AQA Chemistry GCSE - Required Practical 6: Chromatography

Aim

• Use paper chromatography to separate and differentiate between colored substances.
• Calculate Rf values for analysis.

Equipment List

• 250 cm³ beaker
• Wooden spill or pencil (for supporting chromatography paper)
• Paper clip
• Ruler
• Pencil
• Distilled water
• Four known food colorings (labeled A-D)
• Unknown food coloring (labeled U)
• Rectangle of chromatography paper
• Five glass capillary melting point tubes

Method

1. Prepare the Chromatography Paper
   • Draw a horizontal pencil line 2 cm from the edge.
   • Mark pencil spots at equal intervals along the line.
2. Spot the Colorings
   • Use a glass capillary tube to place a small spot of each coloring on the marked spots.
   • Label each spot in pencil.
3. Set Up the Beaker
   • Pour 1 cm³ of water into the beaker.
4. Suspend the Paper
   • Dip the bottom edge of the paper into the water.
5. Develop the Chromatogram
   • Allow the water to travel three quarters up the paper.
   • Mark the solvent front with a pencil.
6. Dry the Paper
   • Hang the paper to dry thoroughly.
7. Calculate Rf Values
   • Use the formula: Rf = distance travelled by substance / distance travelled by solvent.

Key Points

• Ensure the pencil line is above the water to prevent dissolving spots.
• Avoid contact between the paper sides and the beaker walls.

Safety Precautions

• Avoid contact of food colorings with eyes to prevent irritation.

Analysis of Results

• Compare the Rf values of the substances against known Rf values from data books to identify the substances.

Diagram

• (Note: Diagram to illustrate setup, not provided here)