A Liquid That Changes Color with Temperature

The World of Color

Colors surround us on all sides, making the world richer and more engaging. We are one of the few animal species capable of perceiving the full span of visible light, and nature deserves our gratitude for that gift. Color is both a medium and an inspiration across many art forms—not just painting. Literature, too, has long drawn from this vibrant palette; with hues and shades we can convey an astonishing range of meaning. One contemporary novel puts it this way:

Snow was falling. Like a dream, it blurred shapes and colors, hiding everything familiar, burying everything that was real.

Kyle A., The God of Animals

To blur colors? That poetic phrase certainly carries weight—but what does it have to do with chemistry?

After all, chemistry is an extraordinarily colorful science! Virtually every color we perceive is connected in some way to this discipline.

For those who find static colors dull, today we will prepare a liquid whose hue shifts as the temperature rises and falls. The demonstration doubles as an engaging introduction to chemical thermodynamics.

Materials Needed

Fortunately, the list of reagents is short. You will need:

cobalt(II) chloride hexahydrate, CoCl₂·6H₂O,
sodium chloride, NaCl.

Warning: Cobalt(II) chloride hexahydrate (CoCl₂·6H₂O), like most heavy-metal salts, is toxic and likely carcinogenic. Avoid any skin contact. The author accepts no responsibility for any harm that may result; you proceed at your own risk.

You will need to purchase cobalt(II) chloride hexahydrate from a chemical supplier, but ordinary table salt is perfectly adequate for NaCl.

Fully hydrated cobalt(II) chloride crystallizes as an intense red solid:

Kliknij, aby powiększyć

Prepare a solution containing a few percent by mass of cobalt(II) chloride hexahydrate in a sodium-chloride solution saturated at room temperature (≈ 25 °C [77 °F]). The simplest method is to first make a saturated salt solution, add a few crystals of CoCl₂·6H₂O, and—while stirring—gradually add just enough water for the cobalt salt to dissolve. The resulting solution is pink.

Demonstration

Pour a small amount of the prepared solution into a test tube. As noted, the solution is pink—the photo below confirms this.

Kliknij, aby powiększyć

Heat the test tube by any convenient method. Even a slight temperature increase causes a visible shift in shade. Continued heating intensifies the change, and just below the boiling point the liquid becomes deep blue:

Kliknij, aby powiększyć

The color change is completely reversible: once the sample cools, its original hue returns.

You can turn this phenomenon into a clever thermometer by sealing a small quantity of the liquid in a glass ampoule. I made one by pulling and shaping a small test tube in a gas burner flame. The photos below show my chemical thermometer at different temperatures.

24 °C (75 °F)

60 °C (140 °F)

Remember that the ampoule contains a toxic heavy-metal salt, but a properly sealed vessel ensures the device is safe to handle.

Explanation

The observed effect is governed by Le Chatelier’s principle. In aqueous solution, cobalt(II) chloride exists in a dynamic equilibrium between two cobalt complexes, illustrated by the following equation:

The chloro complex is intensely blue, whereas the aquo complex is pink. In pure water the equilibrium lies far to the right, so heating alone causes little visible change. Adding a large excess of chloride ions (from NaCl) shifts the equilibrium slightly to the left, but not enough at room temperature for the blue complex to dominate, so no color change is observed. When the solution is heated, Le Chatelier’s principle drives the equilibrium decisively toward the chloro complex, turning the solution blue. As the temperature falls, the equilibrium shifts back, and the pink aquo complex once again predominates.

Weird Science