Required Practical: Chromatography

This lesson covers paper chromatography, one of the required practicals for the AQA GCSE Chemistry specification (8.1.1). Chromatography is a technique used to separate and identify substances in a mixture, particularly useful for identifying unknown substances or checking the purity of a substance. You need to understand the method, how to calculate Rf values, and how to interpret chromatograms.

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What Is Chromatography?

Chromatography is a separation technique used to separate dissolved substances (solutes) from one another. It works because different substances have different solubilities in the solvent used, so they travel different distances through the stationary phase.

There are two phases in chromatography:

Phase	Description	Example in Paper Chromatography
Stationary phase	The phase that does not move	The chromatography paper
Mobile phase	The phase that moves through the stationary phase	The solvent (e.g., water, ethanol)

The separation depends on the relative attraction of each substance to the stationary and mobile phases:

• A substance that is more soluble in the mobile phase will travel further up the paper.
• A substance that is more attracted to the stationary phase will travel a shorter distance.

Exam Tip: The mobile phase is the solvent that moves up the paper. The stationary phase is the paper itself. Remember: "mobile = moves, stationary = stays still." This is a common 1-mark question.

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Apparatus and Materials

For this required practical, you will need:

Apparatus / Material	Purpose
Chromatography paper	The stationary phase
Pencil (NOT pen)	To draw the baseline — pencil is insoluble in the solvent
Ruler	To draw a straight baseline and measure distances
Beaker or chromatography tank	To hold the solvent
Solvent (water or ethanol)	The mobile phase
Capillary tube or spotting tile	To apply small spots of the samples
Watch glass or cling film	To cover the beaker and prevent solvent evaporation
Known reference samples	To compare with unknown substances

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Method: Step by Step

Follow these steps carefully:

1. Draw a baseline in pencil near the bottom of the chromatography paper (approximately 2 cm from the bottom). Use a ruler to make it straight.

2. Apply spots of the substances to be tested onto the baseline using a capillary tube. Leave gaps between the spots so they do not overlap. Label each spot in pencil below the baseline.

3. Pour a shallow layer of solvent into the beaker — the solvent level must be below the baseline. This is critical: if the solvent touches the spots, the substances will dissolve directly into the solvent rather than travelling up the paper.

4. Place the chromatography paper into the beaker so that the bottom of the paper dips into the solvent, but the baseline remains above the solvent level.

5. Cover the beaker with a watch glass or cling film to prevent evaporation of the solvent.

6. Wait for the solvent to travel up the paper by capillary action. Remove the paper when the solvent front is near the top.

7. Mark the solvent front with a pencil immediately after removing the paper (it will dry quickly).

8. Measure the distances travelled by each substance and by the solvent front from the baseline.

Exam Tip: Always state that the baseline must be drawn in pencil, not pen. Pen ink would dissolve in the solvent and run with the chromatogram, ruining the results. This is a frequently tested point.

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Chromatography Setup Diagram

graph TB
    subgraph Beaker["Chromatography Setup"]
        A["Watch glass / lid (prevents evaporation)"]
        B["Solvent front (mark when removed)"]
        C["Chromatography paper (stationary phase)"]
        D["Separated spots / substances"]
        E["Original baseline (drawn in pencil)"]
        F["Solvent / mobile phase (below baseline)"]
    end

    A --> B
    B --> D
    D --> E
    E --> F

    style A fill:#ecf0f1,color:#2c3e50
    style B fill:#3498db,color:#fff
    style C fill:#f5f5dc,color:#2c3e50
    style D fill:#e74c3c,color:#fff
    style E fill:#95a5a6,color:#fff
    style F fill:#3498db,color:#fff

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Calculating Rf Values

The Rf value (retention factor) is a ratio that allows you to identify substances. It is calculated using the formula:

Rf = distance moved by substance / distance moved by solvent front

Both distances are measured from the baseline (not from the bottom of the paper).

Example Calculation

Measurement	Value
Distance moved by substance	4.8 cm
Distance moved by solvent front	8.0 cm
Rf value	4.8 / 8.0 = 0.60

Key Points About Rf Values

• Rf values are always between 0 and 1 (a substance cannot travel further than the solvent front).
• Rf values have no units — they are ratios.
• Each substance has a unique Rf value in a given solvent under specific conditions.
• Rf values can be used to identify unknown substances by comparing them with known reference values.
• Rf values are only valid if the same solvent and same conditions (temperature, paper type) are used.

Exam Tip: When calculating Rf values, always show your working and measure from the centre of the spot to the baseline. The Rf value must be between 0 and 1 — if you get a value greater than 1, you have measured incorrectly or divided the wrong way round.

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Interpreting Chromatograms

A chromatogram is the pattern of spots produced by chromatography.

What Different Results Tell You

Observation	Interpretation
One spot	The substance is a pure substance (a single compound)
Multiple spots	The substance is a mixture containing multiple compounds
Spot at same height as a known reference	The unknown substance contains the same compound as the reference
Spot does not move from baseline	The substance is insoluble in the mobile phase
Spot at the solvent front	The substance is very soluble in the mobile phase

Identifying Unknown Substances

To identify an unknown substance:

1. Run the unknown substance alongside known reference samples on the same chromatogram.
2. Compare the positions (Rf values) of the spots.
3. If a spot from the unknown has the same Rf value as a known reference, they are likely the same substance.

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Common Errors and How to Avoid Them