Practical: Colours of Transition Metal Salts

The Colour of Transition Metals

Stained glass windows owe their vibrant reds, blues, and greens to tiny amounts of transition metals mixed into the glass. A transition metal is an element found in the central d-block of the periodic table that can form stable ions with an incomplete d-subshell. While the pure metals themselves are typically silvery-grey solids (with exceptions like copper and gold), their compounds are characteristically brightly coloured. This is a key difference from Group 1 metals, which form white solids and colourless solutions. Note that elements like zinc and scandium are found in the d-block but are officially not considered true transition metals at GCSE, as they do not form coloured compounds.

Demonstrating Colours in Solution

To observe the variety of colours produced by transition metal salts, you can carry out a simple step-by-step practical demonstration.

First, place a small spatula measure (around $0.5\text{ g}$ ) of a solid transition metal salt onto a white tile to examine its solid colour clearly.
Next, transfer the solid into a clean test tube and add about $5\text{ cm}^3$ of distilled water.
Finally, stopper the tube and shake it until the solid completely dissolves to form an aqueous solution.

By holding the test tube against a white background, you can easily observe the distinct colour of the dissolved metal cation.

Characteristic Colours and Oxidation States

Transition metals have variable oxidation state values, meaning they can form ions with different positive charges. The colour of an aqueous solution depends entirely on the specific metal and its oxidation state:

Copper(II) ( $Cu^{2+}$ ) solutions are blue.
Iron(II) ( $Fe^{2+}$ ) solutions are pale green or light green.
Iron(III) ( $Fe^{3+}$ ) solutions are yellow or orange-brown.

Identifying Ions Using Precipitation Reactions

Adding a few drops of sodium hydroxide ( $NaOH$ ) solution to these transition metal solutions causes a precipitate to form. A precipitate is an insoluble solid created when two aqueous solutions react. These metal hydroxide precipitates have distinct colours that help identify the transition metal ion present, and importantly, they do not redissolve if excess $NaOH$ is added.

Adding $NaOH$ to a blue $Cu^{2+}$ solution forms a blue precipitate of copper(II) hydroxide:

Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)

Adding $NaOH$ to a pale green $Fe^{2+}$ solution forms a green precipitate of iron(II) hydroxide. If left to stand, the surface often turns orange-brown as the iron(II) oxidises to iron(III):

Fe^{2+}(aq) + 2OH^-(aq) \rightarrow Fe(OH)_2(s)

Adding $NaOH$ to a yellow/orange-brown $Fe^{3+}$ solution forms a reddish-brown (or brown) precipitate of iron(III) hydroxide:

Fe^{3+}(aq) + 3OH^-(aq) \rightarrow Fe(OH)_3(s)

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Exam Tips

Common Mistake
Students often confuse the colour of the solid metal with the colour of its compounds; remember that transition metals themselves are usually silvery-grey solids, but their ions in solution are brightly coloured.
2
Never describe a solution as 'clear' when you mean 'colourless' — a copper(II) sulfate solution is blue and clear (see-through), while pure water is colourless and clear.
3
In exam questions asking for observations, always specify that iron(III) hydroxide forms a 'red-brown' or 'brown' precipitate, while its aqueous solution is typically described as 'yellow' or 'orange-brown'.
4
If an exam question asks why transition metals are useful in the manufacture of paints or stained glass, simply state 'because they form coloured compounds'.

Key Terms(5)

Transition metal: An element located in the d-block of the periodic table that can form one or more stable ions with an incomplete d-subshell, characteristically forming coloured compounds.
Aqueous solution: A solution in which the solvent is water, indicated by the state symbol (aq).
Cation: A positively charged ion, formed when an atom or molecule loses electrons.
Oxidation state: A number indicated by Roman numerals that shows the degree of oxidation or the charge of an atom/ion (e.g., +2 in an Iron(II) ion).
Precipitate: An insoluble solid that emerges from a liquid mixture when two aqueous solutions react together.

