Qualitative analysis is used to identify the cations and anions present in an unknown chemical substance. Cations such as sodium, calcium, and ammonium can be identified using sodium hydroxide and ammonia solutions. Anions like chloride, nitrate, and sulfate can be identified through chemical tests involving silver nitrate, sodium hydroxide with aluminum foil, and barium chloride solutions respectively. These tests produce characteristic precipitates or gas emissions to reveal the ions present. Dilute nitric acid is first added to remove any interfering carbonate ions.

1. Qualitative Analysis
Secondary 4 Express
Chemistry

2. What is Qualitative Analysis?
Qualitative analysis is a
process used to determine and
identify the chemical
properties of an unknown
substance.
In your syllabus, the analysis
will be used to identify the
cations and anions present in
an unknown chemical.

3. What are Cations?
A cation is a positively charged ion.
For example, sodium ion (Na+
) and
ammonium ion (NH4
+
) are cations.

4. What are Anions?
An anion is a negatively charged ion.
For example, chloride ion (Cl-
) and
nitrate ion (NO3
-
) are anions.

5. Salts are ionic compounds.
All salts are ionic compounds and they contain both
cations and anions.
For example, sodium chloride is made up of sodium
ion (cation) and chloride ion (anion).
We can identify the cations and anions in the salt by
adding certain reagents (chemicals).

6. Tests for Anions
Anion How to test? Observations if anion is
present
Nitrate
NO3
-
Add aqueous sodium
hydroxide, then add
aluminium foil and warm
the mixture.
A colourless and pungent
gas which turns moist red
litmus blue is produced.
The gas is ammonia (NH3
).
Carbonate
CO3
2-
Add any dilute acid. Effervescence is observed.
The gas given off forms
white precipitate with
limewater.
The gas is carbon dioxide
(CO2
).

7. Tests for Anions
Chloride
Cl-
Add dilute nitric acid,
then add aqueous silver
nitrate.
White precipitate of silver
chloride (AgCl) is formed.
Iodide
I-
Add dilute nitric acid,
then add aqueous lead
(II) nitrate.
Yellow precipitate of lead
(II) iodide (PbI2
) is formed.
Sulfate
SO4
2-
Add dilute nitric acid,
then add aqueous
barium nitrate
or
Add dilute hydrochloric
acid, then add aqueous
barium chloride
White precipitate of barium
sulfate (BaSO4
) is formed.
White precipitate of barium
sulfate (BaSO4
) is formed.

8. Question to ponder…
Why is dilute nitric acid added before
adding aqueous silver nitrate/ lead (II)
nitrate/ barium nitrate?

9. Question to ponder…
Why is dilute nitric acid added before adding aqueous
silver nitrate/ lead (II) nitrate/ barium nitrate?
This is to remove any carbonate ions that may be
present in the solution which will form insoluble
salts (precipitate) with the reagents added.
Remember that most carbonates are insoluble!

10. Tests for Cations
Cations can be identified by adding aqueous
sodium hydroxide and aqueous ammonia.
Most cations (except H+
, Na+
, K+
and NH4
+
) give
precipitates (insoluble salts) with these
alkalis.

11. Tests for Cations
Ca2+
Cation Effect on adding
aqueous sodium
hydroxide, NaOH (aq)
Effect on adding
aqueous ammonia,
NH4
OH (aq)
Calcium
Ca2+
White precipitate of
calcium hydroxide,
Ca(OH)2
is formed.
Precipitate is insoluble in
excess sodium
hydroxide.
No precipitate formed.

12. Tests for Cations
Aluminium
Al3+
White precipitate of
aluminium hydroxide,
Al(OH)3
is formed.
Precipitate is soluble in
excess sodium hydroxide to
give a colourless solution.
White precipitate of
aluminium hydroxide,
Al(OH)3
is formed.
Precipitate is insoluble in
excess aqueous ammonia.
Lead (II)
Pb2+
White precipitate of lead (II)
hydroxide, Pb(OH)2
is
formed.
Precipitate is soluble in
excess sodium hydroxide to
give a colourless solution.
White precipitate of lead (II)
hydroxide, Pb(OH)2
is
formed.
Precipitate is insoluble in
excess aqueous ammonia.

13. Tests for Cations
Al3+
and Pb2+
give the same observations with aqueous
sodium hydroxide and aqueous ammonia.
How to distinguish them?
Add aqueous potassium iodide (KI)!
If Pb2+
is present, a bright yellow precipitate of lead
(II) iodide (PbI2
) will be formed.
Al3+
will not give precipitate with potassium iodide.

