Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Wet paper demonstration of electrolysis


A magic stylus for colourful writing.

Apparatus and materials

Copper plate, about 20 cm x 20 cm (or similarly sized sheet of aluminium foil)

Stylus (see technical note)

Power supply, variable, 0-12 V, with smoothing capacitor of high ripple rating

Rheostat, (10 - 15 ohms), rated at 4 or 5 A

Filter papers, large

Potassium iodide and starch solution (see technical note)

Potassium sulfate and phenolphthalein solution (see technical notes)

Health & Safety and Technical notes

Read our standard health & safety guidance

The stylus is made from a 15 cm length of 1 cm dowel with two 0.5 cm screw-eyes screwed into one end. A 75 cm length of 26 SWG insulated copper wire is secured to each screw-eye and taped to the dowel with Sellotape or rubber bands. (The copper wire must make good electrical contact with the screw-eye.)

The stylus

0.1 M potassium iodide solution should be adequate. Weigh out 1.66 g of KI in a 250 ml beaker; add 70 ml of distilled water and stir. When dissolved, add 70 ml of distilled water to make up to 100 ml.

Starch indicator is best prepared on the day of use. Mix 1 g of soluble starch with distilled water to form a thin paste in a 250 ml beaker. Bring 80 ml of distilled water to the boil and add the paste, stirring as it goes in. Allow to cool and make up to about 100 ml. Add about 1 ml of starch indicator to 100 ml of KI solution.

0.1 M potassium sulfate solution: Weigh out 1.74 g of K2SO4 in a 250 ml beaker. Add 70 ml of distilled water and stir until dissolved. Pour into measuring cylinder and make up to 100 ml.

Phenolphthalein indicator: Weigh out 0.1 g of solid and dissolve 60 ml industrial methylated spirit; pour into a measuring cylinder and make up to 100 ml with water. Add 0.5 ml to 100 ml of potassium sulfate solution.


a Soak a filter paper in the solution of starch and potassium iodide. 

b Lay it flat on the metal sheet.

c Connect the leads from the screw-eyes in the stylus into a simple series circuit with the smoothed DC supply and a rheostat, set initially at its maximum value.

d Draw the stylus across the dampened paper. One of the screw eyes will leave a brown trace (caused by the arrival of iodine ions), or blue if starch is added.

e Try reversing the DC voltage.

f Try using an AC supply. Remember to remove the smoothing unit before switching to AC.

g Repeat using paper soaked with the potassium sulfate and phenolphthalein solution.


Teaching notes

1 This is a pretty experiment. Electrolysis on wet filter paper containing an indicator shows the product of electrolysis. A full explanation requires a lot of chemical knowledge which might not be appropriate here. Treat the colours as indicators of an electric current, whether AC or DC. 

2 Iodine will make a brown stain where its ions arrive. Phenolphthalein (dissolved in alcohol) is an indicator which, when added to the solution, will create a crimson stain where the potassium ions arrive.

3 The stylus can be used to write and draw patterns on the filter paper. The cell terminals can be reversed and the colours will swap over.

This experiment was safety-checked in January 2007 


Related guidance

Electric charge and current - a short history