A solid–solid reaction between lead nitrate and potassium iodide

This is a very quick demonstration, and should take no longer than two minutes. Show students that two solids can react together as white lead nitrate and white potassium iodide react to make yellow lead iodide.

Equipment

Apparatus

• Eye protection
• Small screw-top jar
• Balance
• Weighing boats or similar, x2

Chemicals

• Lead nitrate (TOXIC, DANGEROUS FOR THE ENVIRONMENT), 10 g
• Potassium iodide, 10 g

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout and wash hands after the demonstration.
• Lead nitrate, Pb(NO₃)₂(s), (TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC057a.
• Potassium iodide, Kl(s) – see CLEAPSS Hazcard HC047b.
• The resulting solid mixture from the demonstration should be retained in a sealed container for professional disposal.

Procedure

1. Weigh out equal masses of both compounds. These are then in approximately the stoichiometric ratio. Between 5 g and 10 g of each is suitable.
2. Mix the solids in a screw-topped jar and shake for several seconds. The yellow colour of lead iodide will be seen.
3. 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 to 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.

This experiment exploring diffusion in liquids is a class practical using the same compounds but as solutions.