Explain why transition metal ions are often coloured.

Transition metal ions are often coloured due to the presence of partially filled d orbitals. When light hits these ions, electrons can be excited from a lower energy d orbital to a higher energy d orbital, absorbing specific wavelengths of light and reflecting others, which gives rise to their characteristic colours.

This answer effectively explains the reason behind the coloration of transition metal ions, linking it to the electronic structure and the absorption of light. The question assesses the student's understanding of the relationship between electronic transitions and colour in transition metals.