# Separation, Purification and Chromatography

> Chemistry · CIE IGCSE 0620
> Source: https://www.owlsprep.com/study/cie-0620-u12-separation-purification-and-chromatography/

This guide covers separation techniques, purity testing, and paper chromatography aligned to CIE IGCSE Chemistry 0620 2026-2028 syllabus points 12.3 and 12.4 for both Core and Extended tiers.

**Prerequisites:** [Understanding of pure substances and mixtures (Unit 1)](https://www.owlsprep.com/study/cie-0620-u1-pure-substances-mixtures/); [Basic laboratory safety procedures](https://www.owlsprep.com/study/cie-0620-u1-experimental-safety/)

## Learning objectives

- Select and describe appropriate separation techniques for common mixtures
- Explain melting and boiling point tests for substance purity
- Describe paper chromatography procedures and interpret core chromatograms
- Calculate and use Rf values to identify substances (Extended only)

## Core Separation Techniques for Mixtures

Mixtures are separated using physical methods that exploit differences in component properties such as solubility, boiling point, and particle size. You will be asked to select and describe the correct method for given mixtures in your exam.

**Separation technique** — A physical process used to isolate individual components of a mixture without changing their chemical identity

- **Filtration**: Separates insoluble solid from liquid (e.g. sand from water)
- **Evaporation**: Separates soluble solid from solvent (e.g. salt from salt water)
- **Simple distillation**: Separates solvent from solution (e.g. pure water from salt water)
- **Fractional distillation**: Separates miscible liquids with different boiling points (e.g. ethanol and water)
- **Crystallisation**: Separates soluble solid from solvent to form large pure crystals (e.g. copper sulfate crystals from solution)

**Worked example:** A student has a mixture of insoluble calcium carbonate and soluble sodium chloride. Outline the steps to separate and obtain pure dry samples of both substances.

1. Add deionised water to the mixture and stir until all sodium chloride dissolves.
2. Filter the mixture: the residue is calcium carbonate, the filtrate is sodium chloride solution.
3. Wash the calcium carbonate residue with cold deionised water and dry in a warm oven.
4. Heat the sodium chloride solution gently in an evaporating dish until crystals form at the edge of the liquid.
5. Leave the solution to cool slowly to form large pure sodium chloride crystals, then filter and dry the crystals.

> **Exam tip:** Always name the specific components of the mixture when describing separation steps, rather than generic terms like 'solid' or 'liquid' to gain full marks.

## Purity Testing Methods

A pure substance contains only one element or compound, with no other contaminants. Impure substances have altered physical properties that we use to test for purity.

**Pure substance** — A single element or compound that melts and boils at fixed, characteristic temperatures

- **Melting point test**: Pure solids melt at a single exact temperature. Impure solids melt over a range of temperatures, at a lower value than the pure substance.
- **Boiling point test**: Pure liquids boil at a single exact temperature. Impure liquids boil over a range of temperatures, at a higher value than the pure substance.

**Worked example:** Pure paracetamol has a melting point of 169°C. A school lab sample of paracetamol melts between 162°C and 167°C. State if the sample is pure, and explain your answer.

1. Conclusion: The sample is impure.
2. Justification: Pure paracetamol melts at a single fixed temperature of 169°C.
3. The sample melts over a wide range of temperatures, and its melting point is lower than the pure value, confirming it contains impurities.

> **Exam tip:** When describing purity tests, always mention comparing your measured melting/boiling point to the known reference value for the pure substance from a data book.

## Core Paper Chromatography

Chromatography separates mixtures of dissolved substances (e.g. food colourings, ink pigments) by exploiting differences in how strongly components attach to a stationary phase (chromatography paper) vs dissolve in a mobile phase (solvent).

**Chromatogram** — The visible output of a chromatography experiment, showing separated mixture components as spots on the stationary phase paper

1. Draw a pencil baseline ~1cm from the bottom of the chromatography paper (pencil graphite is insoluble so it will not move with solvent).
2. Place small spots of the test mixture and reference known substances on the baseline.
3. Place the paper in a beaker with solvent, ensuring the solvent level is below the baseline to avoid dissolving the spots directly.
4. Put a lid on the beaker to saturate air with solvent vapour and prevent fast evaporation.
5. Remove the paper when solvent reaches near the top, and draw a line to mark the solvent front.

> **tip**
>
> A pure substance will only produce one spot on a chromatogram, while an impure substance will produce two or more spots.

