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Simple chemical tests


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.


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.


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.


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.


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.


pH as an indicator


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


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.


State symbols


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.


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)


Which ions are spectators?


Making fireworks

Metal ions can be identified by the unique colours of their flames.

Many metal ions are used to give colour to fireworks.


How do flame tests work?


How to carry out a flame test


Identifying metal ions


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.


Identifying positive ions: metals


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.


Summary – identifying positive ions


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


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?


SO32–

(aq) + 2H+ (aq) SO2 (g) + H2O (l)

blue litmus paper


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)



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.


SO42–

(aq) + Ba2+ (aq) BaSO4 (s)

sodium sulfate

Na2SO4 (aq) +

+

BaCl2 (aq)

barium chloride

BaSO4 (s)

barium sulfate

+

+

2NaCl (aq)

sodium chloride


Chemical analysis


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)


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


Silver halides and photography


Summary – identifying negative ions


Glossary


Summary of chemical techniques


Anagrams


Multiple-choice quiz

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