This practical demonstrates how diffusion occurs in a liquid. Small crystals of potassium iodide and lead nitrate are placed apart in a petri dish which is then filled with deionised (distilled) water. The two substances react to form a yellow crust of solid lead iodide at the point they meet in the dish.
Students should be able to see that the solid lead iodide forms closer to the lead nitrate crystal. This is because the iodide ions are smaller than the lead particles and so move faster.
Equipment required (per set)
- Deionised (distilled) water
- Small lead nitrate crystal (toxic)
- Small potassium iodide crystal
- Petri dish
- Forceps (for handling the crystals)
The formula for this reaction is below:
Pb(NO3)2 + 2KI 
2KNO3 + PbI2