Experimental Techniques

Summary: Methods for separating and purifying substances including chromatography, distillation, filtration, crystallisation, and determining purity. These are essential practical skills tested in Paper 5 (Practical Test) and Paper 6 (Alternative to Practical). Tags: igcse chemistry practical-skills Created: 2026-07-14 Last Updated: 2026-07-16

Learning Objectives

By the end of this topic, you should be able to:

• Describe and explain methods of separation: filtration, crystallisation, simple distillation, fractional distillation, chromatography
• Select an appropriate separation technique for a given mixture
• Describe how to prepare a pure, dry sample of a soluble salt from an insoluble base or carbonate
• Define and distinguish between: solvent, solute, solution, saturated solution, residue, filtrate
• Interpret simple chromatograms and calculate Rf values
• Describe how to test for purity (melting point / boiling point)

Content

1. Key Terminology

2. Filtration

Purpose: To separate an insoluble solid from a liquid.

Equipment: Filter funnel, filter paper, conical flask, beaker

Method:

1. Fold the filter paper into a cone and place it in the filter funnel
2. Pour the mixture into the funnel
3. The liquid (filtrate) passes through the filter paper
4. The insoluble solid (residue) remains on the filter paper

When to use: Separating sand from water; separating a precipitate from a solution after a precipitation reaction; separating an insoluble base (e.g., CuO) from a salt solution.

3. Crystallisation (Evaporation)

Purpose: To obtain a soluble solid (crystals) from a solution.

Equipment: Evaporating dish / beaker, Bunsen burner, tripod, gauze, filter paper

Method:

1. Pour the solution into an evaporating dish
2. Gently heat the solution to evaporate some of the solvent — heat until the solution is saturated (crystals begin to form at the edge, or a drop on a cold glass slide forms crystals)
3. Do NOT heat to dryness — this damages crystals and causes thermal decomposition of some salts
4. Leave the saturated solution to cool at room temperature — crystals form as solubility decreases with temperature
5. Filter off the crystals
6. Wash crystals with a small amount of cold distilled water to remove impurities
7. Dry the crystals by pressing between sheets of filter paper, or in a drying oven / desiccator

Step order (common 6-mark question):

1. Heat/evaporate solution to the point of crystallisation
2. Leave to cool
3. Filter off crystals
4. Wash crystals with cold distilled water
5. Dry crystals on filter paper / in drying oven

4. Simple Distillation

Purpose: To separate a solvent from a solution (obtaining the pure liquid solvent).

Equipment: Round-bottom flask, condenser, thermometer, receiving flask, anti-bumping granules, heat source

Method:

1. Place the solution in a round-bottom flask with anti-bumping granules (to ensure smooth boiling)
2. Heat the flask — the solvent boils, vapour rises into the condenser
3. In the condenser, cold water circulates in the outer jacket, cooling the vapour (water enters at the bottom, exits at the top — ensures the jacket is always full)
4. The vapour condenses back into liquid (the distillate)
5. Pure solvent is collected; the solute remains behind in the flask

Thermometer position: The bulb of the thermometer must be placed at the opening to the condenser (at the boiling point of the vapour entering the condenser).

When to use: Obtaining pure water from salt water; separating a pure liquid from a dissolved solid.

5. Fractional Distillation

Purpose: To separate a mixture of miscible liquids with different boiling points.

Equipment: As for simple distillation, plus a fractionating column packed with glass beads (or a Vigreux column).

Method:

1. Heat the mixture in a flask fitted with a fractionating column
2. The liquid with the lower boiling point boils first — its vapour rises through the column
3. Vapour of the higher-boiling liquid may also rise initially but condenses on the cooler glass beads and drips back into the flask
4. The thermometer reads the boiling point of the vapour entering the condenser
5. When the first liquid has been collected, the temperature rises and the higher-boiling liquid distils over

Key IGCSE Examples:

• Separating ethanol (bp 78°C) from water (bp 100°C)
• Fractional distillation of liquid air: separating N2 (bp −196°C), O2 (bp −183°C), Ar (bp −186°C)
• Fractional distillation of petroleum (crude oil): separating petrol, diesel, kerosene, etc.

6. Paper Chromatography

Purpose: To separate mixtures of soluble coloured substances (dyes, inks, food colourings). Also used for colourless substances (using a locating agent).

Equipment: Chromatography paper (or filter paper), beaker, solvent (water or organic solvent), pencil (NOT pen), ruler, capillary tube

Method:

1. Draw a pencil line (baseline) about 1-2 cm from the bottom of the chromatography paper
2. Place a small spot of the mixture on the pencil line using a capillary tube
3. Place spots of known reference substances alongside for comparison
4. Place the paper in a beaker with a small amount of solvent — the solvent level must be below the pencil line (so spots do not dissolve into the solvent)
5. Cover the beaker and allow the solvent to rise up the paper
6. When the solvent front is near the top, remove the paper and mark the solvent front immediately with a pencil
7. Allow the paper to dry

Why pencil? Pencil lead (graphite) is insoluble and will not run in the solvent. Pen ink runs and contaminates the chromatogram.

