CNC Illustration by Richard Oo

Qualitative analysis

Qualitative analysis is a detective work that unveils the unknown substances in a sample through clever chemistry that gives off characteristic colours, light, sound, and smell.



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 hydroxidesexcept Group I metals
2. Metal carbonatesexcept 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