Here's a fun and easy Valentine's Day chemistry demonstration. Take a pink liquid, heat it up and watch it turn colorless. Upon cooling, the color of the liquid will return to pink. Here's what you do:
Prepare the Hot and Cold Valentine Solution
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.
Perform the Valentine's Day Demo
Present the pink solution. You can heat the liquid over a hot plate or, in the case of the test tube, in a burner flame. Heating the solution causes a shift 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, so dilute it further with water and try again.