AQA A-level Chemistry: Test Tube Reactions (Practical 11)

Test tube reactions of Cu2+, Fe2+, Fe3+, and Al3+.

How Can We Identify Metal Ion Solutions?

Through test-tube reactions where a sample of the metal-ion solution is added into three test tubes, alongside either sodium hydroxide, ammonia, and sodium carbonate.

Outline The Steps In Identifying A Metal Ion Solution Through Test Tube Reactions.

1. Add a sample of the unknown metal ion solution to 3 test tubes.
2. Drop by drop, add NaOH to tube 1 and observe any change. Add more to see if excess NaOH results in further change. Record.
3. Repeat step 2 but using ammonia solution in tube 2. Record observations.
4. In tube 3, add sodium carbonate. Record observations.

What Safety Precautions Should Be Taken When Performing Test Tube Reactions?

1. Wear gloves and goggles as some of the chemicals can cause irritation.
2. Conduct it in a fume cupboard as ammonia gives off pungent fumes.

Which Of The Following Will Form A Precipitate When Reacting With Sodium Hydroxide:

Iron (II), Copper (III), Aluminium (lll), Iron (III).

All of them.

How Can Aluminium (III) Be Distinguished When Reacting With Sodium Hydroxide?

When excess NaOH is added, the aluminium hydroxide precipitate dissolves due to being amphoteric.

Which Of The Following Will Form A Precipitate When Reacting With Ammonia:

Iron (II), Copper (III), Aluminium (lll), Iron (III).

All of them.

How Can Copper (II) Be Distinguished When Reacting Ions With Ammonia?

The copper hydroxide precipitate is the only one that dissolves when excess NH3 is added due to ligand substitution.

The solution also turns into a dark blue colour.

Which Of The Following Will Form A Precipitate When Reacting With Sodium Carbonate:

Iron (II), Copper (III), Aluminium (lll), Iron (III).

All of them.

Fe3+, Fe2+, Cu2+ and Al3+ All Form Precipitates When Reacting With Sodium Carbonate.

How Can They Be Told Apart?

Fe3+ and Al3+: form a precipitate, as well as CO2 gas so bubbling/fizzing observed.

Cu2+ and Fe2+: Form only a precipitate.

Why Do Fe2+ Solutions Need To Be Made “Fresh” / Made Only When Needed?

Because Fe2+ oxidises readily with air to form Fe3+.

What Colour Is [Cu(H2O)6]2+ (Simplified: Cu2+) In Aqueous Solution?

Blue.

What Colour Is [Cu(H2O)6]2+ (Simplified: Cu2+) In Aqueous Solution When Some OH- Or NH3 Is Added?

A pale blue precipitate Cu(OH)2(H2O)4 is formed.

What Colour Is [Cu(H2O)6]2+ (Simplified: Cu2+) In Aqueous Solution When Excess NH3 Is Added To Precipitate?

Dark blue due to partial ligand substitution.

Forms [Cu(NH3)4(H2O)2]2+(aq)

What Colour Is [Cu(H2O)6]2+ (Simplified: Cu2+) In Aqueous Solution When Excess OH- Is Added To Precipitate?

Copper (II) hydroxide is insoluble in excess NaOH (No change; remains pale blue).

What Colour Is [Cu(H2O)6]2+ (Simplified: Cu2+) In Aqueous Solution When Na2CO3 Is Added?

A green-blue precipitate CuCO3(aq) is formed.

What Colour Is [Fe(H2O)6]2+ (Simplified: Fe2+) In Aqueous Solution?

Pale green.

What Colour Is [Fe(H2O)6]2+ (Simplified: Fe2+) In Aqueous Solution When Some OH- Or NH3 Is Added?

A dirty green precipitate Fe(OH)2(H2O)4 (s) is formed.

What Colour Is [Fe(H2O)6]2+ (Simplified: Fe2+) In Aqueous Solution When Excess NH3 Is Added To Its Precipitate?

Iron (II) hydroxide is insoluble in excess NH3 (No change).

What Colour Is [Fe(H2O)6]2+ (Simplified: Fe2+) In Aqueous Solution When Excess OH- Is Added To Its Precipitate?

Iron (II) hydroxide is insoluble in excess NaOH (No Change; remains dark/dirty green).

What Colour Is [Fe(H2O)6]2+ (Simplified: Fe2+) In Aqueous Solution When Na2CO3 Is Added?

A green precipitate FeCO3(s) is formed.

What Colour Is [Fe(H2O)6]3+ (Simplified: Fe3+) In Aqueous Solution?

Yellow.

What Colour Is [Fe(H2O)6]3+ (Simplified: Fe3+) In Aqueous Solution When Some OH- Or NH3 Is Added?

A orange precipitate Fe(OH)3(H2O)3 (s) is formed.

What Colour Is [Fe(H2O)6]3+ (Simplified: Fe3+) In Aqueous Solution When Excess NH3 Is Added?

Iron (III) hydroxide is insoluble in excess NH3 (No Change; remains orange).

What Colour Is [Fe(H2O)6]3+ (Simplified: Fe3+) In Aqueous Solution When Excess OH- Is Added?

Iron (III) hydroxide is insoluble in excess NaOH (No change; remains orange).

What Colour Is [Fe(H2O)6]3+ (Simplified: Fe3+) In Aqueous Solution When Na2CO3 Is Added?

A brown precipitate Fe(OH)3(H2O)3 is formed.

What Colour Is [Al(H2O)6]3+ (Simplified: Al3+) In Aqueous Solution?

Colourless.

What Colour Is [Al(H2O)6]3+ (Simplified: Al3+) In Aqueous Solution When OH- Or NH3 Is Added?

A white precipitate Al(OH)3(H2O)3 is formed.

What Colour Is [Al(H2O)6]3+ (Simplified: Al3+) In Aqueous Solution When Excess OH- Is Added To Its Precipitate, Aluminium Hydroxide?

The solution becomes colourless; [Al(OH)4(H2O)2]- is formed.

What Colour Is [Al(H2O)6]3+ (Simplified: Al3+) In Aqueous Solution When Excess NH3 Is Added To Its Precipitate?

No change.

What Colour Is [Al(H2O)6]3+ (Simplified: Al3+) In Aqueous Solution When Na2CO3 Is Added?

A white precipitate of Al(OH)3(H2O)3 is formed.