When sodium thiosulfate reacts with hydrochloric acid, colloidal sulfur particles form. As the particles grow, they scatter shorter wavelengths of visible light and transmit longer wavelengths, producing a sunset-like sequence of colors when projected with an overhead projector.

1. Prepare a cardboard or heavy paper cutout to cover the projector surface, with a circular hole the size of a crystallization dish. Optionally attach an acetate sheet with a scene drawn in permanent marker.
2. Place 375 mL of deionized water in the crystallization dish.
3. Dissolve 2.8 g of sodium thiosulfate in the water.
4. Place the dish on the projector cutout. Darken the room and turn on the projector. The solution should appear colorless, and the projected light appears white.
5. Add 25 mL of 1.0 M hydrochloric acid and briefly stir with a glass rod.
6. Observe the changes: the projected light shifts from white to yellow, orange, red, and finally black, while the solution viewed from the side changes from transparent to white due to scattered light.

Chemical Sunset - David Moyer:

Aloha Chemical Sunset - FlinnScientific:

📄 Chemical sunset - University of Waterloo: https://uwaterloo.ca/chem13-news-magazine/february-2016/activities/sharing-chemistry-community-chemical-sunset

• Draw different images on acetate sheets (e.g., haunted house, beach scene) to enhance the effect.
• Adjust concentration of chemicals, dish size, or temperature to change the reaction rate.

• Wear safety glasses and gloves.
• Handle hydrochloric acid with care; it is corrosive.
• Sodium thiosulfate is an irritant; avoid ingestion and contact.
• Perform in a well-ventilated area; sulfur dioxide gas may be released.
• Avoid high temperatures, as hydrogen sulfide can also be released.

• Why does the color of the transmitted light change during the demonstration? (Because colloidal sulfur particles grow in size and scatter different wavelengths of light.)
• How is this demonstration similar to a real sunset? (Shorter wavelengths like blue are scattered first, leaving longer wavelengths like red to pass through, just as in atmospheric scattering.)
• What factors influence the rate of color change? (Concentration of reactants, path length of light, and particle growth rate.)
• How could you design an inquiry to synchronize the “sunset” with different pieces of music? (By adjusting chemical amounts, solution depth, or reaction conditions to control timing.)