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Key Terms(5)

Transition metal: An element located in the d-block of the periodic table that can form one or more stable ions with an incomplete d-subshell, characteristically forming coloured compounds.
Aqueous solution: A solution in which the solvent is water, indicated by the state symbol (aq).
Cation: A positively charged ion, formed when an atom or molecule loses electrons.
Oxidation state: A number indicated by Roman numerals that shows the degree of oxidation or the charge of an atom/ion (e.g., +2 in an Iron(II) ion).
Precipitate: An insoluble solid that emerges from a liquid mixture when two aqueous solutions react together.

Practical: Colours of Transition Metal Salts

The Colour of Transition Metals

Demonstrating Colours in Solution

To observe the variety of colours produced by transition metal salts, you can carry out a simple step-by-step practical demonstration.

First, place a small spatula measure (around $0.5\text{ g}$ ) of a solid transition metal salt onto a white tile to examine its solid colour clearly.
Next, transfer the solid into a clean test tube and add about $5\text{ cm}^3$ of distilled water.
Finally, stopper the tube and shake it until the solid completely dissolves to form an aqueous solution.

By holding the test tube against a white background, you can easily observe the distinct colour of the dissolved metal cation.

Characteristic Colours and Oxidation States

Copper(II) ( $Cu^{2+}$ ) solutions are blue.
Iron(II) ( $Fe^{2+}$ ) solutions are pale green or light green.
Iron(III) ( $Fe^{3+}$ ) solutions are yellow or orange-brown.

Identifying Ions Using Precipitation Reactions

Adding $NaOH$ to a blue $Cu^{2+}$ solution forms a blue precipitate of copper(II) hydroxide:

Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)

Adding $NaOH$ to a pale green $Fe^{2+}$ solution forms a green precipitate of iron(II) hydroxide. If left to stand, the surface often turns orange-brown as the iron(II) oxidises to iron(III):

Fe^{2+}(aq) + 2OH^-(aq) \rightarrow Fe(OH)_2(s)

Adding $NaOH$ to a yellow/orange-brown $Fe^{3+}$ solution forms a reddish-brown (or brown) precipitate of iron(III) hydroxide:

Fe^{3+}(aq) + 3OH^-(aq) \rightarrow Fe(OH)_3(s)

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Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often confuse the colour of the solid metal with the colour of its compounds; remember that transition metals themselves are usually silvery-grey solids, but their ions in solution are brightly coloured.
2
Never describe a solution as 'clear' when you mean 'colourless' — a copper(II) sulfate solution is blue and clear (see-through), while pure water is colourless and clear.
3
In exam questions asking for observations, always specify that iron(III) hydroxide forms a 'red-brown' or 'brown' precipitate, while its aqueous solution is typically described as 'yellow' or 'orange-brown'.
4
If an exam question asks why transition metals are useful in the manufacture of paints or stained glass, simply state 'because they form coloured compounds'.

Key Terms(5)

Transition metal: An element located in the d-block of the periodic table that can form one or more stable ions with an incomplete d-subshell, characteristically forming coloured compounds.
Aqueous solution: A solution in which the solvent is water, indicated by the state symbol (aq).
Cation: A positively charged ion, formed when an atom or molecule loses electrons.
Oxidation state: A number indicated by Roman numerals that shows the degree of oxidation or the charge of an atom/ion (e.g., +2 in an Iron(II) ion).
Precipitate: An insoluble solid that emerges from a liquid mixture when two aqueous solutions react together.

Previous Subtopic: Uses of MetalsUses of Metals Next NotePractical: Investigating the Rusting of Iron

Back to Suggested Practicals

Put your knowledge into practice — try past paper questions for Chemistry

Key Terms(5)

Transition metal: An element located in the d-block of the periodic table that can form one or more stable ions with an incomplete d-subshell, characteristically forming coloured compounds.
Aqueous solution: A solution in which the solvent is water, indicated by the state symbol (aq).
Cation: A positively charged ion, formed when an atom or molecule loses electrons.
Oxidation state: A number indicated by Roman numerals that shows the degree of oxidation or the charge of an atom/ion (e.g., +2 in an Iron(II) ion).
Precipitate: An insoluble solid that emerges from a liquid mixture when two aqueous solutions react together.