Why Some Aqueous Solutions Have Color

By Angela Huang

Color of Solutions

In our daily life, we often see a lot of colored aqueous solutions, which could be things such as a cough medicine your doctor prescribed, your tasty drink, or the water around you. Also sometimes, we may notice our silver steel guardrail turning a distinctive flaky red and our shining golden copper door knob turning green after some time. We always wonder what makes these colors change. The simple answer is “transition metal ions''.

Color of solutions

Transition Metals 

Transition metals, also called transition elements, are elements that have partially filled d orbitals. An atom is composed of protons, neutrons and electrons.  Protons and neutrons together are called nuclei. Electrons orbit around a nucleus, following a certain track that is called an orbital. There may be numerous orbitals around a nucleus, which are named as s, p, d and f based on their distance from the nucleus. s orbital is closest to the nucleus and f orbital is farthest from the nucleus. Transition metals refer to the elements, of which the d orbital is partially filled with electrons, or which has the ability to form stable cations with an incompletely filled d orbital according to International Union of Pure and Applied Chemistry (IUPAC). In general, the transition metals appear in the d-block of the modern periodic table (which consists of groups 3-12), as shown in yellow below. When these transition metal elements become ions complexes and compounds, they appear in different unique colors. Some of the examples are shown later on. 

Transition metals

Applications and Significance

The color feature of transition metals is not only utilized to produce various colorful aqueous solutions and paints, but also very helpful when performing a qualitative analysis or assessment to identify the composition of a sample. For example, copper ion (Cu2+) is green, which has been widely used to create green color. If you see your golden copper doorknob become green, you should know that the chemical reaction has turned copper into copper ion, and you should take some proactive measurement to prevent further reaction. 

Transition metal colors in aqueous solutions

Sources

1. Steven S. Zumdahl and Susan A. Zumdahl (2013). Chemistry, 9th edition, Cengage Learning, 1200p. 

2. Theodore E. Brown, H. Eugene LeMay, Bruce E. Bursten, Catherine Murphy, Patrick Woodward, Matthew E. Stoltzfus (2017). Chemistry: The Central Science, 14th edition, Pearson, 1248p. 

3. Morris Hein, Susan Arena, and Cary Willard (2021). Foundations of College Chemistry, Wiley; 16th edition, 608p. 

4. Anne Marie Helmenstine, Ph. D. (2020b, February 12). Why the transition metals form Colored Solutions. ThoughtCo. https://www.thoughtco.com/transition-metal-colors-in-aqueous-solution-608173 

5. BYJU’S. (2022, November 14). Transition elements - general properties and trends with faqs. BYJUS. https://byjus.com/chemistry/transition-elements/ 

6. Helmenstine, A. (2024, May 1). Transition metal ion colors. Science Notes and Projects. https://sciencenotes.org/transition-metal-ion-colors/