Our students in grades 4-7 at the Las Cruces Academy, our non-profit private school, enjoyed tracking the decolorization of phenolphtalein by hydroxide ion. Phenolphthalein is that beautiful rose-pink indicator dye used in acid-base titrations. It changes structure a bit in base but then slowly undergoes a permanent decolorization. The kinetics (rate and order of reaction) are interesting and fun to follow. A write-up from the 2018-19 school year is here.