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© Boardworks Ltd 20091 of 41
© Boardworks Ltd 20092 of 41
© Boardworks Ltd 20093 of 41
Simple chemical tests
© Boardworks Ltd 20094 of 41
What is chemical analysis?
During a chemical analysis scientists aim to identify an unknown substance. There are two major analytical methods:
Chemical techniques: an unknown substance is reacted with a range of different substances. The substance is identified by characteristic reactions, such as those that produce a colour or state change.
Instrumental techniques: an unknown substance is analysed using scientific instruments, which provide numerical data on the substance or its properties. This allows it to be identified.
Chemical techniques often destroy the test substance in a reaction, so a large sample is needed. This is not the case for instrumental techniques.
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Qualitative or quantitative?
A qualitative analysis obtains non-numerical information about a substance and its properties.
A quantitative analysis obtains numerical measures of a substance and its properties.
Both can be used to identify an unknown substance.
In general, chemical techniques tend to be qualitative, while instrumental techniques tend to be quantitative.
Modern chemists tend to use instrumental techniques to identify compounds, as they are very accurate and preserve the original sample, unlike chemical techniques.
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How do you perform qualitative analysis?
When carrying out any analysis, it is crucial to carefully plan your method, ensuring every test improves your knowledge.
Chemical tests can be split into two groups:
general tests – these help you to identify the nature of the chemical. Is it acid or alkali; covalent or ionic?
specific tests – these identify the molecules that make up a substance.
General tests should be used early in your analysis. They should be informative, whatever the result, helping to narrow down your search.
A specific test should be unique for that substance to prevent confusing results.
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General tests
What tests would you use to classify a substance into the following categories?
Reactivity: If you have a reactivity series, displacement reactions could be used to give a clearer idea of the metal present in an ionic compound.
Ionic or covalent:
Ionic substances conduct electricity when liquid or when in solution, unlike covalent compounds.
Acid or base:
universal indicator
neutralization: acid + base → salt + water
Acids will react with most metals to produce hydrogen gas.
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Acids and alkalis
There are many ways to test a substance’s acidity, universal indicator being the most straight forward and informative.
Identifying the pH of a mystery substance can act as both a general and specific test:
General – knowing the pH gives you a much smaller range of options for the substance’s identity.
Specific – a low pH indicates the presence of H+ ions, while a high pH reveals the presence of OH– ions in solution.
Hydrochloric acid (HCl) in solution, showing free H+ ions.
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pH as an indicator
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Precipitation reactions
An insoluble solid that forms during an aqueous reaction is called a precipitate. A reaction which forms a precipitate is called a precipitation reaction.
The limewater test for carbon dioxide 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:
calcium hydroxide
Ca(OH)2
+ carbon dioxide
calcium carbonate
+ water
+ CO2 CaCO3 + H2O
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Uses of precipitation reactions
Precipitation reactions have a number of other uses:
production of coloured pigments for paints and dyes
removal of toxic chemicals from water
separation of reaction products.
Most precipitation reactions are very fast reactions that occur between ions.
This makes them very useful for identifying specific ions based on the type of precipitate formed.
A lead iodide precipitate.
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State symbols
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Isolating the precipitate
The precipitate from a precipitation reaction can be separated from the reaction mixture by filtration.
Buchner funnel
filter paper
vacuum pump
Buchner flask
A Buchner funnel and flask can be used to accelerate the process.
This apparatus uses a vacuum pump to draw the mixture through the filter.
The filtrate is finally washed and dried.
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Spectator ions
In ionic precipitation reactions there are often ions that are not involved in the reaction. These are known as spectator ions.
The spectator ions are easily identified using the ionic equation.
This equation shows that the silver and the iodine ions have reacted, joining together to make the precipitate.
The sodium (Na+) and nitrate (NO3–) ions are spectator ions.
This means the ionic equation can be simplified to:
NaI (aq) AgNO3 (aq) AgI (s) NaNO3 (aq)+ +
Na+ (aq)
AgI (s)+ I– (aq)+Ag+
(aq)+NO3–
(aq)+ Na+
(aq) + NO3–
(aq)
I– (aq) + Ag+ (aq) AgI (s)
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Which ions are spectators?
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© Boardworks Ltd 200918 of 41
Making fireworks
Metal ions can be identified by the unique colours of their flames.
Many metal ions are used to give colour to fireworks.
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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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Identifying positive ions
However, precipitation reactions can also be used to identify positive ions, as many metals form hydroxide precipitates with characteristic colours.
For example, when sodium hydroxide solution is added to a solution of iron (III) chloride, the reaction produces a brown precipitate of iron (III) hydroxide.
+ +
FeCl3 (aq) 3NaOH (aq) Fe(OH)3 (s) ++ 3NaCl (aq)
iron (III)chloride
sodiumhydroxide
iron (III)hydroxide
sodiumchloride
Many metal ions can be identified by flame tests.
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Identifying positive ions: metals
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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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Summary – identifying positive ions
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© Boardworks Ltd 200928 of 41
Identifying negative ions: carbonate
Carbonates, which contain the CO32– ion, are identified by
the addition of a few drops of dilute hydrochloric acid.
A colourless gas is given off as the carbonate ions react with the hydrogen ions.
How would you check the identity of the gaseous product?
What are the other products of this reaction?
The ionic equation for this reaction is:
CO32–
(aq) + 2H+ (aq) CO2 (g) + H2O (l)
limewater: goes cloudy
with CO2
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Identifying negative ions: sulfite
To identify the sulfite ion (SO32–), add a small amount of
dilute hydrochloric acid.
The acidic gas, sulfur dioxide, is produced when the sulfite ions react with the acid’s hydrogen ions.
How could you show that the gas is acidic?
The ionic equation for this reaction is:
SO32–
(aq) + 2H+ (aq) SO2 (g) + H2O (l)
blue litmus paper
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Identifying negative ions: nitrate
The nitrate ion (NO3–) can be identified by heating the solution
with aluminium powder and sodium hydroxide solution.
The aluminium reduces the nitrate ion to ammonia on heating. This is released as a gas and can be easily identified.
How could you confirm that the gas given off is ammonia?
How can you tell that this is a reduction reaction?
3NO3– (aq) + 8Al (s) + 5OH–
(aq) + 18H2O (l)
3NH3 (g) + 8Al(OH)4– (aq)
The ionic equation for this reaction is:
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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:
SO42–
(aq) + Ba2+ (aq) BaSO4 (s)
sodium sulfate
Na2SO4 (aq) +
+
BaCl2 (aq)
barium chloride
BaSO4 (s)
barium sulfate
+
+
2NaCl (aq)
sodium chloride
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Chemical analysis
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Identifying negative ions: halides
Halide ions are formed from the Group VII 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 before adding the silver nitrate solution.
The precipitates formed are silver halides:
If halides are present, a precipitate will form.
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 precipitate
yellow AgI precipitate
chloride bromide iodide
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Silver halides and photography
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Summary – identifying negative ions
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Glossary
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Summary of chemical techniques
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Anagrams
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Multiple-choice quiz