🔬 Understanding Chromatography (IGCSE Chemistry 0620: Section 12.3)

Hello future Chemists! This chapter is all about a super useful technique called Chromatography. This big word just describes a clever method scientists use to separate the tiny components that make up a mixture.

Think of it like being a forensic scientist or a food quality inspector. How do you check if the ink used in a ransom note is a cheap mixture of dyes, or how do you ensure the food colouring in a drink is safe? Chromatography gives us the answer!

By the end of these notes, you will know exactly how this separation works, how to perform the most common type (paper chromatography), and how to interpret the results.

1. The Fundamental Principle of Chromatography

Chromatography is essentially a race! You are racing the different components of a mixture (like different colours in an ink pen) to see which one moves fastest.
The separation works because the components in the mixture have different levels of attraction to two things:

A simple Analogy: The Sticky Race Track

Imagine four people (A, B, C, D) running a race.

  • The ground (the paper) is slightly sticky.
  • The solvent (the water) is the carrier that helps them move.

Runner A is very attracted to the ground (sticky). Runner B is very attracted to the carrier (water).

Result: Runner A will stick to the starting line and move slowly. Runner B will be carried easily by the water and move quickly. They separate!

Key Terms in Chromatography (The Two Phases)

For the exam, you must know these two phases:

  1. The Stationary Phase (The Track): This phase does not move.
    In paper chromatography, this is the chromatography paper.
  2. The Mobile Phase (The Carrier): This phase moves, flowing through the stationary phase, carrying the components of the mixture with it.
    In paper chromatography, this is the solvent (often water or ethanol).

The core principle is that the components separate based on their differing solubility in the mobile phase and their differing adsorption (stickiness) to the stationary phase.

Quick Tip: More soluble in the mobile phase (solvent) = moves further and faster!


Key Takeaway:

Chromatography separates mixtures because different components travel at different speeds due to differing attractions to the stationary phase (paper) and the mobile phase (solvent).

2. Paper Chromatography: Separating Coloured Substances (Core)

Paper chromatography is the simplest and most common form you will use in the lab. We use it primarily to separate mixtures of coloured substances, like dyes in food or inks.

Step-by-Step Procedure:

  1. Preparation: Draw a light line near the bottom of the chromatography paper using a pencil.
    (Why pencil? Pencil lead (graphite) is insoluble in the solvent and will not dissolve or run. If you use ink, the ink itself would separate and ruin the test!)
  2. Spotting: Place a small, concentrated spot of the mixture (e.g., black ink) exactly on the pencil line. Let it dry completely.
  3. Setting up the Apparatus: Place the paper in a beaker or jar with a small amount of the chosen solvent (mobile phase).
    Crucial Rule: The solvent level must be below the pencil line/sample spot. If the spot is submerged, the mixture will dissolve into the solvent pool instead of travelling up the paper.
  4. Running the Chromatogram: Allow the solvent to move up the paper (this process is called capillary action). This is the "mobile phase" doing its job.
  5. Finishing the Run: When the solvent front (the wet edge) is near the top, remove the paper immediately.
  6. Marking and Drying: Immediately mark the position of the solvent front with a pencil (this spot will evaporate quickly). Then let the paper (now called the chromatogram) dry.

Interpreting Simple Chromatograms (Core)

Once the chromatogram is dry, we can analyse the results:

Identifying Pure and Impure Substances (Mixtures)
  • A Pure Substance shows only one spot on the chromatogram.
  • An Impure Substance (a mixture) shows two or more spots, as the different components separate.
Identifying Unknown Substances

To identify an unknown substance (U), you run it alongside known reference substances (A, B, C).

  • If the spot from substance U travels the exact same distance as a spot from substance A, then U likely contains substance A.
  • If U shows three spots, it is a mixture of three components. If one of those three spots matches the distance travelled by pure substance B, then B is one of the components of the mixture U.

Key Takeaway:

Paper chromatography uses a moving solvent (mobile phase) and fixed paper (stationary phase) to separate components based on solubility and adsorption. Pure substances show one spot; mixtures show multiple spots.

3. Extended Content: Quantitative Analysis and Colourless Substances

If you are studying the Extended curriculum, you need to understand how to quantify the results and how to deal with substances that aren't coloured.

3.1 The Retention Factor (\(R_f\)) (Supplement)

The distance a substance travels can be measured accurately and converted into a value called the Retention Factor (\(R_f\)).

The \(R_f\) value is a ratio that helps scientists identify unknown substances accurately, because for a given substance and solvent, the \(R_f\) value is always constant.

The formula is:

$$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}$$

Let's break down the calculation:

  • Distance travelled by substance: This is measured from the centre of the original pencil line (the start line) to the centre of the separated spot.
  • Distance travelled by solvent: This is measured from the original pencil line (the start line) to the marked position of the solvent front (the finish line).

Important facts about \(R_f\) values:

  • The \(R_f\) value will always be less than 1 (or equal to 1 if the substance is extremely soluble and moves exactly with the solvent front).
  • It has no units (because it is a distance divided by a distance).
  • The more soluble a component is in the solvent, the closer its \(R_f\) value will be to 1.

3.2 Separating Colourless Substances (Supplement)

What if you are separating colourless mixtures, such as amino acids? Once the chromatogram is dry, you won't be able to see any spots!

To solve this, we use a Locating Agent (also called a developing agent).

Definition: A locating agent is a chemical substance sprayed onto a chromatogram (after it has dried) that reacts with the colourless separated substances to produce visible coloured spots.

(You do not need to memorise the names of specific locating agents for the exam, only the concept and the necessity of using one when dealing with colourless samples.)


Key Takeaway (Extended):

For colourless mixtures, a locating agent is needed to make the separated spots visible. The \(R_f\) value is a vital quantitative measure used for accurate identification.

📝 Quick Review Box: Chromatography

What is it?

  • A technique to separate soluble mixtures.

The Two Phases:

  • Stationary Phase: Paper (does not move).
  • Mobile Phase: Solvent (moves, carries components).

How to Read Results:

  • One spot = Pure substance.
  • Multiple spots = Mixture (impure).
  • Matching height = Same substance.

Extended Knowledge:

  • Colourless substances need a locating agent.
  • The \(R_f\) value is the distance of the substance divided by the distance of the solvent front, used for precise identification.