ScienceAtoms, elements and mixtures30 min★★★ difficulty

Pure substances and separating mixtures

In chemistry, 'pure' does not mean natural or healthy - it means one substance only. This lesson covers what pure means and how to choose the right separation method for a mixture.

Part of your national curriculum

Chemistry: pure and impure substances: Understand the concept of a pure substance and how mixtures can be separated through filtration, distillation and chromatography

Lesson overview

What you'll learn in this lesson

Understand the concept of a pure substance and how mixtures can be separated through filtration, distillation and chromatography

Key learning points

• The chemist's definition of pure
• Filtration and evaporation
• Distillation
• Chromatography

This lesson at a glance

30 minutes
19 parts to scroll through
4 quick checks
Marked quiz at the end
Gentle pace: short sittings with pauses

Words to know

conceptsubstancemixturesseparatedfiltration

Scroll down — the lesson carries on below

Part 1 of 19 · Discover5%

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Part 1 of 19

Watch first

Pure substances and separating mixtures

Clear up the difference between an element, a compound and a mixture.

Do it for real

The practicals below use what the film shows, with a full method you can follow at home.

Separating the colours in a felt-tip pen

In a nutshell

Understand the concept of a pure substance and how mixtures can be separated through filtration, distillation and chromatography

What you already know

Part 2 of 19

Before we start

What you already know

You should know the difference between a compound and a mixture.

Recap: Atoms, elements, compounds and formulae

Key words

Part 3 of 19

Key words

Words you'll need today

Pure substance: A single element or compound with a fixed melting and boiling point.
Filtration: Separating an insoluble solid from a liquid using filter paper.
Distillation: Separating a liquid from a mixture by evaporating and then condensing it.
Chromatography: Separating dissolved substances by how far they travel up paper in a solvent.
Soluble: Able to dissolve in a given solvent.

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

What pure means

A short piece of teaching, then a check to make sure it has landed.

Explore the idea

Part 5 of 19

Learn

The chemist's definition of pure

A pure substance contains only one element or compound. Pure water melts at exactly 0 degrees C and boils at exactly 100 degrees C. A mixture melts and boils over a range, so a sharp melting point is a test for purity - which is how a chemist checks a product without needing a label.

Explore the idea

Part 6 of 19

Learn

Filtration and evaporation

Sand and water: the sand is insoluble, so filter paper traps it while water passes through. Salt and water: the salt is dissolved, so filtering changes nothing - evaporate or crystallise the water away instead. Ask first whether the solid dissolved; that decides the method.

Quick check

Part 7 of 19

Quick check

A pure substance has...

Quick check

Part 8 of 19

Quick check

To separate sand from water, use...

Reset break

Part 9 of 19

Pause

That's sitting 1 of 2 done

Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.

Stop here for now

Learning cycle

Part 10 of 19

Learning cycle 2 of 2

Choosing a separation method

A short piece of teaching, then a check to make sure it has landed.

Explore the idea

Part 11 of 19

Learn

Distillation

To recover the water rather than the salt, use distillation: heat the solution, the water evaporates, then a condenser cools the vapour back to liquid and it is collected. Fractional distillation separates two liquids with different boiling points, such as ethanol and water, using a column so the lower-boiling liquid comes over first.

Explore the idea

Part 12 of 19

Learn

Chromatography

Put a spot of ink near the bottom of paper, stand it in solvent with the spot above the solvent level, and the dyes travel at different rates. More soluble dyes travel further. Several separate spots mean the ink was a mixture; a single spot suggests a pure substance. Use a pencil for the start line, because pen ink would separate too.

Quick check

Part 13 of 19

Quick check

To recover pure water from salt solution, use...

Quick check

Part 14 of 19

Quick check

Four spots on a chromatogram from one ink shows the ink was...

Common mix-ups

Part 15 of 19

Common mix-ups

Things people often get wrong

'Pure' in chemistry means one substance, not natural or clean.
Filtration cannot separate a dissolved solid from its solvent.
In distillation the liquid collected is the one that evaporated, not the residue.
In chromatography the start line must be drawn in pencil and sit above the solvent.

Challenge round

Part 16 of 19

Game · Sort it

Which of these are true?

Drag each card into the right column. Tap a card first if dragging is fiddly.

True

Not true

Challenge round

Part 17 of 19

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

A pure ____ contains only one element or compound.

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: Pure substances and separating mixtures

6 questions, marked with the reasoning shown. No timer.

1. Pure water boils at...
2. Fractional distillation separates liquids by...
3. In chromatography, more soluble dyes travel...
4. Crystallisation is used to recover...
5. A mixture's melting point is...
6. Why use pencil for the chromatography start line?

Lesson round-up

Part 19 of 19

Lesson round-up

Ready when you are

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Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

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Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Case study

Why the school greenhouse failed

A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.

The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.

The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.

Written for Good Learning Co.

Reading questions

Name the variable the club changed the second time, and one variable they kept the same.
Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
Write a conclusion for the club's report, saying how far the evidence supports it.

Method and results
Writing
Write the aim, what you changed, what you measured and what you kept the same, then record your results in a table.
1 page + results table
Conclusion
Writing
Explain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.
6–8 sentences
Labelled diagram
Diagram
Draw the apparatus or the process in pencil, label it fully, and photograph it.
1 labelled diagram
Reading and notes
Reading
Read the lesson text again and write the key ideas in your own words.
Half a page of notes

Write your answer, notes or working here

Saved as you type.

Photos of paper work, diagrams or working outHow many minutes did the written work take?

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Understand the concept of a pure substance and how mixtures can be separated through filtration, distillation and chromatography