Chemical Analysis
What's covered
Key facts
Flame Tests
Calcium gives a brick-red (orange-red) flame in a flame test.
Testing for Ions
Adding sodium hydroxide solution to a metal-ion solution gives a coloured hydroxide precipitate: copper(II) blue, iron(II) green, iron(III) brown.
Instrumental Methods
Advantages of instrumental methods are that they are fast, accurate, and sensitive, and can use very small samples.
Purity and Chromatography
Chromatography identifies an unknown substance by running it alongside known references; a matching Rf indicates the same compound.
Separating Mixtures (Edexcel)
Evaporation or crystallisation separates a soluble solid from its solvent.
Testing for Gases
Damp red litmus paper turns blue near ammonia gas (because ammonia is alkaline).
Flame Tests
Copper gives a green (blue-green) flame in a flame test.
Testing for Ions
To test for carbonate ions, add dilute acid: the carbonate fizzes and releases carbon dioxide, which turns limewater milky.
Instrumental Methods
Flame emission spectroscopy is an instrumental method used to analyse metal ions in solution.
Purity and Chromatography
In paper chromatography the moving liquid is called the mobile phase.
Practice questions with answers
A taste of the 60 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
- ✓Evaporation or crystallisation
- •Filtration through paper
- •Using a separating funnel
What is the test for carbon dioxide gas?
- ✓Bubble through limewater: turns milky if CO₂ present
- •Damp red litmus turns blue if CO₂ is 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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