M.2 Pure Substances, Mixtures & Separation

UNIT SYLLABUS

MYP

Chemists constantly need to answer two questions: is this substance pure, and if not, how do I separate it? A pure substance contains only one element or compound and has sharp, fixed melting and boiling points; mixtures melt and boil over a range. The separation toolkit — filtration, evaporation, crystallisation, distillation, and chromatography — works because it exploits differences in physical properties: particle size, boiling point, solubility. Choosing the right technique for a given mixture is one of the most-tested skills in MYP chemistry.

Guiding Questions

? How can we tell whether a substance is truly pure?
? What property differences let us pull a mixture apart without chemical reactions?

✓ What the IB expects you to master

Define elements, compounds and mixtures, and classify substances into these categories.
Use melting and boiling points to test purity: pure substances change state at sharp, fixed temperatures.
Select and describe filtration for insoluble solids in liquids.
Select and describe evaporation and crystallisation for dissolved solids.
Select and describe simple and fractional distillation for liquids with different boiling points.
Select and describe paper chromatography for mixtures of dissolved substances, and calculate $R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$ .
Distinguish physical changes (no new substance) from chemical changes.

1 Key Formulas

Retention factor

R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}

2 Exam Preparation & Topic Explanations

Choosing the right separation technique (Criteria A & B)

Exam questions describe a mixture and ask you to plan the separation. Decide from the properties: insoluble solid + liquid → filter; dissolved solid → evaporate/crystallise; two liquids → distil (fractional if boiling points are close); coloured dissolved substances → chromatography.

In Criterion B investigation tasks, justify the choice by naming the property difference being exploited.

Pro Exam Strategy

Filtrate = liquid that passes through; residue = solid left behind. Use the words correctly.
Crystallisation beats boiling dry when the solid could decompose — gentle evaporation, then leave to crystallise.
Chromatography needs the start line in PENCIL — ink would dissolve and run.
Distillation diagrams: thermometer bulb at the junction, water enters the condenser at the bottom.

3 MCQ Practice

Q1. Which technique separates salt from a salt-and-sand mixture after adding water and filtering?

Chromatography
Distillation
Evaporation and crystallisation
Magnetic separation

Q2. A liquid melts between 52 °C and 58 °C. This tells you the liquid is:

A pure compound
A pure element
A mixture
An alloy

Q3. In paper chromatography, a dye spot moves 4.0 cm while the solvent front moves 8.0 cm. The $R_f$ value is:

0.25
0.50
2.0
4.0

4 Short Answer Questions

PDF

Download the practice worksheet

All questions from this topic + answer key — free, printable.

↓

MYP Physics Guide →

The companion topic guide.

Criterion B Guide →

Design investigations that score.

Daily Question →

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