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What chemical reaction has color change?

Chemical reactions that involve a color change are some of the most exciting and engaging demonstrations of chemistry principles. The addition or removal of certain chemicals can cause dramatic shifts in color that captivate students and help explain complex concepts. In this article, we’ll explore some of the most common chemical reactions that feature striking color changes and examine the chemical principles that drive these colorful transformations.

Color changes in chemical reactions occur due to the rearrangement of electrons within molecules during the reaction. When reactant molecules are converted into product molecules, the structure and energy levels of the electrons within the molecules are altered. Electrons absorb certain wavelengths of visible light and impart color. When the arrangement of electrons changes, the wavelengths of light absorbed and reflected change as well, resulting in an observable shift in color.

Some chemical reactions involving color change are simple redox reactions. Redox reactions involve the transfer of electrons between reactant species, resulting in a change in oxidation state. Oxidation states strongly influence color because they affect the energy levels of electrons within a molecule. Other reactions induce color changes due to the breakdown of large pigmented molecules into smaller colorless molecules or vice versa. pH induced color changes are also common. pH influences color because it changes the protonation state of molecules, which alters their electronic properties.

Below we describe some of the most common and visually striking chemical reactions that have color transitions:

Oxidation of Metals

The oxidation of metals is a classic example of a redox reaction that induces color change. Metals tend to be shiny and silvery in their metallic or oxidized state. When oxidized, the electrons within the metal atoms become excited to higher energy levels. This gives the metallic elements their lustrous and reflective appearance. However, when metals oxidize, they lose electrons to oxygen, resulting in a positive metal ion. This changes the wavelength of light the metallic elements reflect, often imparting vibrant and varied colors.

Some vivid examples of metal oxidation color changes include:

  • Copper: Copper metal has a red-orange metallic sheen. When oxidized, it turns green as copper (II) oxide or blue as copper (II) carbonate forms.
  • Iron: Iron metal appears shiny and grey. Oxidized iron turns red-brown due to iron (III) oxide rust formation.
  • Chromium: Chromium in its metallic state has a bright, silvery white hue. Oxidized chromium (III) compounds impart a deep green color.

The extent of oxidation can also affect color. Thin or partially oxidized iron appears yellow or gold, while thick rust deposits look dark red-brown. The thickness of the oxidized layer affects what wavelengths of light are absorbed or reflected, changing the perceived color.

Redox Dye Indicators

Certain organic dyes change color in response to redox reactions and are used as redox indicators. They are often employed to signal when a reaction reaches completion, such as titrations. Common dye indicators include:

  • Methyl orange – Red in oxidized/basic form, yellow in reduced/acidic form
  • Phenolphthalein – Colorless in reduced/acidic form, fuschia in oxidized/basic form
  • 2,6-dichlorophenolindophenol – Blue in oxidized/basic form, colorless in reduced/acidic form

These dyes change from their oxidized to reduced state or vice versa when reacting with the analyte solution during a titration. This induces an obvious color change signaling the endpoint.

Acid-Base Indicators

pH sensitive dyes are also commonly used to indicate when an acid-base neutralization reaction is complete. Common acid-base indicators include:

  • Litmus – Red in acidic conditions, blue in basic
  • Phenol red – Yellow in acidic conditions, red in basic
  • Bromothymol blue – Yellow in acidic conditions, blue in basic

These dyes contain certain functional groups that can accept or donate protons. The protonation state of the dye molecule affects its electronic properties and what wavelengths of light it absorbs/reflects. Shifting between acidic and basic conditions protonates and deprotonates the indicator, resulting in dramatic color changes.

Coordination Complexes

Many transition metal cations form vividly colored coordination complexes in the presence of ligands. Common examples include:

  • Copper complexes – Blue
  • Chromium complexes – Green, orange, red, blue, or violet depending on the ligands
  • Cobalt complexes – Blue, green, red-orange, or pink depending on the ligands
  • Nickel complexes – Green, blue, or red-violet depending on the ligands

The ligands alter the electron density around the transition metal cation, changing the energies of the metal’s d electrons. This allows them to absorb different wavelengths of visible light. Adding certain ligands or adjusting conditions like pH can induce color changes in coordination complexes. For example, cobalt complexes are blue under alkaline conditions but turn pink when acidified.

Anthocyanin Pigments

Anthocyanin plant pigments change color based on pH. They appear red, purple, or blue in acidic environments. But under neutral/basic conditions, anthocyanins convert to colorless or pale yellow structures. Anthocyanins are found in foods like red cabbage, purple grapes, blueberries, and more. The dramatic color shifts can be observed by adding acid or base to extracts from these foods.

The color changes occur because anthocyanin structures are pH sensitive. Under acidic conditions, the molecule is fully protonated and appears red/purple/blue. But at increasing pH, protons are lost, changing the electronics and destroying the color. This gives a visual cue about the acidity of the solution.

Other Reaction Examples

Some other chemical reactions featuring bold color changes include:

  • Iodine clock reaction – Colorless to amber/black as iodine forms
  • Thermal decomposition of sodium bicarbonate – White to black as carbon dioxide evolves
  • Potassium permanganate redox reactions – Purple/pink to colorless upon reduction
  • Iron(III) and thiocyanate – Colorless to blood red upon complex formation

Many inorganic coordination complexes, acid-base indicators, and redox sensitive dyes not described here also exhibit dramatic and educationally useful color changes. Discovering new chemical reactions that have brilliant color transitions provides an excellent opportunity to better understand chemical principles.

Observing Color Changes

To safely observe color-changing chemical reactions:

  • Wear goggles and gloves to protect your eyes and skin.
  • Work slowly and deliberately when combining chemicals.
  • Use small quantities of reactants.
  • Perform reactions over a tray to catch spills.
  • Follow all safety warnings and disposal guidelines.

Reactions should be run in a ventilated space away from flames or sparks. Never consume or taste chemicals. Dispose of compounds properly. With responsible practices, chemical color changes can be observed in illuminating classroom demonstrations or home experiments.


Chemical reactions with color transitions provide visually stimulating ways to explore chemical principles. Oxidation-reduction, acid-base neutralization, and equilibrium shifts can all result in dramatic color changes that signal the progression of the reaction. Understanding the molecular basis for these color changes provides deeper insight into chemistry concepts. With proper safety precautions, color changing chemical reactions can be applied engagingly across many educational settings.

Type of Reaction Example Reaction Color Change Observed
Metal oxidation 2Cu + O2 → 2CuO Copper: silvery grey to black copper(II) oxide
Redox indicator Methyl orange + reductant → Reduced form Methyl orange: red to yellow
Acid-base indicator Litmus + HCl → Acidic form Litmus: blue to red
Coordination complex [Co(H2O)6]2+ + 4Cl- → [CoCl4]2- + 6H2O Cobalt: pink to blue
Anthocyanin pigment Anthocyanin pigment + OH- → Colorless form Red/blue/purple to yellow or colorless

This table summarizes some representative examples of different reaction types featuring striking color changes along with the specific color transitions observed.