Tests
and analysis
Chemical tests are used to confirm that some
anions (negative ions) or cations (positive ions) are present in an unknown,
also to identify unknown gases or liquids.
If a bottle or a container in the lab lost its
label you must have a way to know what the chemical in this bottle is.
If you made a chemical reaction that produces
fumes or gases or something you must have a way to identify this produced gas
or fumes.
Flame
test
An
old way of identifying the presence of cations (positive ions) in an ionic
compound is the flame test.
Description:
Dip
a platinum (or nichrome) wire in the salt, then put it in the blue part of the
flame, the flame color will change.
Result:
Color
of the flame will change, according to the metal ion (positive ions) in the
compound.
Tests
for cations
A newer way of
identifying cations than flame test and more accurate is using aqueous sodium
hydroxide or aqueous ammonia.
Note: you can
perform with sodium hydroxide and then confirm with aqueous ammonia.
This test depends
on the precipitation of the solid hydroxide of the cation
e.g.
- precipitation of calcium hydroxide of
the calcium ion (Ca2+)
- precipitation of copper hydroxide of
the copper ion (Cu2+)
- precipitation of aluminium hydroxide
of the aluminium ion (Al3+)
Description:
Add
a few drops of sodium hydroxide to the unknown salt solution, then after the
precipitate is formed add excess of the sodium hydroxide.
Result:
A
precipitate is formed on adding drops, this precipitate may be soluble or
insoluble in excess sodium hydroxide according to the cation.
Those
precipitates formed are the hydroxides of the cations,
·
If the formed hydroxide is basic so it will be insoluble in excess sodium
hydroxide,
·
If the formed hydroxide is amphoteric so it will be soluble in excess sodium
hydroxide.
The
above reaction can be repeated with aqueous ammonia (known as ammonium
hydroxide), which will give other observations.
these
can be summarized in the following table:
|
Name
(ion) |
Action
of Sodium hydroxide (NaOH) |
Action
of excess Sodium hydroxide (NaOH) |
Action
of aqueous ammonia. |
Action
of excess aqueous ammonia |
|
Aluminium ion (Al3+) |
White ppt |
soluble in excess |
White ppt |
insoluble in excess |
|
Calcium ion (Ca2+) |
White ppt |
insoluble in excess |
No ppt or very
slight white ppt |
|
|
Copper (II) ion
Cu2+ |
Light blue
/turquoise ppt |
insoluble in excess |
dark Blue ppt |
soluble in excess and gives dark blue solution |
|
Iron (II) ions
Fe2+ |
Dark green ppt |
insoluble in excess. |
green ppt |
insoluble in excess |
|
Iron (III) ions
Fe3+ |
Red-Brown ppt |
insoluble in excess. |
Red-Brown ppt |
insoluble in excess. |
|
Zinc ions (Zn2+) |
White ppt |
soluble in excess. |
White ppt |
soluble in excess |
It
is clear that these tests only test for 6 cations, so what about other cations
like ammonium ion (NH4+), silver ions (Ag+) or
hydrogen ions in acids (H+)…
These
cations will have a special test for each not by sodium hydroxide or ammonia.
Test
for ammonium ions (NH4+):
Description:
Add sodium hydroxide solution and warm carefully.
Result:
A gas of pungent smell evolves (ammonia) turns red
litmus paper to blue.
Test
for silver ions (Ag+):
Description:
Add sodium chloride solution or any metal
chloride.
Result:
White precipitate of silver chloride will form and
it darkens with light.
Test
for hydrogen ions (H+) in acids:
Description:
Add magnesium carbonate or any solid carbonate
Result:
Effervescence
occurs and colorless gas evolves (carbon dioxide) which turns lime water milky.
Tests
for anions
Test
for chloride (Cl-), bromide (Br-) and iodide (I-)ions:
Description:
Add aqueous silver nitrate acidified by drops of
nitric acid.
Result:
·
with chloride : white precipitate of silver
chloride.
·
with bromide : cream precipitate of silver
chloride.
·
with iodide : yellow precipitate of silver
chloride.
As silver nitrate is expensive it can be replaced
by lead nitrate giving the same results.
·
with chloride : white precipitate of lead
chloride.
·
with bromide : cream precipitate of lead chloride.
·
with iodide : yellow precipitate of lead chloride.
Test
for sulphate (SO42-) ions:
Description:
Add nitric acid then Barium nitrate solution or
barium chloride.
Results:
Heavy white precipitate of Barium sulphate is
formed.
