Engaging questions:

  • Why are the solutions of iron(III), iron(II), copper(II), cobalt(II) coloured, and the solutions of e.g. sodium and potassium ions not?
  • What determines the colour of the coordination compounds?


CoCl2×6H2O CAS: [7791-13-1]

HCl 12 mol/dm3 CAS: [7647-01-0]

ethanol 95% CAS: [64-17-5]

Equipment: 5 test-tubes, technical scales (giving results in two decimal places), a spatula, measuring cylinder.

Description of the Activity :

On the technical scales, students weigh out 0.1 g CoCl2×6H2O for each of the test-tubes and for each of them they add 10 drops of concentrated hydrochloric acid. Then, they dilute obtained solutions with a solvent of  a composition presented in the Table 1.

Table 1. The composition of a solvent

Number of the test-tube

Solvent [% vol.]


100 %  of deionised water


50% of water - 50% of ethanol


20% of water - 80% of ethanol


5% of water - 95% of ethanol


100% ethanol


  • What are the differences in the solution colours in the test-tubes?
  • Is there any regularity in the change of colour?
  • What ions  are present in the test-tube 1 and 5?
  • What is the structure of the formed coordination compounds?
  • In Figure III.9 the spectra of solutions number 1 and 5 are presented:
    • What determines the position of the peak in the spectrum?
    • What determines the height of the peak in the spectrum?
    • How will the spectra for the test-tubes number 2-4 look like?
    • Are Sc and Zn included to transition metals?
    • Why +II is the typical degree of oxidation for many d-block elements?

Figure III.9. UV-Vis spectrum for the samples 1 to 5. [1]

[1] Interestingly, a local maximum for the solution 1 spectrum at about 520 nm comes from the water of hydration.