Chemical Analysis
What's covered
Key facts
Calcium gives a brick-red (orange-red) flame in a flame test.
Advantages of instrumental methods are that they are fast, accurate, and sensitive, and can use very small samples.
Chromatography identifies an unknown substance by running it alongside known references; a matching Rf indicates the same compound.
Crystallisation separates a soluble solid from its solution.
Damp red litmus paper turns blue near ammonia gas (because ammonia is alkaline).
Test for carbonate (CO₃²⁻): add dilute acid; carbon dioxide is released. Pass the gas through limewater — it turns cloudy/milky, confirming CO₂ and thus carbonate.
Chlorine gas bleaches damp litmus paper white — used as the diagnostic test for chlorine.
Flame emission spectroscopy is an instrumental method used to analyse metal ions in solution.
In paper chromatography the moving liquid is called the mobile phase.
Evaporation or crystallisation separates a soluble solid from its solvent.
Sample questions
A taste of the 71 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
Which metal gives a lilac flame?
- •Calcium
- •Lithium
- ✓Potassium
- •Sodium
What are the main advantages of instrumental methods over manual chemical tests?
- •They always need a very large sample to work
- •They are cheaper but far less accurate than manual tests
- ✓They are fast, sensitive and use very small samples
- •They are slower but easier for students to perform
How can chromatography be used to distinguish a pure substance from a mixture?
- •A mixture gives a colourful pattern; a pure substance shows no colour
- •A pure substance travels further up the paper than a mixture
- ✓Pure gives one spot at a unique Rf; a mixture gives several
- •Pure substances cannot be analysed by chromatography
Which method separates a soluble solid from a solvent?
- •Chromatography across paper
- •Distillation only when boiling
- ✓Evaporation or crystallisation
- •Filtration through paper
What is the test for carbon dioxide gas?
- •Blue litmus turns red if CO₂ is present
- ✓Bubble through limewater — turns milky if CO₂ present
- •Glowing splint relights if CO₂ is present
- •Lit splint gives a squeaky pop if CO₂ present
Which test identifies sulfate ions?
- ✓Add barium chloride; white precipitate forms
- •Add hydrochloric acid; gas bubbles form
- •Add silver nitrate; white precipitate forms
- •Add sodium hydroxide; blue precipitate forms
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