Distillation

Section 1 of 7The idea behind distillation

Separating by boiling point

• Separating by boiling point Distillation separates a liquid from a mixture by evaporating it and then condensing the vapour back to a liquid.
• It works because the components of the mixture boil at different temperatures.

Which technique to use

• Which technique to use Simple distillation separates a liquid from a solution containing a dissolved solid, such as pure water from salt water.
• Fractional distillation separates a mixture of liquids with different, fairly close boiling points.

Section 2 of 7Simple distillation of salt water

1

Heat the salt solution in a flask fitted with a thermometer and a delivery tube leading to a condenser.

2

The water boils at 100 °C and turns into steam, which leaves the flask while the salt stays behind.

3

Cold water flows through the outer jacket of the condenser, entering at the bottom and leaving at the top, so the steam cools and condenses.

4

The pure liquid water drips into a collecting flask.

5

Stop heating before the flask boils dry, leaving the solid salt as the residue.

Section 3 of 7Fractional distillation of ethanol and water

1Heat the mixture

• Heat the mixture Ethanol boils at about 78 °C and water at 100 °C.

2Ethanol evaporates first

• Ethanol evaporates first The vapour rises up a fractionating column packed with glass beads, which provides a large surface area.

3Vapour condenses and rises again

• Vapour condenses and rises again Water-rich vapour condenses on the cooler beads and drips back, while the more volatile ethanol travels to the top.

4Collect each fraction

• Collect each fraction When the thermometer reads about 78 °C the ethanol passes into the condenser and is collected.
  • After the temperature rises to 100 °C the flask is changed to collect water.

Section 4 of 7Limits of the method (HT only)

Pure ethanol is not possible (HT only)

• Pure ethanol is not possible (HT only) Some water evaporates even below its boiling point,
  • so a little water vapour always travels with the ethanol and the distillate is never completely pure.

Energy and cost

• Energy and cost Distillation needs a great deal of energy to heat and boil large volumes of liquid, so producing pure water this way is expensive.

Section 5 of 7Comparing the two methods

Feature	Simple distillation	Fractional distillation
Separates	A solvent from a dissolved solid	Liquids with different boiling points
Fractionating column	Not used	Used to improve separation
Example	Pure water from seawater	Ethanol from a fermentation mixture; crude oil fractions

Section 6 of 7Key terms

Term	Meaning
Distillation	Separating liquids by evaporating then condensing them.
Condenser	A cooled tube in which vapour is turned back into liquid.
Boiling point	The temperature at which a liquid changes to vapour throughout the liquid.
Fractionating column	A column filled with beads or plates that helps separate liquids with similar boiling points.
Distillate	The liquid collected after the vapour has condensed.

Section 7 of 7Exam technique

1

Saying the salt evaporates along with the water: it has a very high boiling point and stays in the flask.

2

Running the condenser water the wrong way: cold water should enter at the bottom and leave at the top.

3

Placing the thermometer bulb in the liquid: it should be at the side arm opening to read the vapour temperature.

4

Claiming fractional distillation gives perfectly pure ethanol (HT only): some water vapour always escapes.

5

Describing the process as filtrationdistillation separates by boiling point, not by particle size.