Demonstrate dramatic colour changes as a result of redox and precipitation reactions.

In this experiment redox and precipitation reactions take place, including two colour changes.


A mixture containing iodate(V) ions, hydrogensulfate(IV) (hydrogensulfite) ions, mercury(II) ions and starch suddenly turns orange after a few seconds, with the formation of a precipitate of mercury(II) iodide. A few seconds later the mixture suddenly turns blue-black with the formation of the starch-iodine complex. Orange and black are the colours of the House of Nassau. The reactions taking place involve redox and precipitation.

Lesson organisation

This experiment has a good visual impact and would be one way of stimulating interest in chemistry, perhaps via an Open Day. The reactions involved are not too complex and could be introduced during an advanced level course when studying kinetics.

The demonstration lasts about 3 - 5 minutes but about 20 minutes is needed for the preparation beforehand.

Apparatus Chemicals

For each demonstration:

Eye protection: goggles

Disposable gloves (preferably nitrile)

Volumetric flasks (1 dm3), 3

Beakers (250 cm3), 3

Measuring cylinders (50 cm3 or 100 cm3), 3

Stopwatch (optional)

Soluble starch, 4 g

Sodium metabisulfite (HARMFUL), 13.7 g


Potassium iodate(V) (OXIDISING), 15 g

Deionised or distilled water, 3 dm3

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear goggles and protective gloves.

Soluble starch - see CLEAPSS Hazcard.

Sodium metabisulfite, Na2S2O5(s), (HARMFUL) - see CLEAPSS Hazcard. 

Mercury(II) chloride, HgCl2(s), (VERY TOXIC, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard. 

Potassium iodate(V), KIO3(s), (OXIDISING) - see CLEAPSS Hazcard. 

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 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  
a Make a paste of 4 g of soluble starch with a few drops of water.

b Pour 500 cm3 of boiling water on to this, and stir.

c Cool to room temperature

d Add 13.7 g of sodium metabisulfite and stir.

e Make up to 1 dm3 with water in a volumetric flask.

Solution B  
a Dissolve 3 g of mercury(II) chloride in some water.

b Make up the solution to 1 dm3 in a volumetric flask. This solution should be labelled HARMFUL.

Solution C  
a Dissolve 15 g of potassium iodate(V) in water.

b  Make up the solution to 1 dm3 in a volumetric flask.

The demonstration
HEALTH & SAFETY: Wear gloves and eye protection

a Use two measuring cylinders to mix 50 cm3 of solution A with 50 cm3 of solution B in a beaker.

b Use the third measuring cylinder to pour 50 cm3 of solution C into a second beaker.

c Pour the contents of the first beaker into the second.

d After about 5 seconds the mixture turns an opaque orange colour.

e After another few seconds the mixture suddenly turns blue-black.

Teaching notes

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 following steps 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)

and the intense blue-black starch-iodine complex is formed.

Health & Safety checked, 2016


This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated 2016