Purity, Formulations & Chromatography (AQA GCSE Chemistry): Revision Notes
Core practical - Chromatography
What is this practical about?
This practical helps you identify different substances in a mixture. You use paper chromatography to separate coloured substances and measure how far they travel.
The aim is to:
- Separate substances in a mixture
- Calculate Rf values to identify what the substances are
Equipment you need
Always wear eye protection when working with solvents and chemicals in the laboratory.
- Eye protection (safety glasses)
- Beaker
- Chromatography paper
- Pencil and ruler
- Solvent (liquid that moves up the paper)
- Coloured substances to test
- Pipettes
How chromatography works
Chromatography has two important parts:
- Stationary phase: The paper that stays still
- Mobile phase: The solvent that moves up the paper
Different substances are attracted to these phases differently. This is why they separate out.
Understanding the difference between the stationary phase and mobile phase is crucial - this is the fundamental principle that makes chromatography work!
Method - step by step
-
Draw a pencil line near the bottom of the chromatography paper (use pencil, not pen!)
-
Add your sample: Use a pipette to put a small drop of the coloured mixture on the pencil line
-
Set up the beaker: Put some solvent in a beaker and place the paper so the bottom touches the solvent (but the pencil line stays above the solvent)
-
Wait and watch: The solvent rises up the paper and carries the different substances with it
-
Mark the solvent front: When the solvent has moved up the paper, mark where it stopped
Never let the pencil line go below the solvent level - this would wash your sample away and ruin the experiment!
Understanding your results
Different substances travel different distances because:
- Some are more attracted to the paper (stationary phase)
- Others are more attracted to the solvent (mobile phase)
Example: Separating a Coloured Mixture
When you test a mixture of food colourings:
- Red substance might travel 60mm
- Green substance might travel 27mm
- Blue substance might travel 12mm
Each substance has different attractions to the paper and solvent, so they separate clearly.
If a substance is pure, it will only make one spot. If it's a mixture, you'll see multiple spots.
Calculating Rf values
The Rf value helps you identify what each substance is.
Formula:
Rf values are like fingerprints for substances - each pure substance has its own characteristic Rf value in a particular solvent.
Key facts about Rf values:
- They're always less than 1
- Different substances have different Rf values
- You can look up Rf values in data books to identify substances
- Rf values depend on which solvent you use
Sample calculation
Worked Example: Calculating Rf Values
If the solvent front travels 75mm:
Step 1: Measure the distance each substance travelled
- Red substance: 60mm
- Green substance: 27mm
- Blue substance: 12mm
Step 2: Apply the formula
- Red substance:
- Green substance:
- Blue substance:
Key Points to Remember:
- Chromatography separates mixtures by making different substances travel different distances
- The stationary phase is the paper, the mobile phase is the solvent
- Pure substances make only one spot, mixtures make multiple spots
- Use the formula to identify substances
- Always use pencil for the start line - ink would interfere with your results