Which physical characteristic of a solution may indicate the presence of a transition element?

The presence of a transition element in a solution is often indicated by its color due to the unique electronic configurations of transition metals. Transition metals have partially filled d-orbitals, which allow them to absorb certain wavelengths of light. This absorption results in the emission of complementary colors, giving rise to the vivid colors often observed in solutions containing these metals. For instance, copper sulfate appears blue, while potassium dichromate shows an orange color.

Colors arise from d-d electronic transitions in the metal ions when they are in solution, which is a distinct characteristic of many transition metals. This is in contrast to other physical characteristics of solutions such as density, which does not directly correlate with the presence of transition elements, or the effects on litmus or phenolphthalein, which are related to the acidity or basicity of the solution rather than the specific presence of transition metals. Thus, the coloration of a solution serves as a clear indicator of the involvement of transition elements.