Practical Chemistry
Practical Chemistry covers qualitative salt analysis, functional-group detection and titration technique - the marks most often lost through careless recording rather than lack of knowledge.
Core rules
- Cation groups: Group I (Pb²⁺, Ag⁺, Hg₂²⁺) with dilute HCl; Group II (Pb²⁺, Cu²⁺, Bi³⁺, Hg²⁺) with H₂S in dilute HCl; Group III (Al³⁺, Fe³⁺, Cr³⁺) with NH₄OH in the presence of NH₄Cl; Group IV (Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺) with H₂S in ammoniacal medium; Group V (Ba²⁺, Sr²⁺, Ca²⁺) with (NH₄)₂CO₃.
- Anions: CO₃²⁻ gives effervescence with acid and turns lime water milky; SO₃²⁻ gives SO₂, which decolourises KMnO₄; SO₄²⁻ gives a white precipitate with BaCl₂ that is insoluble in concentrated HCl; Cl⁻ gives a white precipitate with AgNO₃ that dissolves in NH₄OH; NO₃⁻ gives the brown ring test.
- Flame tests: Na golden yellow, K lilac, Ca brick red, Sr crimson, Ba apple green, Cu blue-green.
- Functional groups: phenol with neutral FeCl₃ (violet), carboxylic acid with NaHCO₃ (CO₂), aldehyde with Tollens' or Fehling's, ketone with 2,4-DNP (orange), 1° amine with the carbylamine test.
- Titration: rinse the burette with the solution it will hold, read the lower meniscus for colourless liquids, and accept readings agreeing within 0.1 mL.
Worked example
A white salt gives brisk effervescence with dilute HCl and the gas turns lime water milky, so the anion is carbonate. The flame test is golden yellow, identifying Na⁺. Hence the salt is Na₂CO₃. To confirm the carbonate, pass excess CO₂ through the milky suspension: it clears as soluble bicarbonate forms, distinguishing carbonate from sulphite.
Traps that cost marks
- Using concentrated instead of dilute HCl for Group I, precipitating groups together.
- Forgetting to boil off H₂S before adding NH₄OH for Group III.
- Reporting a precipitate without a confirmatory test.
- Reading the upper meniscus in a colourless titration.
- Recording a precipitate as a "white solution".