O Level Qualitative Analysis: Test for Ions and Gases

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Identifying unknown ions and gases in qualitative analysis

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

Precipitation is the formation of insoluble solid products when two solutions are mixed

Precipitation in Qualitative Analysis

Mixing solutions to produce an insoluble solid product, called a precipitate (ppt).

Mn+(aq) + nOH(aq) ⟶ M(OH)n(s)



Insoluble compounds do not dissolve in water, and are formed as precipitate

Insoluble Compounds

Some ionic compounds do not dissolve in water like:

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

In qualitative analysis, we add sodium hydroxide to identify cations

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

In qualitative analysis, we add ammonia to identify cations

Identify Cations with NH3

Add a few drops of NH3, and then in excess to check if ppt redissolves.

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

In qualitative analysis, we add chloride solution to distinguish lead(II) ion from aluminium ion

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




In qualitative analysis, we add silver nitrate to identify halide anions

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

We add acid to aqueous carbonate ion or carbonate solid to test its presence

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)


Sulfate is an anion that can be identified by adding barium nitrate

Identify SO42- with Ba(NO3)2

Add dilute HNO3 and aqueous Ba(NO3)2, which forms white ppt.

Ba2+(aq) + SO42-(aq) ⟶ BaSO4


A Bunsen burner, that we use to warm the reaction mixture, to reduce nitrate into ammonia

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+

Litmus paper, to test for alkaline ammonia or chlorine gas

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)

Burning splint, to test for hydrogen gas

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)

Sulfur dioxide gas, that can be identified by bubbling it into purple acidified potassium manganate solution

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)

Bubbling of carbon dioxide into limewater produces white precipitate

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)


A rainbow of colours that we get in a qualitative analysis practical

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

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