Chemical reactions come in a variety of colors: green, yellow, red, and brown. They are all shades of the same fundamental colors which arise from their respective energy levels. These colors have been used for millennia in math, art, and craft-making to represent different qualities or categories. In fact, there is an ancient Greek myth of the Fisher King whose land was immersed in a simmering sea. If nai is added, will agi(ksp=8.3×10−17) or pbi2(ksp=7.9×10−9) precipitate first? Even history used color to represent different qualities: the Roman Empire was green. The British flag is red. Yellow has been associated with wealth, danger and fertility. Red has been connected to blood and suffering. Brown has been connected to earth and age.
Green is associated with nature, life and fertility. Blue is associated with healing and tranquility. All of these shades have been connected with certain qualities, but they have also been used to represent different types of relationships. When two atoms bond, they do so with a certain strength. When one atom forms two bonds to the same atom, that bond is particularly strong. This phenomenon is known as hypervalency and can be easily represented by green or yellow shades. The force that binds the atoms together in a molecule or crystal might be illustrated by a spring connecting them (as in this picture from C&EN).
There are three manifestations of color in chemistry which include color-change reactions, absorption and emission spectra, and by using color to calculate energies.
1. Color-Change Reactions:
Color-change reactions are categorized as redox reactions. Redox reactions are chemical reactions in which electrons are transferred between different species, resulting in a change of oxidation state of atoms. In other words, in a redox reaction, electrons move from one atom to another.
Here is an example of a color-change reaction: When chromium metal is added to concentrated sulfuric acid, the solution will turn from clear to green/blue and then green. Scientists call this process “oxidation.” When you add a reducing agent to the same solution, the color will shift from green/blue back to clear. Scientists call this process “reduction.”
2. Absorption and Emission Spectra:
The electromagnetic spectrum is a continuum of electromagnetic radiation frequencies. Visible light is but one small part of it, making up only 0.4% of all electromagnetic radiation frequencies. Light is a form of electromagnetic radiation which ranges from gamma rays and x-rays at one end, to radio waves at the other end. However, the majority of all electromagnetic radiation is in the infrared and ultraviolet range. By detecting only certain wavelengths, we can see certain spectrums of light. These same principles apply to different parts of the spectrum.
For example, the visible spectrum of light includes wavelengths from approximately 380 to 780 nanometers. These wavelengths correspond to colors like red and orange, but different wavelengths can be attributed to different other colors. For example, when you look at a rainbow, you are actually seeing all of the light in the entire visible spectrum.
The same principle applies for X-rays and radio waves. The electrons in an atom can absorb a given wavelength (quantum number) of x-rays and scatter or reject it depending on the energy of the x-ray wave. By measuring this energy level, scientists can determine what element is present in a given substance.
3. Using Color to Calculate Energy:
In quantum mechanics, electrons are described by waves which are in a constant state of motion. When an electron absorbs energy (e.g. x-rays) it increases its energy level and if it loses energy it decreases its energy level. The amount of energy absorbed or released depends on the difference between the initial and final states of the electron.
The colors that you see around you are ultimately caused by the transitions in atomic levels of electrons within molecules, atoms, and solids. In other words, colors are manifestations of changes in atoms’ total energies as they ground to higher or lower levels. Color is therefore the response to changes in the energy of a crystal and a molecule.
Color changes are linked to specific wavelengths and at certain energies, electrons will be more likely to change from one level to another (colored) state. Many people are confused about how color works because many different substances can be represented as having one spectral data, but when they look under the microscope or use a spectrometer, they realize that there is actually very little that is “the same”.
4. What is the Color of Your Molecule?
There are many theories about color. The most widely accepted theory about the nature of color was proposed by physicists in the early 1900’s. This theory was described by Max Planck in a paper titled “The Constitution of Radiation and the Quantum Hypothesis.” In this paper, Planck describes a microscopic structure that has been dubbed “quanta” which he believes is responsible for all “emissions and absorptions” we observe in chemistry and physics. Later, Albert Einstein took this quantum model to explain why light is emitted or absorbed in packets or quanta (discrete units).