If you enjoy growing crystals, up your game with this easy project that produces large crystals that change color from yellow to green to blue, depending on light and temperature. The crystals take from a few hours to overnight to grow.
Color Change Crystals Materials
You need two chemicals and water to grow these crystals:
- 10 grams potassium alum (potassium aluminum sulfate)
- 3 grams red prussiate [potassium hexacyanoferrate(III)]
- 50 milliliters hot water
Alum is readily available at grocery stores and pharmacies, but you’ll probably have to go online to find red prussiate. If you can’t find it, the color change crystal kit from Thames and Kosmos has everything you need, plus three fun experiments to try with the crystals.
Make the Solution and Grow Crystals
- Dissolve the potassium alum and red prussiate in the hot water in a small container. If the salts don’t completely dissolve within a few minutes, set the small container inside a larger container of very hot water (a homemade hot water bath) to add heat.
- Once the chemicals dissolve, set the small container of crystal-growing solution in a location where it won’t be disturbed. Tiny crystals will appear within 30 minutes to a couple of hours and crystal growth should be complete overnight to a couple of days. The rate of crystal growth depends on the humidity and temperature. Dry air aids water evaporation, which concentrates the liquid and encourages crystal growth. Because salt solubility is affected by temperature, cooler temperatures also promote crystal growth.
- When you are satisfied with the crystals, use a spoon to remove them from the liquid. You can pour leftover crystal growing solution down the drain. Set the crystals on a dish to dry. Initially, the crystals will be yellow or green, depending on the temperature.
- The easiest way to explore the color change is to divide the crystals into different containers and observe the color under different conditions. For example, place some crystals in a dark closet and others in a sunny window. See what happens when you place the crystals in the refrigerator or freezer.
What to Expect
When you change the light or temperature conditions, it usually takes an hour or two to see the crystals change color. However, it can take as long as a day. In the dark, crystals may be canary yellow or greenish yellow (depending on the temperature). Sunlight changes the color to bright green and finally blue. Cold promotes the yellow color (especially in the dark), while heat promotes the blue color (especially in bright light).
How Color Change Crystals Work
The crystals start out yellow, but light or heat provide energy that drives a chemical reaction between alum and red prussiate that forms Prussian blue or Berlin blue [iron(III) ferrocyanide: C18Fe7N18]. Prussian blue is used in paint and ink cartridges. As the reaction occurs, the mixture of the original chemicals and the blue pigment makes the crystals appear green. Continued exposure to light or heat converts all of the reactants to the blue pigment, turning the crystals blue. Over time, the color of the crystals may continue to change.
Despite the “cyanide” in the chemical name, the compounds in this project are non-toxic. In fact, Prussian blue is used as an antidote for certain types of poisoning and is a common laundry bluing agent. It is used in other safe science projects, including the blue ink recipe and crystal garden.
However, the red prussiate and Prussian blue contain iron, which is toxic in high doses. You should wash your hands after handing the chemicals or crystals to prevent accidental ingestion. Keep the crystals away from pets and young children so they don’t lick them. Keep the crystal chemicals away from food and rinse and cookware used in this project before using it again.
The project uses hot water, so take care not to get burned.
- Crystallography Open Database (1934). “Crystal structure of the alums.”
- Dunbar, K. R.; Heintz, R. A. (1997). “Chemistry of Transition Metal Cyanide Compounds: Modern Perspectives.” Progress in Inorganic Chemistry. 45. pp. 283–391. doi:10.1002/9780470166468.ch4. ISBN 9780470166468.
- Völz, Hans G. et al. (2006) “Pigments, Inorganic” in Ullmann’s Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. doi:10.1002/14356007.a20_243.pub2