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AS level Qualitative Analysis (inorganic chemistry experiments)
Qualitaitive analysis concerns the detection and identification of ions present in substances or
in mixtures. These can solids or solutions. Simple tests with suitable reagents are carried out
in test tubes to identify the ions in the mixture.
A systematic approach to salt analysis may involve the following steps;
(a)
(b)
(c)
(d)
(e)
The initial examination of the substance.
Heating the solid
Adding concentrated sulphuric acid and heating
Making a stock solution of the substance.
Detecting the ions by carrying out prescribed tests.
A. Initial examination or observation of the sample.
The physical appearance of the sample can give an indication of its nature and identity. Its
colour, smell and for solids whether crystalline or powdered can be used to draw tentative
initial conclusions about the sample. Some guidelines are given below.
(a) Solid samples
Colour of sample
Brown
Pink
yellow
Light green
Dark green
Blue
Black
Purple
Orange
White
Probable ions
Fe3+
Mn2+
PbO, ZnO(yellow hot & white cold), CrO42Cu2+, Fe2+
Cr3+
Cu2+
Cu2+(in CuO), Mn4+ (in MnO2)
Cr3+, MnO4Cr2O72-, Pb2+ (in PbO)
compound of s-block element or no transition metal cation
(b) Aqueous solutions.
Colour of solution
Brown- yellow
Orange
yellow
Green
Purple
Pink
Blue
colourless
Inference
I2, Fe3+
Br2, Cr2O72CrO42Cr3+, Fe2+, Cu2+
MnO4-, Cr3+
MnO4-(dilute), Mn2+
Cu2+
No transition metal cations.
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B. Heating the solid
The solid is at first gently heated and then more strongly heated. Observations may be
made as the solid is heated. These include;
(i)
(ii)
(iii)
Whether the solid melts then decomposes or simply decomposes without melting.
Colour changes during heating and final colour of residue when cold.
Evolution of gas or vapour. This should be tested and a summary of test that gives
positive result given.
The table below gives some observations and inferences;
Observation
Solid appears to melt
White sublimate
Charring, solid becomes
black
Colourless gas which turns
limewater milky
SO2 evolved, turns
dichromate green
O2, rekindles glowing splint
Brown gas, NO2
Residue is yellow when hot
and white when cold
Yellow residue
Solid cracks and decrepitates
NO2 and O2
Inference
Hydrated salt (had water of crystallisation).
NH4+ may be present
May be an organic substance, contain CH3COO- or C2O42ions.
CO32- or HCO3- present
SO32- present or HSO32Group 1 metal nitrate or oxide
Heavy metal nitrate
Zn2+
Pb2+
Pb(NO3)2
Pb(NO3)2, Group 2 nitrates.
C. Warming with concentrated sufuric acid.
This is used mainly to test for the presence of halide ions in solid samples.
The sample is moistened with two or three drops of the acid and gently warmed.
Observation
White fumes
White fumes followed by reddish brown
vapour
Very little white fumes, violet vapour and
black solid
Brown fumes which become denser with a
few Cu turnings.
Deduction or inference
Cl- present, gas evolved is HCl
Br- present, HBr and Br2 evolved
INO3-
present, HI, H2S and I2 produced.
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D. Addition of dilute HCl to solid or solution.
1. This usually results in the evolution of gases from samples with the following anions;
Gas produced
CO2
SO2
H2S
NO2
O2
H2
Cl2
Ions present
CO32- or HCO3SO32- or HSO3- or S2O32S2NO2O22Reactive metal (put in acid)
MnO4-
2. A white precipitate can be formed after the addition of dilute HCl if the solution
contains Pb2+ or Ag+ ions. These are precipitated as their respective chlorides which
are white in colour.
3. A yellow ppt is formed from solutions with S2O32- ions. The ppt is sulphur.
4. A colour change from yellow to orange occurs with solutions that have chromate VI
ions, (CrO42- to Cr2O72-).
E. Adding dilute sulphuric acid to solution or solid.
1. Gases can be liberated as given under dilute HCl.
2. Precipitates can be formed with solutions that contain Pb2+, Ba2+, Ca2+, and Ag+.
(ppts will be sulphates). A pale yellow ppt of sulphur can be formed from
solutions with S2O32-.