14. Tests for Cations
Copper
(II)
Cu2+
Blue precipitate of
copper(II) hydroxide,
Cu(OH)2
is formed.
Precipitate is insoluble in
excess sodium hydroxide.
Blue precipitate of
copper(II) hydroxide,
Cu(OH)2
is formed.
Precipitate is soluble in
excess aqueous ammonia
to give a deep blue
solution.
Iron (II)
Fe2+
Green precipitate of iron(II)
hydroxide, Fe(OH)2
is
formed.
Precipitate is insoluble in
excess sodium hydroxide.
Green precipitate of iron(II)
hydroxide, Fe(OH)2
is
formed.
Precipitate is insoluble in
excess aqueous ammonia.
On standing, the green Fe(OH)2
precipitate oxidises to form red-brown
Fe(OH) precipitate.

15. Tests for Cations
Iron (III)
Fe3+
Red brown precipitate of
iron(III) hydroxide, Fe(OH)3
is formed.
Precipitate is insoluble in
excess sodium hydroxide.
Red brown precipitate of
iron(III) hydroxide, Fe(OH)3
is formed.
Precipitate is insoluble in
excess aqueous ammonia.
Zinc
Zn2+
White precipitate of zinc
hydroxide, Zn(OH)2
is
formed.
Precipitate is soluble in
excess sodium hydroxide
to give a colourless
solution.
White precipitate of zinc
hydroxide, Zn(OH)2
is
formed.
Precipitate is soluble in
excess aqueous ammonia
to give a colourless
solution.

16. Tests for Cations
Ammoni
um
NH4
+
No precipitate is formed.
On heating, a gas which
turns moist red litmus blue
is produced. The gas is
ammonia.
---

17. Tests for Cations
Note that Al3+
and Pb2+
can be distinguished by adding
aqueous potassium iodide (KI).
If Pb2+
is present, a bright yellow precipitate of lead (II)
iodide (PbI2
) will be formed.
H+
(acid) can be identified by adding carbonate.
Carbon dioxide gas will be produced.
K+
and Na+
ions do not form any precipitate as all their
salts are soluble.

18. Flowchart on identifying Cations
(A) Adding aqueous sodium
hydroxide
Ca2+
Al3+
Pb2+
Zn2+

19. Flowchart on identifying Cations
(B) Adding aqueous ammonia
Al3+
Pb2+

20. Tests for Gases

21. Tests for Gases

22. Tests for Water
There are 2 chemical tests to detect the presence of
water or water vapour:
Use anhydrous copper (II) sulfate
Water will change the colour of anhydrous copper (II)
sulfate from white to blue.
Use dry cobalt (II) chloride paper
Water will change the colour of dry cobalt (II) chloride
paper from blue to pink.

23. Important Concepts for QA
Important Reactions from Acids, Bases & Salts
Acid + metal → salt + hydrogen
Acid + metal oxides/ hydroxide (base) → salt + water
Acid + carbonate → salt + water + carbon dioxide
Alkali + ammonium salt → salt + water + ammonia

24. Decomposition by heat (1)
Some metal carbonates are decomposed by
heat to form metal oxides & carbon dioxide.
Eg: copper (II) carbonate is decomposed by heat to
form copper (II) oxide and CO2.
CuCO3 (s) CuO (s) + CO2 (g)

25. Decomposition by heat (2)
Most metal nitrates (except Group I) decompose to
form metal oxides, nitrogen dioxide and oxygen gas.
Eg: decomposition of calcium nitrate
2 Ca(NO3)2 (s) 2 CaO (s) + 4 NO2 (g) + O2 (g)
Nitrogen dioxide, NO2, is a brown gas. It can be seen as
brown fumes given off during the heating of solid metal
nitrates.

26. Decomposition by heat (2)
Group I metal nitrates decompose to form metal
oxides and oxygen only. No brown nitrogen dioxide
gas is given off.

27. Confusion…
Ammonia (NH3) is an alkaline gas that turns
moist red litmus blue.
Aqueous ammonia (NH4OH) is a weak alkali
that is used to test for cations.
Ammonium (NH4
+
) is a cation found in
ammonium salts & aqueous ammonia.

28. White Precipitate in Cation Test
When asked to identify white precipitate when sodium
hydroxide/aqueous ammonia is added, look out for Zn2+
,
Ca2+
, Pb2+
and Al3+
.
Zn2+
is the only cation that forms white precipitate that is
soluble in excess in both sodium hydroxide and aqueous
ammonia.
Ca2+
is the only cation that forms white precipitate that is
insoluble in excess NaOH.

29. Solubility of Salts
All nitrates are soluble.
Most chlorides are soluble except silver chloride and lead
(II) chloride.
Most sulfates are soluble except barium sulfate, lead (II)
sulfate and calcium sulfate.
All sodium/potassium/ammonium salts are soluble.
Most carbonates are insoluble except
sodium/potassium/ammonium carbonate.