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

Lead tree

Demonstration

A delightful demonstration of electrolysis.

Apparatus and materials

Variable voltage supply, 0-12 V

Leads, 4 mm, 2

Ammeter, 0-1 A

Cell to contain liquid - sized to suit projector

Electrodes, one pair

Lead acetate (ethanoate) solution

Projection microscope or slide projector

Health & Safety and Technical notes


Lead salts are toxic and must be handled with care. Hands must be washed thoroughly after handling them and their solutions.

ApparatusThe cell should have flat sides. It should be of glass or Perspex.

If you use a projection system, it should be an old optical projector or 'flexcam' and display screen.

The electrodes can be of carbon (for example, two pencil leads) but platinum wire is to be preferred. Lead electrodes can also be used.

A suitable strength is 30 g of lead acetate to 100 g of water. Should difficulty be experienced in dissolving the lead acetate, add a few drops of glacial acetic acid.
 

Procedure


a Put the solution of lead acetate into the cell and arrange the two electrodes suitably. In the case of wire electrodes, one wire should run down one side and round the bottom as shown. This is the anode. The central wire is the cathode. 

b Place the projection microscope next to the cell, and ensure that a clear image of the electrodes is projected. If using a slide projector, remove the slide holder and support the cell in its place.

c Connect the power supply to the electrodes. (Positive terminal to anode, negative terminal to cathode.)

d Pass a small current, preferably less than 50 mA. About 10 volts DC may be necessary. A beautiful ‘tree’ of crystalline lead will be grown.

e The tree can be made to dwindle away by reversing the current. A new tree will grow at the other electrode.

 

Teaching notes


A forest of lead crystals grows on the negative electrode. On reversing the potential difference, that forest shrinks and a new one grows on the other electrode. 

Make sure that students can correctly identify the electrodes by attaching a ‘+’ label to the anode, and ensure that this is visible in the projected image. Check carefully for optical reversion of the image.

This experiment was safety-checked in January 2007 

 

Related guidance


Electric charge and current - a short history