# O Level Qualitative Analysis: Test for Ions and Gases

The complete repository for the O Level topic of Qualitative Analysis

## SKIM OVER

#### 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 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

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

#### 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

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

#### 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

## GO FURTHER

#### Useful Websites

1. BBC Bitesize: Test for Ions and Gases