Test
for nitrate (NO3 -) ions:
Description:
Add sodium hydroxide solution and aluminium foil
then warm carefully.
Results:
A gas of pungent smell (ammonia gas) that turns
red litmus paper to blue
Test
for carbonate (CO32-) ions:
Description:
Add to any dilute acid (hydrochloric acid).
Result:
Effervescence occurs and colorless gas evolves (CO2)
that turns lime water milky
Summary
for tests for anions
|
Anion |
Test description |
Observation
we see |
|
Chloride
|
add dilute nitric acid and silver nitrate solution Or add dilute nitric acid and lead (II) nitrate solution |
white precipitate of
silver chloride |
|
Bromide |
cream precipitate of
silver or lead bromide, soluble in aqueous ammonia. |
|
|
Iodide |
yellow precipitate of
silver or lead iodide insoluble in aqueous ammonia. |
|
|
Sulphate
|
1.
add dilute hydrochloric acid and a few
drops of barium chloride or barium nitrate solution |
white precipitate of
barium sulphate |
|
nitrate |
2.
add aqueous sodium hydroxide
then aluminium foil. 3.
warm carefully. |
Ammonia
is produced (pungent gas that turns red litmus paper to blue) |
|
Carbonate |
1.
add dilute hydrochloric acid
to the suspected carbonate 2.
colorless gas evolved, test with limewater |
1.
An effervescence and colorless gas evolves. 2.
The gas turns lime water milky. |
Tests
for covalent gases and liquids
Test
for ammonia gas (NH3):
Test 1: It has a Strong pungent odour.
Test 2: turns red litmus paper to blue.
Test 3: fumes concentrated hydrochloric acid will
give white cloud of ammonium chloride.
Test
for carbon dioxide gas (CO2):
Description:
Bubble through lime water (calcium hydroxide
solution):
Result:
Turns milky. (this is due to the formation of
calcium carbonate).
Tests
for chlorine gas (Cl2):
Description:
Test with damp blue litmus paper
Result:
Turns red then bleaches
Tests
for hydrogen gas(H2):
Description:
Lit splint
Result:
Pop sound with blue flame.
Tests
for oxygen gas(O2):
Description:
Glowing splint
Result:
Relights.
Tests
for sulphur dioxide gas (SO2):
Description:
Expose to a paper wet with potassium dichromate.
Result:
Turns from orange to green (this is because
sulphur dioxide is a reducing agent).
Test
for water(H2O):
Description:
Drip on white anhydrous copper sulphate powder;
Result:
Turns blue (hydrated copper sulphate).
Or
Description:
Drip on blue anhydrous cobalt chloride powder;
Result:
Turns deep rose (hydrated cobalt chloride).
Summary
for tests for covalent gases and liquids
|
Gas |
Test description |
Observation we see |
|
Ammonia |
1.
Odour appears (take care). |
1.
Strong pungent Odour. |
|
2.
Red litmus paper. |
2.
Litmus paper turns blue. |
|
|
3.
fumes conc. hydrochloric acid |
3.
White clouds with HCl fumes. |
|
|
Carbon dioxide |
bubble into limewater (aqueous calcium hydroxide Ca(OH)2
solution) |
turns cloudy - fine milky white precipitate of calcium carbonate |
|
Chlorine gas |
|
pungent
green gas, |
|
blue litmus |
(i) litmus turns red
and then is bleached white, |
|
|
Hydrogen |
lit splint |
Pop sound (might see condensation on test tube) |
|
Oxygen |
glowing splint |
re-ignites
it - flame |
|
|
freshly made potassium dichromate(VI) paper |
paper
changes from orange to green |
Test for water : by adding white powder of
anhydrous copper sulphate turns blue hydrated copper sulphate.
Or
blue powder of anhydrous cobalt chloride turns deep rose (pink) hydrated cobalt
chloride.
Colors
Compounds of alkali
metals are white or colorless.
Compounds of
transition metals are colored:
|
CuO black |
Cu(OH)2
blue ppt |
CuSO4
white solid |
CuSO4.5H2O
blue crystals |
|
CuCO3
green solid |
CuSO4
blue solution |
Cu(NO3)2
blue solution |
|
|
Fe2O3
brown solid |
Fe(OH)3 brown ppt |
Fe(OH)2
green ppt |
|
All gases are colorless
or of very pale color except:
Chlorine green gas,
fluorine yellow gas, NO2 brown fumes, bromine red brown vapour,
iodine purple vapour.