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.)