Hot and Cold Pink Color Change Chemistry Demonstration

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Hot and Cold Pink Valentine's Day Chemistry Demo
The hot and cold Valentine’s day chemistry demonstration is simple. All you need is water, phenolphthalein indicator, and a few drops of ammonia.

Valentine’s Day Color Change Chem Demo

Watch a solution change from pink to clear and back again with the hot and cold demonstration. (Ty Nigh)
Watch a solution change from pink to clear and back again with the hot and cold demonstration. (Ty Nigh)

The hot and cold reaction is a fun color change chemistry demonstration, just right for Valentine’s Day. Make a pink solution and watch it turn colorless upon heating. When the solution cools it becomes pink again. Here’s what you do:

Prepare the Hot and Cold Valentine Solution

All you need is a phenolphthalein solution and a drop of concentrated ammonia.

Add a few drops of phenolphthalein indicator and a drop of concentrated ammonia to 500 ml of water in a beaker, flask or large test tube. You can, of course, use a bit more of a less-concentrated ammonia solution instead.

Perform the Valentine’s Day Demo

Present the pink solution. Heat the liquid over a hot plate or, in the case of the test tube, in a burner flame. Heating the solution shifts in the equilibrium between the unionized ammonia and the ionized ammonium hydroxide. The change in pH makes the pink-colored indicator turn colorless. If you do not experience a color change, there is too much ammonia in the solution. Add more water and try again.

How the Hot and Cold Color Change Demo Works

At negative pH, the indicator is in its H3In+ form. Between pH 0 and 8.3, phenolphthalein is in its colorless lactone form (H2In). From pH 8.3 to 10.0 phenolphthalein converts to a pink In2− form. However, under strongly basic conditions, it converts to In(OH)3− and slowly fades until it is completely colorless again above pH 12. The pKa values of phenolphthalein are 9.05, 9.50 and 12. So, while the demonstration is simple, the color changes depend on achieving a slightly basic pH for the pink color and then shifting the pH to turn the indicator colorless.

Ammonia (NH3) works perfectly for this, as it readily ionizes in water, accepting a hydrogen from a water molecule and forming an ammonium ion (NH4+) and a hydroxide ion (OH–) in solution. The reaction is reversible.

NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH– (aq)

It is a weak base reaction, as only a small fraction of ammonia molecules ionize. Shifting the temperature shifts the equilibrium between the ammonia, ammonium ion, and hydroxide ion. It decreases ammonia ionization, meaning there are more ammonia molecules than ammonium ions.

More Valentine’s Day Science Experiments

The hot and cold demonstration is one of several Valentine’s Day science experiments to try. For more holiday fun, try the vanishing valentine demonstration or make a borax crystal heart.

References

  • Kunimoto, Ko-Ki (2001). “Molecular structure and vibrational spectra of phenolphthalein and its dianion”. Spectrochimica Acta Part A. 57 (2): 265–271. doi:10.1016/S1386-1425(00)00371-1
  • Wittke, Georg (1983). “Reactions of phenolphthalein at various pH values”. Journal of Chemical Education. 60 (3): 239. doi:10.1021/ed060p239
  • Zumdahl, Steven S. (2009). Chemical Principles (6th ed.). New York: Houghton Mifflin Company.