3. Solutions with CrO42- change colur from yellow to orange due to formation of
dichromate ions, Cr2O72-.
F. Dilute nitric acid has the same effects as dilute HCl except that no precipitates are
formed as all nitrates are soluble.
G. Adding solutions of K2CrO4 or K2Cr2O7 to test solutions.
1. A pale yellow ppt can be formed with solutions that have Ba2+ and Pb2+ ions.
These ppts are BaCrO4 and PbCrO4 respectively. A brick red ppt of Ag2CrO4 is
produced with solutions that have Ag+ ions.
2. A colour change from yellow to orange occurs with solutions that have H+ ions
and the reverse occurs with solutions that have OH- ions.
3. A green colour appears with solutions that have reducing agents. (Cr2O72- reduced
to Cr3+).
H. Adding Ag+(aq) to test solution.
This usually results in precipitates being formed.
1. White ppt. This can be AgCl formed with Cl- ions in test solution (ppt is AgCl
which is soluble in dilute NH3 solution) or it can be Ag2CO3 formed with
carbonate ions in solution. (ppt is soluble in HNO3)
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2. Cream ppt. Ppt is AgBr which is soluble in concentrated NH3 and insoluble in
dilute NH3.
3. Pale yellow ppt. Ppt is AgI which is insoluble in both dilute and conc NH3.
4. Grey- brown ppt. Ppt is Ag2O which shows presence of OH- ions.
5. Black ppt. Ppt is Ag2S or metallic Ag formed with reducing agents.
I. Adding aqueous KI to test solution.
1. The colour of the solution changes to red-brown or yellow if the test solution has
an oxidant and I- ions will be oxidised forming I2.
2. Precipitates can be formed if the solution has Ag+, Pb2+ and Cu2+ ions.
Colour of ppt
inference
Pale yelllow
Ag+ ions (ppt is AgI –insol in NH3 and HNO3)
Bright yellow
Pb2+ (ppt isPbI2 – sol in excess and boiling)
Muddy brown
Cu2+ (grey ppt is CuI settles on bottom of brown
solution of I2.
Black ppt
Oxidising agent. (ppt is free I2)
J. Adding aqueous lead nitrate to test solution.
This usually results in precipitation of insoluble Lead salts.
Colour of precipitate
inference
White
Cl- (ppt PbCl2 which dissolves on boiling and recrystallise
on cooling.
Br- (ppt PbBr2).
SO42- (ppt PbSO4, does not dissolve on boiling).
Yellow
yellow
CrO42- (ppt PbCrO4).
I-, ppt is PbI2 which dissolves on boiling and recrystallise
on cooling.
K. Adding aqueous sodium carbonate to test solution.
1. CO2 is evolved from acidic solutions with H+, Al3+ and Fe3+ ions.
2. The following precipitates can be formed.
Colour of ppt
inference
white
Mg2+, Al3+, Zn2+, Ba2+, Ca2+, Pb2+ (ppt is carbonate)
Buff
Mn2+, ppt is MnCO3 which turns brown in air
green
Fe2+, (ppt is FeCO3 ,turns brown in air).
blue
Cu2+, (ppt is CuCO3, turns black on heating/ boiling).
