CNC Illustration by Richard Oo
SUMMARY
Qualitative Analysis
In qualitative analysis, you identify ions or gases, by adding reagents and observing:
1. Colour of solution
2. Colour of precipitate and if it redissolves in excess
3. Effervescence, colour and smell of gas
Observing Precipitation
Mixing solutions to produce an insoluble solid product, called a precipitate (ppt).
Mn+(aq) + nOH–(aq) ⟶ M(OH)n(s)
Insoluble Compounds
Precipitates are ionic compounds that do not dissolve in water, such as:
1. Metal hydroxides, except Group I metals
2. Metal carbonates, except Group I carbonates
3. Some halides: AgX, PbX2
4. Some sulfates: BaSO4, CaSO4, PbSO4
Identify Cations with NaOH
Add a few drops of NaOH, and then in excess to check if ppt redissolves.
Al3+, Pb2+, Zn2+: white ppt > redissolve into colourless sol
NH4+: no ppt > produces NH3(g) on warming
Ca2+: white ppt > insoluble
Cu2+: light blue ppt > insoluble
Fe2+: green ppt > insoluble
Fe3+: red-brown ppt > insoluble
Identify Cations with NH₃
Al3+, Pb2+: white ppt > insoluble
Zn2+: white ppt > redissolve into colourless sol
Ca2+: no ppt
Cu2+: light blue ppt > redissolve into dark blue sol
Fe2+: green ppt > insoluble
Fe3+: red-brown ppt > insoluble
Differentiate Pb²⁺ from Al³⁺
To differentiate between Al3+ and Pb2+, add a few drops of chloride solution.
Al3+: no ppt, as AlCl3 is soluble
Pb2+: white ppt of insoluble PbCl2
Identify Halides with AgNO₃
Add silver nitrate solution, which forms white, cream and yellow ppt with Cl–, Br– and l– respectively.
Ag+(aq) + Cl–(aq) ⟶ AgCl(s), white ppt
Ag+(aq) + Br–(aq) ⟶ AgBr(s), cream ppt
Ag+(aq) + l–(aq) ⟶ Agl(s), yellow ppt
Identify CO₃²⁻ with Acids
Add acid, which causes the effervescence of CO2 that forms white ppt in limewater.
CO32-(aq) + 2H+(aq) ⟶ CO2(g) + H2O(l)
Identify SO₄²⁻ with Ba(NO₃)₂
Add dilute HNO3 and aqueous Ba(NO3)2, which forms white ppt.
Ba2+(aq) + SO42-(aq) ⟶ BaSO4
Identify NO₃⁻ with Redox
Add NaOH and Al foil. On warming, NO3– is reduced to form colourless, pungent NH3 that turns damp red litmus blue.
NO3_ is reduced to NH3
Al is oxidised to Al3+
Identify Gases with Litmus
Colourless, pungent NH3 turns damp red litmus blue. Cl2 with a sharp, choking smell turns damp blue litmus red and bleaches it white.
NH3(g) + H2O(l) ⟶ NH4+(aq) + OH–(aq)
Cl2(g) + H2O(l) ⟶ HCl (aq) + HClO(aq)
Identify Gases with Splint
Colourless, odourless H2 extinguishes a lighted splint with a ‘pop’. Colourless, odourless O2 relights a glowing split.
H2(g) + O2(g) ⟶ H2O(l)
O2(g) + CxH2y(s)⟶ xCO2(g) + yH2O(l)
Identify SO₂ with Redox
Colourless, pungent SO2 reduces purple aqueous acidified KMnO4 to colourless Mn2+.
SO2(g) + 2MnO4–(aq) + 4H+(aq) ⟶
SO42-(aq) + Mn2+(aq) + 2H2O(l)
Identify CO₂ with Limewater
Colourless, odourless CO2 forms white ppt when it is bubbled into limewater.
CO2(g) + Ca(OH)2(g) ⟶ CaCO3(s) + H2O(l)
Other Colour Changing Reactions
Halogen displacement reaction also causes colour changes.
Cl2 + colourless 2Br– ⟶ 2Cl– + brown Br2
Cl2 + colourless 2I– ⟶ 2Cl– + blue-black I2
Br2 + colourless 2I– ⟶ 2Br– + blue-black I2
Resources
Test for ions in qualitative analysis
The definitive guide to qualitative analysis
Solubility of ionic compounds quiz
How well do you know the solubility table?
Virtual lab: Test for ions
How do you identify the unknown ions present in a mystery solution?
Qualitative analysis TYS questions – Combined Chemistry
Go with the flow and solve flowchart questions that are commonly featured in Combined Science QA
Virtual lab: Identifying an unknown gas
What gas forms when we add acid to the carbonate ions in a seashell?