well under its own boiling temperature, and it does not decompose.

MethodSteam distillation

  1. Heat the water in the steam generator and pass the steam into the mixture.
  2. The compound and the water vapour pass into the condenser together.
  3. The distillate collects as two layers: water and the compound, which do not mix.
  4. Separate the layers in a separating funnel, or extract with a solvent, then dry the compound.

Worked exampleWhy steam distillation for limonene

Limonene is the main compound in orange peel oil. It has a boiling temperature of about 176 °C (illustrative value) and is damaged by heating that far, and it does not dissolve in water.

  1. Steam passes through crushed orange peel and carries limonene vapour with it.
  2. The mixture distils at below 100 °C, so the limonene is not damaged.
  3. The condensed liquid separates into two layers. Limonene is less dense than water and forms the upper layer.
  4. Run off the water, dry the limonene with an anhydrous salt, and keep it.

Check yourself

  1. How do you know when enough anhydrous drying agent has been added to an organic liquid?
  2. Why must the liquid be dried before the final distillation?
  3. Explain why the thermometer bulb is placed level with the side arm in a distillation.
  4. Ethanal boils at 21 °C, ethanol at 78 °C and ethanoic acid at 118 °C. Which would distil first from a mixture, and why?
  5. State three properties of a compound that make steam distillation suitable.
  6. The distillate from a steam distillation is cloudy and forms two layers. Explain this and say how you would isolate the product.

Answers cover these until you have tried the questions

  1. The liquid becomes clear and some of the drying agent stays free-flowing, no longer clumping together.
  2. Water left in the liquid would distil over with the product, or alter the boiling temperature, so the product would be wet and the boiling temperature would not show purity.
  3. The bulb must sit in the vapour entering the condenser, so that it reads the boiling temperature of the liquid that is distilling. A bulb in the liquid would read a temperature that is too high; one above the side arm would read too low.
  4. Ethanal distils first, because it has the lowest boiling temperature. It cannot hydrogen bond to itself, while ethanol and ethanoic acid can, so they need more energy to boil.
  5. The compound must be volatile, immiscible with water, and one that would decompose (or have too high a boiling temperature) if heated to its own boiling temperature.
  6. The compound and water do not mix, so the distillate separates into two layers; the cloudiness is tiny droplets of one layer in the other. Run the layers apart in a separating funnel, keeping the layer that holds the product, then dry it with an anhydrous salt.