GCSE · Chemistry · Edexcel · Spec 1CH0

Concentrating ethanol by fractional distillation

Ethanol boils at 78 °C and water at 100 °C, just 22 °C apart. So how do you coax the ethanol out of the mixture without dragging the water along?

Chemistry · The fractionating column

Follow the vapour up the column

A fractionating column drawn top to bottom (a schematic, not to scale). Start at the flask, zone 5, and climb. At each zone ask one question: is it still hot enough to stay a gas?

↑ Top of the column: coolest, next to the condenser

↓ Bottom of the column: hottest, nearest the heat source

Zone 1 of 5

Top of the column, by the condenser

Make it all the way up and you reach the coolest part, right beside the condenser. Once the gradient has developed, the component with the lowest boiling point is the one that arrives here as a gas. In an ethanol–water mixture that is the ethanol-rich vapour (ethanol boils at 78 °C), and it goes straight into the condenser.

Ethanol-rich vapour carries on
Passes into the condenser

Hotter at the bottom, colder towards the top. Water condenses in the column and drips back; ethanol-rich vapour reaches the condenser.

Chemistry · The apparatus

The distillation apparatus, part by part

Showing 1 layer: Apparatus

Explore

Tap a part to see what it does.

Simple or fractional?

Simple distillationvsFractional distillation

Start with the top row: it is the reason ethanol and water get the column.

Focus

Mixtures it suits

Simple distillation

Two components whose boiling points differ greatly

Fractional distillation

Boiling points within about 25 °C of each other, or more than two components

The insight

Ethanol (78 °C) and water (100 °C) are 22 °C apart. That is inside the fractional range, so ethanol and water suit the fractional method.

Apparatus

Simple distillation

No fractionating column

Fractional distillation

A fractionating column

What you collect

Simple distillation

One distillate

Fractional distillation

Several distillates, each called a fraction

When the other method is used

Simple distillation

Inefficient and impractical for close boiling points or more than two components: it would have to be repeated many times, using time and energy

Fractional distillation

An extension of simple distillation, used for exactly those mixtures

Chemistry · Heating

Does heating harder separate better?

An ethanol–water mixture is distilling, and it feels slow. Your lab partner says: "Just turn the heat right up and leave it. More heat, better separation."

Which idea is closest to what you think?

Heating strongly and for a long time separates the mixture better.Heating has to be carefully controlled, so only one substance at a time is in the condenser.The thermometer shows the temperature of the liquid in the flask.The column does all the separating, so the heating hardly matters.

How sure are you?

I'm sureI think soI'm guessing

Heating strongly and for a long time separates the mixture better.

This is a common misconception

It sounds sensible: more heat, more action. But heating strongly and for a long time does not improve the separation. What matters is control, so that only one substance at a time is in the condenser.

Heating has to be carefully controlled, so only one substance at a time is in the condenser.

This matches the evidence

Yes. Control keeps one substance at a time reaching the condenser, and the link between the heat applied, the boiling point and the thermometer reading is essential.

The thermometer shows the temperature of the liquid in the flask.

This is a common misconception

Easy to assume, because the flask is where the heating happens. But the thermometer monitors the temperature of the vapour, which is why its reading can be linked to boiling points.

The column does all the separating, so the heating hardly matters.

This is a common misconception

The gradient in the column is what does the separating, but the heating still has to be carefully controlled so that only one substance at a time is in the condenser.

Turning the heat up does not improve the separation. The column's temperature gradient does the sorting, and your job is control.

Chemistry · Write it, then check it

From fermented mixture to spirit

Spirits are made by fermenting a plant material and then fractionally distilling the fermented mixture. Explain how fractional distillation of a mixture of ethanol and water gives an ethanol fraction, and state what that ethanol fraction is. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Ethanol has the lower boiling point (78 °C, against 100 °C for water), so it boils off first. (1 mark)
The fractionating column is hotter at the bottom and colder towards the top. (1 mark)
Water, with the higher boiling point, condenses in the column and drips back into the flask. (1 mark)
The ethanol-rich vapour reaches the top of the column and enters the condenser. (1 mark)
The ethanol fraction is a more concentrated ethanol–water mixture, not pure ethanol. (1 mark)

Model answer

The mixture is heated and ethanol, which has the lower boiling point (78 °C, against 100 °C for water), boils off first. The fractionating column is hotter at the bottom and colder towards the top, so water, with its higher boiling point, condenses in the column and drips back into the flask. The ethanol-rich vapour reaches the top of the column and enters the condenser, where it condenses and is collected. This ethanol fraction is a more concentrated ethanol–water mixture, not pure ethanol.

Write the chain: lower boiling point, then gradient, then water returns and ethanol-rich vapour reaches the condenser. And never call the ethanol fraction pure ethanol. Careful: not every alcoholic drink is distilled. It is spirits that are made this way.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A tall column that is hot at the bottom and cool at the top sorts vapours by how long they can stay a gas.

