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.

quantitative qualitative

blue solution

bubbles of gas

feels warm

2.6ml solution

0.5ml gas

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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?

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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.

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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?

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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–)

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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.

+



+

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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.

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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

+

H₂O NH₄+

+

OH–

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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) CO

2(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)

+

_dioxidecarbon _carbonatecalcium

+

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 precipitate

yellow AgI precipitate

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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) BaSO

4(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

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