Separate chemistry 1
Qualitative analysis
Qualitative analysis identifies what substances are present (not how much). Key tests at GCSE:
Flame tests (metal cations) — CP7
| Ion | Flame colour |
|---|---|
| Li⁺ | Crimson/red |
| Na⁺ | Yellow/orange |
| K⁺ | Lilac/purple |
| Ca²⁺ | Brick red/orange-red |
| Cu²⁺ | Blue-green/green |
| Ba²⁺ | Green |
Method: clean nichrome wire loop with HCl; dip in sample; hold in blue Bunsen flame; observe colour.
Precipitate tests for metal cations
Add NaOH solution:
- Al³⁺: white precipitate that dissolves in excess NaOH (amphoteric).
- Ca²⁺: white precipitate, insoluble in excess.
- Cu²⁺: blue precipitate, insoluble in excess.
- Fe²⁺: green precipitate, insoluble in excess.
- Fe³⁺: orange/brown precipitate, insoluble in excess.
Tests for anions
| Ion | Reagent | Observation |
|---|---|---|
| Carbonate CO₃²⁻ | Dilute acid | Effervescence; gas turns limewater milky (CO₂) |
| Sulfate SO₄²⁻ | Dilute HCl then BaCl₂ | White precipitate (BaSO₄) |
| Chloride Cl⁻ | Dilute HNO₃ then AgNO₃ | White precipitate (AgCl) |
| Bromide Br⁻ | Dilute HNO₃ then AgNO₃ | Cream precipitate (AgBr) |
| Iodide I⁻ | Dilute HNO₃ then AgNO₃ | Yellow precipitate (AgI) |
Tests for gases
| Gas | Test | Positive result |
|---|---|---|
| H₂ | Burning splint at tube mouth | Squeaky pop |
| O₂ | Glowing splint | Relights |
| CO₂ | Bubble through limewater | Turns milky/white |
| Cl₂ | Damp litmus paper | Bleaches white |
| NH₃ | Damp red litmus paper | Turns blue (alkaline gas) |
Hydrocarbons, polymers and alcohols
Hydrocarbons
A hydrocarbon contains only carbon and hydrogen. Alkanes (general formula CₙH₂ₙ₊₂) are saturated: all C-C single bonds. Alkenes (CₙH₂ₙ) are unsaturated: at least one C=C double bond.
Test for alkene: add bromine water. Alkene decolourises (orange → colourless); alkane does not react.
Combustion of hydrocarbons: Complete: CₓHᵧ + excess O₂ → CO₂ + H₂O Incomplete (insufficient O₂): produces CO (toxic) and/or soot C.
CP6 — Test combustion products: cobalt chloride paper turns pink (water present); limewater turns milky (CO₂ present).
Addition polymers
Polymerisation: many small alkene monomers join to form long chain polymers. Ethene (CH₂=CH₂) → poly(ethene): −[−CH₂−CH₂−]ₙ− The double bond opens and links the monomers; no other product (addition polymerisation).
Identifying the monomer: reverse the polymer — replace single bonds between repeat units with double bonds.
Alcohols
Alcohols contain the −OH (hydroxyl) group. General formula: CₙH₂ₙ₊₁OH. Methanol: CH₃OH; Ethanol: C₂H₅OH; Propanol: C₃H₇OH.
Uses of ethanol: alcoholic drinks, fuel (bioethanol), solvent.
Reactions of alcohols:
- Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
- Oxidation to carboxylic acid: ethanol → ethanoic acid (vinegar) — requires an oxidising agent or bacteria.
⚠Common mistakes
- Sodium yellow flame is very persistent — a trace of Na can mask other flame colours; students mistake Na for the sample cation.
- Alkane vs alkene test: the test is for unsaturation (C=C), not combustion.
- Addition vs condensation polymerisation: addition (alkenes, no other product) vs condensation (water or HCl released — seen in nylon/PET).
- Acidifying before halide/sulfate tests: you MUST acidify with HNO₃ (for halides) or HCl (for sulfates) first to remove interfering carbonate ions.
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