IGCSE Chemistry — Experimental Techniques and Analysis: Notes, Practice Questions & Worked Solutions

IGCSE Chemistry 0620 · Topic 12: Experimental Techniques and Analysis

Experimental Techniques and Analysis

This topic is the practical toolkit of the whole syllabus: how to separate mixtures, and how to identify unknown ions and gases using simple, distinctive chemical tests.

1. Separation Techniques

Technique	Used to separate
Filtration	An insoluble solid from a liquid
Crystallisation	A dissolved solid from a solution (by evaporating solvent)
Simple distillation	A solvent from a solution (e.g. water from salt water)
Fractional distillation	A mixture of miscible liquids with different boiling points
Paper chromatography	Different dissolved substances in a mixture

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

2. Tests for Ions

Cation tests (using sodium hydroxide solution) Add NaOH dropwise, then in excess, and observe the precipitate colour:
Cu²⁺ → blue precipitate. Fe²⁺ → green precipitate. Fe³⁺ → red-brown precipitate. Ca²⁺ → white precipitate (insoluble in excess). NH₄⁺ → no precipitate, but ammonia gas released on warming (turns damp red litmus blue).

Anion tests Carbonate (CO₃²⁻): add dilute acid, gas produced turns limewater milky (CO₂). Sulfate (SO₄²⁻): add dilute HCl then barium chloride solution, forms a white precipitate. Halide (Cl⁻, Br⁻, I⁻): add dilute nitric acid then silver nitrate solution — Cl⁻ gives a white precipitate, Br⁻ a cream precipitate, I⁻ a yellow precipitate.

3. Tests for Gases

Gas	Test and positive result
Hydrogen	Lit splint → “squeaky pop”
Oxygen	Glowing splint → relights
Carbon dioxide	Bubbled through limewater → turns milky
Ammonia	Damp red litmus paper → turns blue
Chlorine	Damp litmus paper → turns white (bleached)

4. Flame Tests

Lithium: red. Sodium: yellow/orange. Potassium: lilac. Calcium: orange-red. Copper: blue-green.

Worked example — calculating Rf value On a chromatogram, a spot moves 4.2 cm while the solvent front moves 6.0 cm. Calculate the Rf value.
Rf = 4.2/6.0 = 0.7

Common mistakes to watch for

Forgetting to add acid BEFORE barium chloride in the sulfate test — this rules out a carbonate giving a false-positive precipitate.
Mixing up the ammonium ion test (no precipitate with NaOH, just gas on warming) with the other cation tests (which do give coloured precipitates).
Measuring Rf from the wrong starting point — always measure from the original pencil baseline, not the edge of the paper.

Key Vocabulary Recap

Rf value — ratio of distance travelled by a substance to distance travelled by the solvent

Precipitate — an insoluble solid formed in a solution during a reaction

Syllabus Reference

This note covers IGCSE Chemistry 0620 Topic 12, Experimental Techniques and Chemical Analysis: measuring apparatus, purification methods, chromatography, and qualitative tests for ions, gases, and flame colours.

Practice questions

IGCSE Chemistry 0620 · Topic 12: Experimental Techniques and Analysis — Practice Question Bank (no answers)

Experimental Techniques and Analysis — Practice

Covers: separation methods, chromatography, and tests for ions and gases. 10 questions.

Q1 Core

State which separation technique would be used to separate: (a) sand from water, (b) salt from salt water, (c) ethanol from a water-ethanol mixture.

Q2 Core

On a paper chromatogram, a spot moves 4.2 cm while the solvent front moves 6.0 cm. Calculate the Rf value.

Q3 Core

Describe the test for carbon dioxide gas and its positive result.

Q4 Core

Describe the test for hydrogen gas and its positive result.

Q5 Core

Describe the test for ammonia gas and its positive result.

Q6 Extended

Describe how you would test a solution for the presence of Cu²⁺ ions, and state the expected observation.

Q7 Extended

Describe how you would test a solution for sulfate ions, and state the expected observation.

Q8 Extended

Describe how you would test a solution for chloride ions, and state the expected observation.

Q9 Core

State the flame test colour for sodium, potassium, and calcium.

Q10 Extended

Explain why dilute acid is added before barium chloride solution in the test for sulfate ions.

Worked solutions

Show worked solutions

IGCSE Chemistry 0620 · Topic 12: Experimental Techniques and Analysis — Solutions

Experimental Techniques and Analysis — Practice: Solutions

Separation techniques for three mixtures.

✓(a) Filtration (b) Simple distillation, or crystallisation to recover the salt (c) Fractional distillation

Rf value: spot 4.2cm, solvent 6.0cm.

Rf = distance moved by spot / distance moved by solvent

4.2÷6.0

✓Rf = 0.7

Test for CO₂.

✓Bubble the gas through limewater. A positive result is the limewater turning milky/cloudy.

Test for H₂.

✓Hold a lit splint at the open end of the test tube. A positive result is a “squeaky pop” sound.

Test for NH₃.

✓Hold a piece of damp red litmus paper near the gas. A positive result is the paper turning blue.

Test for Cu²⁺.

✓Add sodium hydroxide solution dropwise. A blue precipitate forms, which does not dissolve in excess NaOH.

Test for sulfate ions.

✓Add dilute hydrochloric acid, then barium chloride solution. A white precipitate (barium sulfate) forms if sulfate ions are present.

Test for chloride ions.

✓Add dilute nitric acid, then silver nitrate solution. A white precipitate (silver chloride) forms if chloride ions are present.

Flame test colours: Na, K, Ca.

✓Sodium: yellow/orange. Potassium: lilac. Calcium: orange-red.

Q10

Why add acid before BaCl₂ in the sulfate test?

✓The acid reacts with and removes any carbonate ions present (which would otherwise also form a white precipitate with barium chloride, barium carbonate, giving a false positive). This ensures a white precipitate is only seen if sulfate ions are genuinely present.

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