AQA GCSE FOUNDATION Chemistry: Chromatography

AQA GCSE Foundation Chemistry: Chromatography

Chromatography is a technique used to separate and identify different substances in a mixture. It's particularly useful for separating pigments in inks or food dyes. Here's how it works:

Materials:

• Chromatography paper
• A beaker
• A pencil
• A ruler
• A solvent (e.g., water, ethanol)
• Ink or food dye samples

Procedure:

1. Draw a line: Use a pencil to draw a line across the chromatography paper, about 2 cm from the bottom edge. This is called the baseline.
2. Apply samples: Use a capillary tube or a toothpick to apply small dots of your ink or food dye samples onto the baseline. Leave some space between each dot.
3. Prepare the solvent: Fill the beaker with a suitable solvent, ensuring the level is below the baseline on the chromatography paper.
4. Suspend the paper: Carefully suspend the chromatography paper in the beaker, ensuring the baseline is above the solvent level but not touching the top of the beaker.
5. Observe the separation: As the solvent travels up the chromatography paper, the different components of the ink or food dye will separate and travel at different rates. This creates distinct bands of color.
6. Mark the solvent front: Once the solvent has nearly reached the top of the paper, carefully remove the paper from the beaker and mark the solvent front with a pencil.
7. Measure the distances: Use a ruler to measure the distance traveled by each component (from the baseline to the center of the band) and the distance traveled by the solvent (from the baseline to the solvent front).
8. Calculate Rf values: Calculate the Rf value for each component using the following formula:

Rf = distance traveled by solute / distance traveled by solvent

Understanding Rf values:

• Rf values are always between 0 and 1.
• The higher the Rf value, the further the component traveled up the paper, indicating it has a stronger affinity for the solvent.
• Rf values can be used to identify substances by comparing them to known Rf values.

Safety Precautions:

• Handle the solvent carefully, as some can be flammable or toxic.
• Avoid touching the chromatography paper with your fingers, as this can contaminate the samples.
• Always work in a well-ventilated area.

Example:

If a red ink sample is separated into three bands (red, yellow, and blue) and the following distances are measured:

• Red band: 4 cm
• Yellow band: 6 cm
• Blue band: 2 cm
• Solvent front: 10 cm

Then the Rf values for each component would be:

• Red: 4 cm / 10 cm = 0.4
• Yellow: 6 cm / 10 cm = 0.6
• Blue: 2 cm / 10 cm = 0.2

By comparing these Rf values to known values for different pigments, you can identify the components of the red ink.

Conclusion:

Chromatography is a simple yet powerful technique used for separating and identifying different substances in a mixture. By understanding the principles of chromatography and Rf values, you can apply this technique to a wide range of applications in chemistry and beyond.