Many compounds have a colour. Different ions in aqueous solutions have different colour. By observing the colour of the aqueous solution, the ion species in the solution could be predicted preliminarily.
The below are the colours of some ions that you need to memorise:
- Dichromate ion (Cr2O72-)
- Chromate ion (CrO42-)
- Iron(III) ion (Fe3+)
- Iron(II) ion (Fe2+)
- Nickel(II) ion (Ni2+)
- Chromium(III) ion (Cr3+)
- Copper(II) ion (Cu2+)
Very Pale Pink (It can hardly be observed and seems colourless):
- Manganese ion (Mn2+)
- Permanganate ion (MnO4–)
Note that for ions with different charges (different oxidation states), the colour of the ion is different. For example, iron(II) is pale green while iron(III) is yellow. Copper(I) is colourless and Copper(II) is blue.
All main group metal ions are colourless in aqueous solutions. Many of the transition metal ions with different oxygen states have colour. The below is a fascinating video about Vanadium, a period 4 transition metal. Note the colour change as the Vanadium ion is being reduced from +5 to +2.