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1 of 29 © Boardworks Ltd 2012 Qualitative Analysis
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Qualitative Analysis
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Qualitative or quantitative?
A qualitative analysis obtains non-numerical information
about a substance and its properties.
In contrast, quantitative analysis obtains numerical
measures of a substance and its properties.
quantitativequalitative
blue solution
bubbles of gas
feels warm
2.6ml solution
0.5ml gas
30°Celsius
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Chemical techniques
In contrast, quantitative analysis often makes use of
scientific instruments to provide numerical data on the
substance or its properties.
In general, qualitative analysis
uses chemical techniques to
identify an unknown substance.
How are qualitative and quantitative techniques used to
identify an unknown substance?
The substance is identified
through a characteristic result
in a chemical reaction, such as
a colour or state change.
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Qualitative techniques
Which types of reaction can be used for a qualitative analysis?
flame tests – many metals burn with
a coloured flame. Often this colour is
unique to a particular metal.
precipitation reactions – a precipitate
is the insoluble product of an aqueous
reaction. Precipitates often have
properties that are unique to a
particular substance.
Why are these tests good diagnostic tools
for an unknown substance?
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Experimental practice
Successful diagnostic tests rely upon good experimental
practice. This means:
Constructing a well thought-out plan. Every test
should increase the amount known about the
test substance.
Selecting a test sample that is representative of
the bulk material. Why is this important?
Following standard procedures in the collection,
storage and preparation of samples. Which factors
should be controlled?
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How do flame tests work?
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How to carry out a flame test
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Identifying metal ions
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Sometimes when two solutions are mixed, they react to
form an insoluble solid product.
For example, mixing solutions of lead nitrate and sodium
chloride produces a white precipitate of lead chloride.
What is a precipitation reaction?
The insoluble solid product is called a precipitate.
A precipitate is present if the mixture goes cloudy.
lead
nitrate
Pb(NO3)2 (aq)
sodium
chloride
2NaCl (aq)
lead
chloride
PbCl2 (s)
sodium
nitrate
2NaNO3 (aq)
+
+
+
+
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Uses of precipitation reactions
Most precipitation reactions are fast reactions that occur
between ions. This makes them useful for identifying
specific ions based on the type of precipitate formed.
Precipitation reactions will identify either the positive or the
negative ions in a compound:
positive ions
metals:
Cu2+ Fe2+ Fe3+
Al3+ Mg2+ Ca2+
ammonium (NH4+)
carbonate (CO32–)
halide (Cl– Br– I–)
sulfate (SO42–)
negative ions
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Identifying positive ions: metals
Several metals form hydroxide precipitates that have
characteristic colours.
Copper compounds react with sodium hydroxide to
form a blue precipitate.
Iron(II) compounds react with sodium hydroxide to
form a green precipitate.
Iron(III) compounds react with sodium hydroxide to
form a brown precipitate.
+ +
FeCl3 (aq) 3NaOH (aq) Fe(OH)3 (s) ++ 3NaCl (aq)
iron(III)
chloride
sodium
hydroxide
iron(III)
hydroxide
sodium
chloride
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Metals with coloured precipitates
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Identifying hydroxide precipitates
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Identifying positive ions: metals (2)
Several metals form hydroxide precipitates that are white:
aluminium hydroxide is white
calcium hydroxide is white
magnesium hydroxide is white.
How can they be distinguished?
If an excess of sodium hydroxide
is added to each solution, the
aluminium hydroxide precipitate
will dissolve.
Calcium and magnesium hydroxides will not dissolve in
excess sodium hydroxide.
Al3+ Ca2+ Mg2+
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Identifying positive ions: ammonium
The ammonium ion (NH4+) is a positive non-metal ion.
It is found in substances like ammonium chloride (NH4Cl).
The ammonium ion can be identified
by adding sodium hydroxide solution.
Heating the mixture produces ammonia
gas, which turns red litmus paper blue.
NH3 + H2ONH4+ + OH–
This reaction can also be used as a test
for hydroxide ions: addition of ammonium
chloride will produce ammonia gas in the
presence of hydroxide ions.
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Precipitation – true or false?
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Identifying negative ions: carbonate
Carbonates contain the CO32– ion. They react with dilute acids
to form carbon dioxide.
What is the other product of this reaction?
The ionic equation for this reaction is:
CaCl2 (aq)
calcium
chloride+ +
CaCO3 (s) 2HCl (aq) ++ CO2 (g)
calcium
carbonate
hydrochloric
acid
carbon
dioxide+ water
H2O (l)+
CO32– (aq)+2H+(aq) CO2 (g)+H2O (l)
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The limewater test
Carbon dioxide can be detected using the limewater test.
This is a precipitation reaction.
Limewater is actually a dilute
solution of calcium hydroxide.
The calcium hydroxide reacts with
carbon dioxide to form calcium
carbonate, which is insoluble in water.
This turns the limewater cloudy.
calcium
hydroxide
Ca(OH)2 (aq)
+carbon
dioxide
calcium
carbonate+ water
+ CO2 (g) CaCO3 (s) + H2O (l)
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Identifying negative ions: halide
Halide ions are formed from the Group 7 elements (the
halogens). Halides are detected using silver nitrate solution.
The substance to be tested is first acidified with a small
amount of nitric acid. Silver nitrate solution is then added,
and a precipitate will form if the halide ion is present.
The precipitates formed are silver halides:
sodium
chloride+
silver
nitrate
silver
chloride+
sodium
nitrate
NaCl (aq) + AgNO3 (aq) AgCl (s) + NaNO3 (aq)
Cl– (aq) + Ag+ (aq) AgCl (s)
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Silver halides
The different silver halide precipitates can be distinguished
by their differing colours.
white AgCl
precipitate
cream AgBr
precipitateyellow AgI
precipitate
chloride bromide iodide
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Silver halides and photography
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Identifying negative ions: sulfate
Sulfate ions (SO42–) are identified by adding a few drops of
barium chloride solution. The solution must be acidified
first with a few drops of hydrochloric acid.
A white precipitate of barium sulfate forms.
The ionic equation for this reaction is:
sodium
sulfate+
barium
chloride
barium
sulfate+
sodium
chloride
Na2SO4 (aq) + BaCl2 (aq) BaSO4 (s) + 2NaCl (aq)
SO42– (aq) + Ba2+ (aq) BaSO4 (s)
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Wider uses of precipitation reactions
removal of undesirable
substances from drinking
and waste water
checking for the presence of
specific substances in blood
production of coloured pigments
for paints and dyes
separation of reaction products.
In addition to their use in qualitative
analysis, precipitation reactions
form the basis of numerous other
reactions used in society:
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Glossary
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Multiple-choice quiz
