Pure substances and separating mixtures
Chemists choose a separation technique by asking what differs between the substances in the mixture: particle size, boiling point or attraction to a surface.
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
- • What 'pure' means in chemistry
- • Filtration and crystallisation
- • Distillation
- • Chromatography
This lesson at a glance
- 30 minutes
- 21 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 21
Visual introductionPicture this
Pure substances and separating mixtures
Chemists choose a separation technique by asking what differs between the substances in the mixture: particle size, boiling point or attraction to a surface.
In a nutshell
Understand the concept of a pure substance and how mixtures can be separated through filtration, distillation and chromatography
Learning cycle
Part 2 of 21
Learning cycle 1 of 2
Part 1 · What 'pure' means in chemistry
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 21
Learn
What 'pure' means in chemistry
In chemistry a pure substance contains only one element or compound, which is stricter than the everyday meaning of 'pure orange juice'. A pure substance melts and boils at a fixed temperature; a mixture melts over a range, which makes melting point a practical test of purity.
Reset break
Part 4 of 21
Pause
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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.
Explore the idea
Part 5 of 21
Learn
Filtration and crystallisation
Filtration separates an insoluble solid from a liquid, because the solid particles are too large to pass through the filter paper. To recover a dissolved solid, evaporate or crystallise the solution: slow crystallisation gives larger, purer crystals than rapid boiling.
Quick check
Part 6 of 21
Quick check
Part 7 of 21
Reset break
Part 8 of 21
Pause
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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.
Learning cycle
Part 9 of 21
Learning cycle 2 of 2
Part 2 · Distillation
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 10 of 21
Learn
Distillation
Simple distillation separates a solvent from a solution by boiling and condensing it: useful for obtaining pure water from seawater. Fractional distillation separates liquids with different boiling points, using a column in which the substance with the lower boiling point reaches the top first.
Explore the idea
Part 11 of 21
Learn
Chromatography
Chromatography separates dissolved substances by how strongly each is attracted to the paper compared with the solvent. Substances carried further are more soluble in the solvent and less attracted to the paper. The start line must be drawn in pencil, because ink would separate and contaminate the result.
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Part 12 of 21
Pause
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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.
Quick check
Part 13 of 21
Quick check
Part 14 of 21
Explore the idea
Part 15 of 21
Worked example
Worked answer: describe how to obtain pure water from a salt solution (3 marks)
Heat the solution so the water boils and turns to vapour, leaving the salt behind because its boiling point is far higher (1). Pass the vapour through a condenser, where cold water cools it back into a liquid (1). Collect the liquid, which is pure water, in a clean flask; this process is simple distillation (1). Filtration would not work, because dissolved salt passes straight through filter paper.
Reset break
Part 16 of 21
Pause
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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.
Challenge round
Part 17 of 21
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 18 of 21
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
In chemistry a pure ____ contains only one element or compound, which is stricter than the everyday meaning of 'pure orange juice'.
____ separates an insoluble solid from a liquid, because the solid particles are too large to pass through the filter paper.
Challenge round
Part 19 of 21
Game · Recall cards
Which method separates an insoluble solid from a liquid?
Card 1 of 4
Mastery quiz
Part 20 of 21
Marked quiz
End of lesson quiz: Pure substances and separating mixtures
4 questions, marked with the reasoning shown. No timer.
1. Which method separates an insoluble solid from a liquid?
2. How does distillation obtain pure water from salt water?
3. What does a pure substance have that a mixture does not?
4. A pure substance melts
Lesson round-up
Part 21 of 21
Lesson round-up
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Understand the concept of a pure substance and how mixtures can be separated through filtration, distillation and chromatography