What you need to know

Distillation works because liquids have different boiling points: one boils off first, then condenses back into a liquid.
Boiling is a liquid changing to a gas with bubbles forming. Condensation is a gas cooling and changing back to a liquid.
In ethanol and water, ethanol has the lower boiling point (78 °C against 100 °C), so ethanol-rich vapour reaches the condenser first.
Have a goYour mate Sam says water will reach the condenser first, because 100 is a bigger number than 78 and so it must win. What has Sam got backwards?
The lower boiling point goes first. Ethanol (78 °C) boils off before water (100 °C).
On heating, the ethanol-rich component boils off first because it has the lower boiling point, so a bigger number does not mean first.
Simple distillation suits two components with boiling points that differ greatly, and it collects one distillate.
Ethanol and water are only 22 °C apart, which is within about 25 °C, so fractional distillation is the method to use.
A fractionating column is long and upright, hotter at the bottom near the heat source and colder towards the top.
Have a goCondensation is a gas cooling. Compare a spot near the flask with a spot near the top of the column: which one is cooler, and so more likely to make a vapour condense?
The spot near the top of the column.
The column is hotter at the bottom and colder towards the top, and a gas condenses when it cools.
Higher-boiling components condense in the column and drip back to the flask; the lowest-boiling component reaches the condenser as a gas.
Fractional distillation gives several distillates called fractions, and the lowest boiling point is collected first.
Have a goA fractional distillation produces three fractions, A, B and C. A has the lowest boiling point and C the highest. In what order do they come off?
A first, then B, then C.
The component with the lowest boiling point is collected first, so order follows boiling point from lowest upwards.
Heat carefully: strong, long heating does not improve the separation, and the thermometer monitors the vapour's temperature.
Spirits are made by fermenting, then fractionally distilling; the ethanol fraction is a more concentrated ethanol–water mixture, not pure ethanol.

The big picture

Fractional distillation separates liquids whose boiling points are close together, using a fractionating column that is hotter at the bottom and colder towards the top. In an ethanol–water mixture, ethanol (78 °C) reaches the condenser first while water (100 °C) condenses in the column and drips back. Careful heating, not strong heating, gives the separation. Spirits are made by fractionally distilling a fermented mixture, and the ethanol fraction is a more concentrated ethanol–water mixture, not pure ethanol.

Key points

1Distillation uses boiling and condensation to separate liquids, because their boiling points are different.

2Fractional distillation is an extension of simple distillation, used for boiling points within about 25 °C or more than two components. The main apparatus difference is the fractionating column.

3In the column the temperature is hotter at the bottom and colder towards the top; a taller column allows a larger gradient.

4Higher-boiling components condense in the column and drip back; the lowest-boiling component reaches the condenser and is collected first.

5Heating must be carefully controlled so only one substance at a time is in the condenser.

6The distillation claim here is for spirits, and the ethanol fraction is a more concentrated ethanol–water mixture, not pure ethanol.

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Worked example

Problem

Ethanol boils at 78 °C and water at 100 °C. Decide whether a mixture of the two should be separated by simple or fractional distillation, and say which one reaches the condenser first.

⚠ Watch out

Calling the ethanol fraction 'pure ethanol'. Fractionally distilling a fermented mixture gives a more concentrated ethanol–water mixture, not pure ethanol.

🧠

Memory hook

The column sorts vapours by height: water gives up partway, ethanol makes it to the top.

✓

Check yourself

Without looking back, explain to an imaginary friend why the ethanol-rich vapour reaches the condenser while the water drips back. A full answer uses the column's temperature pattern and both boiling points.

Flashcards

(12)

What does distillation exploit to separate the liquids in a mixture?

The different boiling points of the components, using boiling and condensation.

Boiling or condensation: which is a liquid changing to a gas with bubbles forming?

Boiling. Condensation is the opposite change: a gas cooling and changing to a liquid.

Which has the lower boiling point, ethanol or water, and what are the values?

Ethanol: 78 °C, against 100 °C for water. The ethanol-rich component boils off first and is first to reach the condenser.

When is simple distillation used, and how many distillates does it collect?

For a mixture of two components with boiling points that differ greatly. It collects one distillate.

Why is simple distillation a poor choice when boiling points are close together?

It is inefficient and impractical: it would have to be repeated many times, using time and energy.

What is the main apparatus difference between fractional and simple distillation?

Fractional distillation uses a fractionating column.

What is the temperature pattern in a fractionating column?

Hotter at the bottom, near the heat source, and colder towards the top. The taller the column, the larger the gradient that can develop.

What happens to higher-boiling components in the column?

They condense in the column and drip back into the flask.

Why is each distillate from fractional distillation called a fraction, and which comes first?

Each is part of the original mixture. The component with the lowest boiling point is collected first.

What does it tell you if a fraction is collected over a temperature range?

It is a mixture of substances with similar boiling points. A fraction that boils at a specific temperature is pure.

What does the thermometer in distillation apparatus monitor?

The temperature of the vapour, not the liquid in the flask.

Which drinks does the fractional distillation claim cover, and what is the ethanol fraction?

Spirits, made by fermenting then fractionally distilling. The ethanol fraction is a more concentrated ethanol–water mixture, not pure ethanol. Not every alcoholic drink is distilled.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Follow the vapour up the column

Does heating harder separate better?

From fermented mixture to spirit

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