Required practical - Chromatography (AQA GCSE Chemistry Combined Science): Revision Notes
Required practical - Chromatography
What this practical is about
This experiment helps you learn how to separate and identify different substances in a mixture using paper chromatography. You'll measure something called Rf values to work out what substances you have.
The aim
The main goal is to identify unknown substances by separating them on chromatography paper and calculating their Rf values. This technique works because different substances travel different distances up the paper.
Equipment you need
- Safety gear: Eye protection (always wear this!)
- Chromatography paper: This is your stationary phase
- Beaker: To hold the solvent
- Pencil and ruler: For drawing the start line and measuring
- Solvent: This is your mobile phase (often water or ethanol)
- Coloured substances: The mixtures you want to separate
- Pipettes: For putting tiny drops of your sample on the paper
Safety First: Always wear eye protection when working with solvents. Some solvents can be harmful if they splash, so handle them carefully and work in a well-ventilated area.
Step-by-step method
Step 1: Use a pencil and ruler to draw a line near the bottom of your chromatography paper. This is called the start line.
Step 2: Use a pipette to put a small drop of your coloured mixture on the start line. Let it dry completely.
Step 3: Pour a small amount of solvent into your beaker. The level should be below your start line.
Step 4: Place the chromatography paper in the beaker so the bottom edge touches the solvent. Make sure the start line stays above the solvent level.
Step 5: Wait and watch! The solvent will slowly move up the paper, carrying the different substances with it.
Step 6: When the solvent reaches near the top, take out the paper and mark where the solvent front reached.
How chromatography works
Understanding what happens during this experiment is really important. The separation process relies on the different attractions between substances and the two phases in the system.
Understanding the Separation Process
- The paper is the stationary phase - it stays still during the experiment
- The solvent is the mobile phase - it moves up the paper and carries substances with it
- Different substances travel different distances because they have different attractions to the paper and solvent
- Substances that travel further are more attracted to the solvent
- Substances that don't travel far are more attracted to the paper
Calculating Rf values
The Rf value tells you how far each substance travelled compared to the solvent. Here's the formula:
The Rf value is always a number between 0 and 1, and it's unique for each substance in a particular solvent system.
Worked Example: Calculating Rf Values
Let's say your results looked like this:
- Solvent front travelled: 75 mm
- Red substance travelled: 60 mm
- Green substance travelled: 27 mm
- Blue substance travelled: 12 mm
Calculations:
- Rf for red = 60 ÷ 75 = 0.80
- Rf for green = 27 ÷ 75 = 0.36
- Rf for blue = 12 ÷ 75 = 0.16
Identifying substances
Each pure substance has its own Rf value in a particular solvent. Scientists have data books with these values listed. If your calculated Rf value matches one in the data book, you've identified your substance!
Important point: Pure substances give only one spot, but mixtures create multiple spots.
Common errors and improvements
When doing this practical, it's important to avoid common mistakes that can affect your results.
Common Mistakes to Avoid
- Don't use ink pens - use pencil for the start line as ink will run
- Keep the start line above the solvent - if it goes underwater, your sample will dissolve away
- Make sure spots are small and concentrated - large spots give messy results
- Don't move the paper once started - this messes up the separation
- Mark the solvent front immediately - it disappears when the paper dries
Key Points to Remember:
- Paper chromatography separates mixtures based on how different substances are attracted to paper and solvent
- The Rf value is calculated by dividing distance moved by substance by distance moved by solvent
- Each pure substance has a unique Rf value that can help identify it
- Always use a pencil for the start line, not a pen
- Keep safety in mind - wear eye protection and work carefully with solvents