**Worked example:** A student tests 3 food colourings for a banned red pigment using chromatography. The reference banned pigment produces a spot 4cm up the paper. Colouring A has a spot at 4cm, B has no spot at 4cm, C has spots at 4cm and 2cm. State which colourings contain the banned pigment.

1. Compare the position of each colouring's spots to the reference banned pigment spot.
2. Colouring A has a spot at the same position as the reference, so it contains the banned pigment.
3. Colouring C also has a spot matching the reference position, so it also contains the banned pigment.
4. Colouring B has no matching spot, so it does not contain the banned pigment.

## Extended Only: Rf Value Calculation

Extended candidates must calculate retention factor (Rf) values from chromatograms, and use these values to identify unknown substances. Rf values are unique for each substance when using the same solvent and stationary phase, so they can be compared to reference data to identify components.

**Retention factor (Rf)** — The ratio of the distance a substance moves up chromatography paper to the distance the solvent front moves from the baseline

*Notation:* R_f

$$R_f = \frac{\text{Distance moved by substance}}{\text{Distance moved by solvent front}}$$

**Worked example:** In a chromatography experiment, the solvent moves 8.0cm from the baseline. A blue ink spot moves 5.2cm from the baseline. Calculate the Rf value of the blue ink, giving your answer to 2 significant figures.

1. Identify values: distance moved by substance = 5.2cm, distance moved by solvent = 8.0cm
2. $$R_f = \frac{5.2}{8.0}$$
3. Calculate: 5.2 ÷ 8.0 = 0.65
4. Final answer: Rf = 0.65 (no units required)

> **Exam tip:** Rf values are always less than 1. If your calculation gives a value greater than 1, you have swapped the numerator and denominator. No units are needed for Rf as it is a ratio.

## Common pitfalls

- **Wrong:** Placing chromatography solvent level above the baseline of spots
  - Why it fails: Spots dissolve directly into the bulk solvent instead of moving up the paper, producing no usable chromatogram
  - Correct: Ensure solvent level is below the pencil baseline when setting up the experiment
- **Wrong:** Stating impure substances have a higher melting point than pure substances
  - Why it fails: Impurities lower the melting point of a substance and make it melt over a range of temperatures
  - Correct: Remember: impure = lower melting point range, higher boiling point range
- **Wrong:** Using pen to draw the chromatography baseline
  - Why it fails: Pen ink is soluble in most solvents and moves up the paper, interfering with sample spots
  - Correct: Always use pencil to draw the baseline, as graphite is insoluble in common solvents
- **Wrong:** Including units when writing Rf values
  - Why it fails: Rf is a ratio of two distances, so units cancel out entirely
  - Correct: Write Rf values as a decimal less than 1 with no units
- **Wrong:** Using full evaporation to separate copper sulfate crystals from solution
  - Why it fails: Fast direct heating produces small impure crystals and may decompose heat-sensitive solids
  - Correct: Use crystallisation: heat gently until crystals form at the edge, then cool slowly for large pure crystals

## Cheatsheet

| Technique | Use Case | Key Principle |
| --- | --- | --- |
| Filtration | Insoluble solid + liquid | Difference in particle size |
| Evaporation | Soluble solid from solution | Solvent vaporises leaving solid residue |
| Simple distillation | Solvent from solution | Difference in boiling point of solvent and solute |
| Fractional distillation | Miscible liquid mixture | Different boiling points separated via fractionating column |
| Crystallisation | Pure soluble solid from solution | Slow cooling forms large pure crystals |
| Paper chromatography | Dissolved mixture (inks/colourings) | Components move at different rates through stationary phase |
| Melting point test | Test solid purity | Pure solid melts at fixed temperature; impure melts over lower range |
| Boiling point test | Test liquid purity | Pure liquid boils at fixed temperature; impure boils over higher range |
| Rf calculation (Extended) | Identify chromatogram components | Rf = distance moved by substance / distance moved by solvent |

## What's next

Now that you have mastered separation, purification, and chromatography for CIE IGCSE Chemistry 0620, you are ready to move to qualitative analysis, the next key experimental techniques topic. This builds on your practical skills to identify unknown ions and gases using standard chemical tests. You should also practice applying these separation methods to past paper practical questions, focusing on the tier you are sitting (Core Paper 3 or Extended Paper 4). Make sure you revisit purity test rules and chromatography steps regularly, as these are common 2-3 mark question topics across all papers.

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