Filtration, crystallisation and chromatography (AQA GCSE Chemistry): Revision Notes
Filtration, crystallisation and chromatography
What are separation methods?
When we want to separate mixtures, we use physical processes. This means we don't need chemical reactions and no new substances are created. We're just splitting up what's already there.
Physical processes are different from chemical reactions because they only change the arrangement or state of substances, not their chemical composition. This is why we can reverse these separation methods.
Filtration
What is filtration?
Filtration separates insoluble solids from liquids. Think of it like a sieve that catches the big pieces but lets liquids through.
How filtration works
You need a funnel, philtre paper, and a flask to catch the liquid.
The process works like this:
- Pour the mixture into the funnel with philtre paper
- Insoluble solids get trapped in the philtre paper
- Solutions and liquids pass through into the flask below
- The liquid that comes through is called the filtrate
Worked Example: Filtering Muddy Water
If you had muddy water, the mud particles (insoluble solids) would stay in the philtre paper and clean water would drip into the flask as the filtrate.
Crystallisation
What is crystallisation?
Crystallisation helps you get pure solid crystals from a solution. It works by removing the water so crystals can form.
How crystallisation works
You need an evaporating basin and a heat source.
Here's what happens:
- Heat the solution to make it more concentrated
- Some crystals might start forming straight away
- Leave the solution in a warm place
- The remaining water evaporates slowly
- Pure crystals form as the water disappears
Worked Example: Salt Crystallisation
You could use this to get salt crystals from salt water. As the water evaporates, the salt becomes more concentrated until it forms pure white crystals.
Paper chromatography
What is paper chromatography?
This method separates different substances in a mixture. It works because different substances travel different distances up the paper.
Setting up chromatography
You need chromatography paper, a solvent, and a container.
Follow these steps:
- Draw a pencil line near the bottom of the paper (pencil won't dissolve)
- Add small spots of your mixture onto this line
- Put the paper in a container with solvent at the bottom
- Make sure the solvent level is below your pencil line
- The solvent will carry different substances up the paper at different speeds
Always use a pencil to draw the starting line, never a pen! Pen ink would dissolve in the solvent and interfere with your results.
How it works
Soluble substances get carried up the paper by the solvent. Different substances travel different distances, so they separate out into different spots.
Real Example: Testing Food Colourings
Scientists used chromatography to test food colourings in sweets. They found that colours A and C from known samples travelled the same distance as spots from the sweet, showing these colours were present in the sweet.
Why these methods work
Each method works because of different physical properties:
- Filtration: Some particles are too big to fit through philtre paper
- Crystallisation: Solids become less soluble as water evaporates
- Chromatography: Different substances move at different speeds through paper
Understanding the physical properties that make each method work helps you choose the right separation technique for different mixtures.
Key Points to Remember:
- All three methods are physical processes - no new substances are made
- Filtration separates insoluble solids from liquids using philtre paper
- Crystallisation removes water to form pure crystals
- Chromatography separates mixtures because different substances travel different distances
- Each method works because substances have different physical properties