Filtration, crystallisation and chromatography (AQA GCSE Chemistry Combined Science): Revision Notes
Filtration, crystallisation and chromatography
What are separation methods?
These are ways to split up mixtures using physical processes. This means no chemical reactions happen and no new substances are created. You're just separating things that were already mixed together.
Physical processes are different from chemical reactions because they don't create new substances. The substances you end up with are exactly the same as what you started with - they're just no longer mixed together.
Filtration
What is it used for?
Filtration separates insoluble solids from liquids. Think of it like straining pasta - the pasta stays in the strainer but the water goes through.
Equipment you need:
- Funnel
- Philtre paper
- Conical flask
- The mixture you want to separate
How it works:
- Pour the mixture into the funnel with philtre paper
- Insoluble solids get trapped in the philtre paper and stay there
- Solutions and liquids pass through the philtre paper into the flask below
The liquid that comes through is called the filtrate. The solid left behind stays on the philtre paper.
Worked Example: Separating Muddy Water
If you have muddy water:
- The mud (insoluble solid) stays in the philtre paper
- Clean water (the filtrate) passes through and is collected in the flask
- Result: clean water separated from mud
Crystallisation
What is it used for?
Crystallisation helps you get solid crystals from a solution. It's useful when you want to separate a dissolved solid from water.
Equipment you need:
- Evaporating basin
- Heat source
- Solution containing dissolved substances
How it works:
- Heat the solution to make it more concentrated - this means some water evaporates away
- Crystals start to form as the solution becomes more concentrated
- Leave it in a warm place so more water can evaporate slowly
- Crystals form as the water disappears
The key to good crystallisation is slow, controlled evaporation. If you heat too quickly, you might get lots of tiny crystals instead of nice, clear ones.
Worked Example: Making Salt Crystals
Starting with salt water:
- Heat the salt water solution gently
- Water evaporates, leaving behind a more concentrated salt solution
- Continue heating until crystals start to appear
- Result: pure salt crystals separated from water
Paper chromatography
What is it used for?
Paper chromatography separates different coloured substances in mixtures. It's great for finding out what colours are mixed together in inks or food colourings.
Setting it up:
- Draw a pencil line near the bottom of chromatography paper
- Add spots of the substances you want to test on this line
- Put the paper in a tank with a small amount of solvent at the bottom
- Make sure the solvent is below the line where you put your spots
Always use pencil for the baseline, never pen! Pen ink would dissolve and interfere with your results.
How it works:
- Soluble substances get carried up the paper by the solvent
- Different substances travel at different speeds - some go further up than others
- You can see which colours were mixed together by looking at how far each one travelled
Reading the results:
- Substances that travel the same distance are likely to be the same
- If a spot breaks up into different colours, it means it was a mixture
- Pure substances only create one spot that doesn't separate
Worked Example: Testing Sweet Colouring
Testing an "orange" sweet colouring:
- Place a spot of the orange colouring on the baseline
- Run the chromatography experiment
- Observe the results: the spot separates into red and yellow bands
- Conclusion: the "orange" colour was actually made by mixing red and yellow dyes
Critical Safety Information
When doing chromatography experiments, you can't always be sure if something is safe to eat just because the colours match known safe ones. There might be other unsafe substances present that you can't detect using this method.
Key Points to Remember:
- Physical separation means no chemical reactions - you're just separating mixtures
- Filtration separates insoluble solids from liquids using philtre paper
- Crystallisation gets solid crystals from solutions by evaporating water
- Paper chromatography separates different coloured substances based on how far they travel up paper
- Different substances travel at different rates in chromatography, which is how we can separate and identify them