Chemical Analysis
What's covered
Key facts
Calcium gives a brick-red (orange-red) flame in a flame test.
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.
In paper chromatography the moving liquid is called the mobile phase.
Evaporation or crystallisation separates a soluble solid from its solvent.
The test for chlorine: damp litmus paper is bleached (turns white).
Test for chloride (Cl⁻): acidify with dilute nitric acid, then add silver nitrate solution. A white precipitate of silver chloride confirms chloride.
Sample questions
A taste of the 55 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
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
What are the flame test colours for sodium, potassium, calcium, and copper?
- •Sodium → blue; Potassium → orange; Calcium → lilac; Copper → crimson.
- •Sodium → crimson; Potassium → blue; Calcium → yellow; Copper → orange.
- •Sodium → green; Potassium → yellow; Calcium → crimson; Copper → lilac.
- ✓Sodium → yellow/orange; Potassium → lilac/violet; Calcium → orange-red; Copper → green/blue-green.
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