Rf Value (Retention Factor):

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

• Rf is always between 0 and 1
• A substance with a larger Rf value is more soluble in the solvent (travels further)
• Each substance has a characteristic Rf value in a given solvent — useful for identification

7. Choosing a Separation Technique

8. Testing for Purity

Pure substances melt and boil at fixed, sharp temperatures. Impurities:

• Lower the melting point
• Widen the melting point range
• Raise the boiling point

Melting point determination:

1. Place a small amount of solid in a capillary tube
2. Attach to a thermometer and heat slowly in an oil bath or melting point apparatus
3. Record the temperature range over which the solid melts
4. Compare with the known melting point — a pure substance melts within 1°C of the literature value

Worked Examples (Exam-Style)

Example 1: Preparing Copper Sulfate Crystals

Question: Describe how you would prepare a pure, dry sample of copper(II) sulfate crystals from copper(II) oxide and sulfuric acid. [6 marks]

Solution (model answer):

• Add excess copper(II) oxide (black solid) to warm dilute sulfuric acid and stir ✓
• Filter the mixture to remove unreacted (excess) copper(II) oxide ✓
• The filtrate is copper(II) sulfate solution ✓
• Heat the filtrate to evaporate some water — until the point of crystallisation (saturated) ✓
• Leave the solution to cool — blue crystals of CuSO4.5H2O form ✓
• Filter off the crystals, wash with a little cold distilled water, and dry between filter paper ✓

Example 2: Chromatography

Question: A spot of black ink is placed on chromatography paper. The chromatogram shows three spots: red (Rf = 0.8), blue (Rf = 0.5), and yellow (Rf = 0.3). Which dye is most soluble in the solvent? Explain. [2 marks]

Solution (model answer):

• The red dye is most soluble ✓
• Because it has the highest Rf value (0.8), meaning it spent more time dissolved in the mobile phase (solvent) than adsorbed to the stationary phase (paper) ✓

Example 3: Separating a Mixture

Question: A mixture contains zinc powder (insoluble) and sulfur powder (soluble in organic solvents). Suggest a method to separate the mixture. [4 marks]

Solution (model answer):

• Add an organic solvent (e.g., methylbenzene) to the mixture and stir until all sulfur dissolves ✓
• Filter the mixture — zinc remains as residue on the filter paper ✓
• Wash the zinc residue and dry it ✓
• Evaporate the solvent from the filtrate to obtain solid sulfur ✓

Practice Questions

1. State the piece of apparatus used to measure the temperature during distillation and state where its bulb should be positioned. [2 marks]

2. Describe the steps to obtain pure, dry crystals of a soluble salt from its aqueous solution. [5 marks]

3. Explain why the baseline in chromatography is drawn with a pencil and not a pen. [2 marks]

4. Two liquids, X and Y, have boiling points of 78°C and 100°C. Name the technique used to separate them and explain how it works. [4 marks]

5. An impure sample of a solid melts over the range 115-122°C. The pure solid melts at 125°C. Explain why the impure sample has a lower and wider melting point range. [2 marks]

Key Facts to Memorise

• Filtration: separates insoluble solid from liquid
• Crystallisation: obtain soluble solid from solution — evaporate to saturation, cool, filter, wash with cold water, dry
• Simple distillation: obtain pure solvent from solution — solvent boils, vapour condenses, solute stays behind
• Fractional distillation: separates miscible liquids using fractionating column (different boiling points)
• Chromatography: separates dissolved substances by differential solubility — Rf = spot distance / solvent distance
• Impure substances: lower MP, wider MP range, higher BP than pure substance
• Thermometer bulb: must be at the opening to the condenser (to measure BP of vapour entering the condenser)
• Anti-bumping granules: ensure smooth, even boiling (prevent large bubbles forming suddenly)
• Water in condenser: enters at bottom, exits at top (ensures jacket is always full)

Common Misconceptions

Key Concepts from Past Papers

Definitions You MUST Know (Exact Mark Scheme Wording)

• Saturated solution: a solution containing the maximum amount of dissolved solute at that temperature
• Filtrate: the liquid that passes through the filter paper
• Residue: the solid that remains on the filter paper
• Rf value: distance moved by spot divided by distance moved by solvent front

Recurring Mark Scheme Answers

• Step 4: evaporate filtrate to point of crystallisation / evaporate some of the water and leave / evaporate to form saturated solution
• Step 6: dry crystals on filter paper / dry crystals in drying oven
• Add (organic) solvent to the mixture (and stir until all sulfur dissolves)
• Filter off the zinc / filter off the residue
• Let solvent evaporate from zinc / residue
• Evaporate the sulfur solution / evaporate the solvent from the solution / evaporate solvent from filtrate

Common Question Types

• Extended writing (describe/explain/suggest): “Describe how to prepare a pure, dry sample of…”
• Short recall (state/give/name): “Name a suitable piece of apparatus to measure…”
• Diagram/labelling task: “Label the condenser, thermometer, and flask on the diagram”
• Multiple choice: “Which technique is used to separate…”

Exam Tips

• This topic appears in 3 papers in the database
• Total Q+A entries: 3
• Average marks per question: 2.7
• The crystallisation question is almost always 4-6 marks and follows the same 5-step pattern every time
• “Heat to dryness” will lose you marks — always say “saturated” or “point of crystallisation”
• In chromatography, a locating agent is needed for colourless substances (e.g., ninhydrin for amino acids)
• For fractional distillation, be able to name real examples (ethanol/water, liquid air, crude oil fractions)

Keywords from Past Papers

evaporate, step, solution, dry, crystals, filter, solvent, filtrate, point, crystallisation, water, leave, form, saturated, drying

Related Notes

• Chemical Tests — Tests for gases, cations, anions, and water
• Making Salts — Preparation methods for soluble and insoluble salts
• Water of Crystallisation — Hydrated salts and finding x
• IGCSE-Chem-Index — Full IGCSE Chemistry index

Sources

• OpenStax Chemistry 2e — [Chapter 1: Essential Ideas (Measurements and Laboratory Techniques)], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Experimental Techniques and Apparatus], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 2: Experimental Techniques, Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 2: Laboratory Techniques], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/32 Feb/March 2019: Q33(c)(iii) (4m)
• 0620/33 October/November 2022: Q66(d)(i) (2m), Q66(d)(i) (2m)