Illustrate dramatic colour changes as a result of redox and precipitation reactions in this vivid demonstration

In this experiment, based on the ‘Old Nassau’ or ‘Halloween’ clock reaction, students observe as a mixture containing iodate(V) ions, hydrogensulfate(IV) (hydrogensulfite) ions, mercury(II) ions and starch suddenly turns orange after a few seconds, forming a precipitate of mercury(II) iodide. A few seconds later the mixture suddenly turns blue-black with the formation of the starch-iodine complex.

The reactions taking place are not too complex, but involve redox and precipitation, and could be introduced to post-16 students studying kinetics.

This experiment has a good visual impact and would be one way of stimulating interest in chemistry, perhaps via an open day. The demonstration lasts about 3–5 minutes but about 20 minutes is needed for the preparation beforehand.



  • Eye protection (goggles)
  • Disposable gloves (preferably nitrile)
  • Volumetric flasks, 1 dm3, x3
  • Beakers, 250 cm3, x3
  • Measuring cylinders, 50 cm3 or 100 cm3, x3
  • Stopwatch (optional)


  • Soluble starch, 4 g
  • Sodium metabisulfite (HARMFUL), 13.7 g
  • Potassium iodate(V) (OXIDISING), 15 g
  • Deionised or distilled water, 3 dm3

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection (goggles) throughout and use protective gloves.
  • Soluble starch – see CLEAPSS Hazcard HC040c.
  • Sodium metabisulfite, Na2S2O5(s), (HARMFUL) – see CLEAPSS Hazcard HC092. 
  • Mercury(II) chloride, HgCl2(s), (VERY TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC062. 
  • Potassium iodate(V), KIO3(s), (OXIDISING) – see CLEAPSS Hazcard HC080. 
  • To dispose of reaction mixtures after the demonstration, filter off the insoluble mercury(II) iodide and place the filter paper in a sealed plastic bag or a screw top bottle. Any unused solution B (see below) should be treated with excess sodium hydroxide to precipitate insoluble orange mercury hydroxide. This should be filtered off and the filter paper combined with the first residue. The mercury residues should then be retained for professional disposal. 


Before the demonstration

Using deionised or distilled water, make up three solutions, in graduated flasks as follows:

Solution A

  1. Make a paste of 4 g of soluble starch with a few drops of water.
  2. Pour 500 cm3 of boiling water on to this, and stir.
  3. Cool to room temperature.
  4. Add 13.7 g of sodium metabisulfite and stir.
  5. Make up to 1 dm3 with water in a volumetric flask.

Solution B

  1. Dissolve 3 g of mercury(II) chloride in some water.
  2. Make up the solution to 1 dm3 in a volumetric flask. This solution should be labelled HARMFUL.

Solution C

  1. Dissolve 15 g of potassium iodate(V) in water.
  2. Make up the solution to 1 dm3 in a volumetric flask.

The demonstration

Health and safety note

Wear gloves and eye protection.

  1. Use two measuring cylinders to mix 50 cm3 of solution A with 50 cm3 of solution B in a beaker.
  2. Use the third measuring cylinder to pour 50 cm3 of solution C into a second beaker.
  3. Pour the contents of the first beaker into the second.
  4. After about 5 seconds the mixture turns an opaque orange colour.
  5. After another few seconds the mixture suddenly turns blue-black.

Teaching notes

As well as being associated with Halloween, orange and black are the colours of the House of Nassau. A white background helps to make the colour changes more vivid. A white laboratory coat is ideal.

Scale up the volumes if the demonstration is being shown to a large audience.

Diluting all the solutions by a factor of two increases the time taken for the colour changes to occur.

The mechanisms of the various reactions taking place are complex and it is wise initially to keep the volume ratio as 1:1:1. Altering this ratio leads to unpredictable results.

Using a smaller volume of B speeds up the reaction.

If the volume of solution B is doubled, the appearance of the orange colour is delayed and the blue colour fails to appear at all.

The reactions occurring are fairly complex but the steps below explain the observations satisfactorily.

At first the metabisulfite ions react with water to form hydrogensulfite ions:

S2O52–(aq) + H2O(l) → 2HSO3(aq).

These hydrogensulfite ions reduce the iodate(V) ions to iodide ions:

IO3(aq) + 3HSO3(aq) → I(aq) + 3SO42–(aq) + 3H+(aq)

As soon as the concentration of iodide ions is large enough to exceed the solubility product of HgI2 (4.5 x 10–29 mol3 dm– 9), orange mercury(II) iodide solid is precipitated until all of the Hg2+ ions are used up (provided that there is an excess of I ions):

Hg2+(aq) + 2I(aq) → HgI2(s)

This precipitate is responsible for the initial orange cloudiness in the mixture.

If at this stage there are still I and IO3 ions in the mixture, a redox reaction producing molecular iodine takes place:

IO3(aq) + 5I(aq) + 6H+(aq) → 3I2(aq) + 3H2O(l)

Here, the intense blue-black starch-iodine complex is formed.