Red-brown
Fe3+, ppt is Fe(OH)3
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Solubility Rules Chart
Negative Ions
(Anions)
Positive Ions
+
=
(Cations)
Solubility of
Compounds in
water
Example
Alkali Ions
any anion
+
any anion
+
(Li+, Na+, K+, Rb+, Cs+,
Fr+)
=
soluble
=
soluble
hydrogen chloride,
HCl, is soluble
=
soluble
ammonium
chloride, NH4Cl, is
soluble
=
soluble
potassium nitrate,
KNO3, is soluble
=
soluble
sodium acetate,
CH3COONa, is
soluble
=
low solubility
(insoluble)
silver chloride, AgCl,
forms a white
precipitate
=
soluble
potassium bromide,
KBr, is soluble
barium sulfate,
BaSO4, forms a
white precipitate
copper sulfate,
CuSO4, is soluble
hydrogen ion
[H+
(aq)]
ammonium ion
any anion
+
(NH4)
nitrate
NO3-
+
any cation
acetate
(CH3COO-)
any cation
+
(except Ag)
Ag+,
Pb2+,
Hg2+,
Cu+,
Tl+
Chloride (Cl-),
+
Bromide (Br-),
Iodide (I-)
+
any other cation
+
Ca2+, Sr2+, Ba2+, Ag2+,
Pb2+, Ra2+, Hg2+
=
low solubility
(insoluble)
+
any other cation
=
soluble
Sulfate
(SO42-)
Sodium fluoride,
NaF, is soluble
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alkali ions (Li+, Na+, K+,
Rb+, Cs+, Fr+),
+
sulfide
(S2-)
alkali earth metals (Be2+,
Mg2+, Ca2+, Sr2+, Ba2+,
Ra2+),
=
soluble
magnesium sulfide,
MgS, is soluble
=
low solubility
(insoluble)
zinc sulfide, ZnS, is
insoluble
soluble
strontium
hydroxide, Sr(OH)2,
is soluble
low solubility
(insoluble)
silver hydroxide,
AgOH, is insoluble
(forms a precipitate)
=
soluble
ammonium
phosphate,
(NH4)3PO4, is soluble
=
low solubility
(insoluble)
and H+(aq), and NH4+
+
any other cation
alkali ions (Li+, Na+, K+,
Rb+, Cs+, Fr+),
+
Sr2+, Ba2+, Ra2+, Tl+,
=
and H+(aq), and NH4
hydroxide
OH-
Phosphate
(PO43-),
+
any other cation
=
alkali ions (Li+, Na+, K+,
Rb+, Cs+, Fr+),
+
and H+(aq), and NH4
Carbonate
(CO32-),
Sulfite (SO32-)
+
any other cation
alkali ions (Li+, Na+, K+,
Rb+, Cs+, Fr+),
Chromate
CrO42-
+
+
Ca2+, Sr2+, and NH4+,
any other cation
=
soluble
=
low solubility
(insoluble)
magnesium
carbonate, MgCO3,
is insoluble
sodium chromate,
Na2CrO4, is soluble
General Solubility Trends:



All compounds of the ammonium ion (NH4+), and of the Alkali metal (Group IA)
cations, are soluble.
All nitrates and actetates are soluble.
All chlorides, bromides, and iodides are soluble EXCEPT those of silver, lead, and
mercury(I).
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




All sulfates are soluble EXCEPT those of silver, lead, mercury(I), barium, strontium,
and calcium.
All carbonates, sulfites, and phosphates are insoluble EXCEPT those of ammonium
and Alkali metal (Group IA) cations.
All hydroxides are insoluble EXCEPT those of ammonium, barium, and alkali metal
(Group IA) cations.
All sulfides are insoluble EXCEPT those of ammonium, Alkali metal (Group I) cations,
and Alkali earth metal (Group II) cations.
All oxides are insoluble EXCEPT those of calcium, barium, and Alkali metal (Group I)
cations; these soluble ones actually react with the water to form hydroxides.
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Key reactions associated with the anions prescribed in the AS syllabus.
1. Carbonate (CO32-) and hydrogen carbonate (HCO32-).
These evolve carbon dioxide in the presence of acids.
CO32-(aq) + 2H+(aq) → H2O(l) + CO2(g).
HCO3-(aq) + H+(aq) →H2O(l) + CO2(g).
The gas gives a white precipitate, CaCO3, when bubbled through lime water. The ppt
dissolves in excess CO2.
CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l).
In excess,
CaCO3(s) +CO2(g) +H2O(l) → Ca(HCO3)2(aq).
2. Sulfate(VI) ions, SO42- and Sulphite ions,SO32When dilute HCl is added to sulphite ions and the mixture warmed, sulphur dioxide
is given out.
SO32- + 2H+ →H2O + SO2.
Sulfur dioxide is acidic and has a pungent smell. It decolourise a dilute solution of
acidified potassium mangate(VII) and turns an acidified solution of potassium
dichromate (VI) from orange to green/blue.
A white precipitate of barium sulphate or sulphite is produced when a solution of
barium nitrate or chloride is added to a solution with sulphate or sulphite ions.
Barium sulphite dissolves in excess dilute HCl, but barium sulphate does not dissolve.
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