Halloween Clock Reaction

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff

Categories: Chemical Reactions, Colour Changes, Science Shows, Special Occasions

Alternative titles: Old Nassau Reaction

Summary

The Halloween or Old Nassau reaction is a dramatic clock reaction in which a clear solution first turns opaque orange as mercury(II) iodide precipitates, then changes to a deep blue-black as a starch-iodine complex forms.

Procedure

See method in links below.

Links

Halloween clock reaction - Hegelrast:

Halloween Clock Reaction - ChemEx LK:

📄 Halloween Reaction or Old Nassau Reaction - Anne Marie Helmenstine, Ph.D.: https://www.thoughtco.com/halloween-or-old-nassau-reaction-604253

Variations

Dilute the solutions to slow the reaction and make the timing of color changes longer.

Safety Precautions

Mercury(II) chloride is highly toxic and must be handled with extreme care; wear gloves, goggles, and a lab coat, and conduct the experiment in a fume hood.
Dispose of mercury-containing waste according to institutional hazardous waste guidelines—do not pour down the sink.
Sodium metabisulfite is an irritant; avoid inhaling dust or vapors.
Potassium iodate is an oxidizer; store away from reducing agents and flammable materials.
Use small-scale quantities only and keep students at a safe viewing distance.

Questions to Consider

Why is this reaction classified as a “clock reaction”? (Because there is a timed delay before the dramatic color change occurs.)
What chemical species is responsible for the orange color? (Mercury(II) iodide, HgI₂, precipitate.)
What causes the final blue-black color? (Iodine forming a complex with starch.)
How does changing the concentration of reagents affect the timing of the reaction? (More dilute solutions slow it down; more concentrated solutions speed it up.)