H₂In + 2H₂O 2H₂O⁺ + In^2-

where H₂In is phenophthalein. The addition of 3–4 drops of phenolphthalein indicator solution followed immediately by 3–4 drops of 50% (w/w) NaOH to the vortex of the HCl solution results in a shift to the right in the equilibrium owing to the reaction of OH^- + H₃O⁺ to form water. This shift is accompanied by the vortex becoming visible by the appearance of a pinkish-red color caused by an increase in In^2- concentration within the localized region of the vortex. The demonstration also provides one an excellent opportunity to discuss the topics of limiting reagent and reagent in excess. Some insight regarding the extent to which uniform mixing is achieved when using a magnetic stirrer is also provided.

Included is a note from the Feature Editor, Ed Vitz.