Practical: Colours of Transition Metal Salts

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:

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):

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