Question

why transition metal ions are coloured in water

Answer 1

Transition metal ions form hydrated complex with water molecules and thus exist as hydrated complex ions in water. During the formation of complex ions the d orbitals of the transition metals ions split into higher and lower energy levels. The electrons in the partially filled d orbitals of the transition ion absorb light energy and are promoted to higher energy levels and reflect light complementary to the colour of the light absorbed due to which the aqueous solution appears coloured.