This is a very quick demonstration showing that two solids can react together. White lead nitrate and white potassium iodide react to make yellow lead iodide.


Lesson Organisation


This demonstration is very quick and will take no more than 2 minutes.


Apparatus and Chemicals


For one demonstrationEye protection (goggles)A small screw-top jarBalanceWeighing boats or similar, 2
Lead nitrate (Toxic, Dangerous for the environment), 20gPotassium iodide (Low hazard), 20g


Technical Notes


Lead nitrate (Toxic, Dangerous for the environment) Refer to CLEAPSS Hazcard 57APotassium iodide (Low hazard) Refer to CLEAPSS Hazcard 47B
1 The resulting solid mixture from the demonstration should be retained in a sealed container for professional disposal.


Procedure


HEALTH & SAFETY: Wear goggles
a Weigh out equal masses of both compounds. These are then in approximately the stoichiometric ratio. Between 10 g and 20 g of each is suitable.
b Mix the solids in a screw topped jar and shake for several seconds. The yellow colour of lead iodide will be seen.


c Make a little more of the mixture and place it quickly into a beaker containing a little water. The reaction will be much more rapid.


Teaching notes


The demonstration might have more impact if the jar is opaque and the yellow product can be poured out and shown to the unsuspecting audience. Have a white background available.
Point out that for a reaction ot occur, particles of the reactants must meet. This is much easier in solution (where the particles are free to move) than in the solid state.
The reaction is:
Pb(NO3)2(s) + 2KI(s) → 2KNO3(s) + PbI2(s)
All of these compounds are white except lead iodide, which is yellow.
Lead ethanoate can be substituted for lead nitrate, but the reaction is much slower.
The experiment Diffusion in liquids is a class practical using the same compounds but as solutions.